Radar Subsystems

NEETS MODULE 18-Radar Principles

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2 RADAR SUBSYSTEMS

Learning Objectives

After you finish this chapter, you should be able to do the following: 1. Describe, in general terms, the function of a radar synchronizer. 2. State the basic requirements and types of master synchronizers. 3. Describe the purpose, requirements, and operation of a radar modulator. 4. Describe the basic operating sequence of a keyed-oscillator transmitter. 5. Describe the basic operating sequence of a power-amplifier transmitter. 6. State the purpose of a duplexer. 7. State the operational principles of tr and atr tubes.

8. Describe the basic operating sequence of series and parallel connected duplexers. 9. List the basic design requirements of an effective radar receiver. 10. List the major sections of a typical radar receiver. 11. Using a block diagram, describe the operational characteristics of a typical radar receiver. 2.1 INTRODUCTION TO RADAR SUBSYSTEMS Any radar system has several major subsystems that perform standard functions. A typical radar system consists of a SYNCHRONIZER (also called the TIMER or TRIGGER GENERATOR), a TRANSMITTER, a DUPLEXER, a RECEIVER, and an INDICATOR. These major subsystems were briefly described in chapter 1. This chapter will describe the operation of the synchronizer, transmitter, duplexer, and receiver of a typical pulse radar system and briefly analyze the circuits used. Chapter 3 will describe typical indicator and antenna subsystems. Because radar systems vary widely in specific design, only a general description of representative circuits is presented in this chapter.

2.1.1 Synchronizers The synchronizer is often referred to as the "heart" of the radar system because it controls and provides timing for the operation of the entire system. Other names for the synchronizer are the TIMER and the KEYER. We will use the term synchronizer in our discussion. In some complex systems the synchronizer is part of a system computer that performs many functions other than system timing. NEETS MODULE 18-Radar Principles

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2.1.2 Synchronizer Function The specific function of the synchronizer is to produce TRIGGER PULSES that start the transmitter, indicator sweep circuits, and ranging circuits. Timing or control is the function of the majority of circuits in radar. Circuits in a radar set accomplish control and timing functions by producing a variety of voltage waveforms, such as square waves, sawtooth waves, trapezoidal waves, rectangular waves, brief rectangular pulses, and sharp peaks. Although all of these circuits can be broadly classified as timing circuits, the specific function of any individual circuit could also be wave shaping or wave generation. The operation of many of these circuits and associated terms were described in detail in NEETS, Module 9, Wave-Generation and Wave- Shaping Circuits.

Q1. What is the purpose of the synchronizer in a radar system? Q2. What is the purpose of the majority of circuits in a radar system? 2.1.3 Synchronizer Operation Radar systems may be classified as either SELF-SYNCHRONIZED or EXTERNALLY SYNCHRONIZED systems. In a self-synchronized system, the timing trigger pulses are generated in the transmitter. In an externally synchronized system, the timing trigger pulses are generated by a MASTER OSCILLATOR, which is usually external to the transmitter. The master oscillator in an externally synchronized system may be a BLOCKING OSCILLATOR, a SINE-WAVE OSCILLATOR, or an ASTABLE (FREE-RUNNING) MULTI-VIBRATOR. When a blocking oscillator is used as a master oscillator, the timing trigger pulses are usually obtained directly from the oscillator. When a sine-wave oscillator or an astable multivibrator is used as a master oscillator, pulse-shaping circuits are required to form the necessary timing trigger pulses. In an externally synchronized radar system, the pulse repetition rate (prr) of the timing trigger pulses from the master oscillator determines the prr of the transmitter.

In a self-synchronized radar system, the prr of the timing trigger pulses is determined by the prr of the modulator or transmitter. NEETS MODULE 18-Radar Principles

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Associated with every radar system is an indicator, such as a cathode-ray tube, and associated circuitry. The indicator can present range, bearing, and elevation data in visual form so that a detected object may be located. Trigger pulses from the synchronizer are frequently used to produce gate (or enabling) pulses. When applied to the indicator, gate pulses perform the following functions: 1. Initiate and time the duration of the indicator sweep voltage 2. Intensify the cathode-ray tube electron beam during the sweep period so that the echo pulses may be displayed 3. Gate a range marker generator so that range marker signals may be superimposed on the indicator presentation Figure 2-1 shows the time relationships of the various waveforms in a typical radar set.

The timing trigger pulses are applied to both the transmitter and the indicator. When a trigger pulse is applied to the transmitter, a short burst of transmitter pulses (rf energy) is generated. Figure 2-1 Time relationship of waveforms NEETS MODULE 18-Radar Principles

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This energy is conducted along a transmission line to the radar antenna. It is radiated by the antenna into space. When this transmitter energy strikes one or more reflecting objects in its path, some of the transmitted energy is reflected back to the antenna as echo pulses. Echo pulses from three reflecting targets at different ranges are illustrated in figure 2-1. These echoes are converted to the corresponding receiver output signals as shown in the figure. The larger initial and final pulses in the receiver output signal are caused by the energy that leaks through the duplexer when a pulse is being transmitted.

The indicator sweep voltage shown in figure 2-1 is initiated at the same time the transmitter is triggered. In other applications, it may be more desirable to delay the timing trigger pulse that is to be fed to the indicator sweep circuit. Delaying the trigger pulse will initiate the indicator sweep after a pulse is transmitted. Note in figure 2-1 that the positive portion of the indicator intensity gate pulse (applied to the cathode-ray tube control grid) occurs only during the indicator sweep time. As a result, the visible cathode-ray tube trace occurs only during sweep time and is eliminated during the flyback (retrace) time. The negative portion of the range-marker gate pulse also occurs during the indicator sweep time. This negative gate pulse is applied to a range-marker generator, which produces a series of range marks.

The range marks are equally spaced and are produced only for the duration of the range- marker gate pulse. When the range marks are combined (mixed) with the receiver output signal, the resulting video signal applied to the indicator may appear as shown at the bottom of figure 2-1. Q3. A self-synchronized radar system obtains timing trigger pulses from what source? Q4. What type of multivibrator can be used as a radar master oscillator? Q5. In an externally synchronized radar, what determines the prr of the transmitter?

Q6. In figure 2-1, what causes the initial and final pulses on the receiver output signal? NEETS MODULE 18-Radar Principles

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2.1.4 Basic Synchronizer Circuits The basic synchronizer circuit should meet the following three basic requirements: 1. It must be free running (astable). Because the synchronizer is the heart of the radar, it must establish the zero time reference and the prf (prr). 2. It should be stable in frequency. For accurate ranging, the prr and its reciprocal, pulse- repetition time (prt), must not change between pulses. 3. The frequency must be variable to enable the radar to operate at different ranges. Three basic synchronizer circuits can meet the above mentioned requirements. They are the SINEWAVE OSCILLATOR, the SINGLE-SWING BLOCKING OSCILLATOR, and the MASTERTRIGGER (ASTABLE) MULTIVIBRATOR.

Figure 2-2 shows the block diagrams and waveforms of these three synchronizers as they are used in externally synchronized radar systems. In each case, equally spaced timing trigger pulses are produced. The prr of each series of timing trigger pulses is determined by the operating frequency of the associated master oscillator. Figure 2-2 Timers used in externally synchronized radar systems NEETS MODULE 18-Radar Principles

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2.1.4.1 Sine-Wave Oscillator Synchronizer In the sine-wave oscillator synchronizer (figure 2-2, view A), a sine-wave oscillator is used for the basic timing device (master oscillator). The oscillator output is applied to both an overdriven amplifier and the radar indicator. The sine waves applied to the overdriven amplifier are shaped into square waves. These square waves are then converted into positive and negative timing trigger pulses by means of a short-time- constant RC differentiator. By means of a limiter, either the positive or negative trigger pulses from the RC differentiator are removed. This leaves trigger pulses of only one polarity. For example, the limiter in view A of figure 2-2 is a negative-lobe limiter; that is, the limiter removes the negative trigger pulses and passes only positive trigger pulses to the radar transmitter.

A disadvantage of a sine-wave oscillator synchronizer is the large number of pulse- shaping circuits required to produce the necessary timing trigger pulses. 2.1.4.2 Master Trigger (Astable) Multivibrator Synchronizer In a master trigger (astable) multivibrator synchronizer (view B, figure 2-2), the master oscillator generally is an astable multivibrator. The multivibrator is either ASYMMETRICAL or SYMMETRICAL. If the multivibrator is asymmetrical, it generates rectangular waves. If the multivibrator is symmetrical, it generates square waves. In either case, the timing trigger pulses are equally spaced after a limiter removes undesired positive or negative lobes.

There are two transistors in an astable multivibrator. The two output voltages are equal in amplitude, but are 180 degrees out of phase. The output of the astable multivibrator consists of positive and negative rectangular waves. Positive rectangular waves are applied to an RC differentiator and converted into positive and negative trigger pulses. As in the sine-wave synchronizer, the negative trigger pulses are removed by means of a negative-lobe limiter, and the positive pulses are applied to the transmitter. Both positive and negative rectangular waves from the astable multivibrator are applied to the indicator. One set of waves is used to intensify the cathode-ray tube electron beam for the duration of the sweep. The other set of waves is used to gate (turn on) the range marker generator.

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2.1.4.3 Single-Swing Blocking Oscillator Synchronizer In the single-swing, blocking-oscillator synchronizer, shown in view C of figure 2-2, a free-running, single-swing blocking oscillator is generally used as the master oscillator. The advantage of the single swing blocking oscillator is that it generates sharp trigger pulses without additional shaping circuitry. Timing trigger pulses of only one polarity are obtained by means of a limiter. Gating pulses for the indicator circuits are produced by applying the output of the blocking oscillator to a one-shot multivibrator or another variable time delay circuit (not shown). Crystal-controlled oscillators may be used when very stable frequency operation is required.

Q7. What basic circuits meet the requirements of an externally synchronized master oscillator? Q8. Name a disadvantage of sine-wave oscillator synchronizers. Q9. Which of the basic timing circuits produces sharp trigger pulses directly? 2.2 TRANSMITTERS The TRANSMITTER produces the short duration high-power rf pulses of energy that are radiated into space by the antenna. Two main types of transmitters are now in common use. The first is the KEYED-OSCILLATOR type. In this transmitter one stage or tube, usually a magnetron, produces the rf pulse. The oscillator tube is keyed by a high-power dc pulse of energy generated by a separate unit called the MODULATOR (discussed in the following section). The second type of transmitter consists of a POWER- AMPLIFIER CHAIN. This transmitter system begins with an rf pulse of very low power.

This low-level pulse is then amplified by a series (chain) of power amplifiers to the high level of power desired in a transmitter pulse. In most power-amplifier transmitters, each of the power-amplifier stages is pulse modulated in a manner similar to the oscillator in the keyed-oscillator type. Because the modulator is common to both types of transmitter systems, the operation of a typical modulator will be discussed first. NEETS MODULE 18-Radar Principles

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2.2.1 Radar Modulator The modulator controls the radar pulse width by means of a rectangular dc pulse (modulator pulse) of the required duration and amplitude. The peak power of the transmitted rf pulse depends on the amplitude of the modulator pulse. Figure 2-3 shows the waveforms of the trigger pulse applied by the synchronizer to the modulator, the modulator pulse applied to the radar transmitter, and the transmitted rf pulse. As you can see in the figure, the modulator pulse is applied to the transmitter the instant the modulator receives the trigger pulse from the synchronizer (T1, T2). The modulator pulse is flat on top and has very steep leading and trailing edges. These pulse characteristics are necessary for the proper operation of the transmitter and for the accurate determination of target range. The range timing circuits must be triggered the instant the leading edge of the transmitted rf pulse leaves the transmitter. In this way, the trigger pulse that controls the operation of the modulator also synchronizes the cathode- ray tube sweep circuits and range measuring circuits.

Figure 2-3 Transmitter waveforms NEETS MODULE 18-Radar Principles

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MAGNETRON OSCILLATORS are capable of generating rf pulses with very high peak power at frequencies ranging from 600 to 30,000 megahertz. However, if its cathode voltage changes, the magnetron oscillator shifts in frequency. To avoid such a frequency change, you must ensure that the amplitude of the modulator (dc) pulse remains constant for the duration of the transmitted rf pulse. That is, the modulator pulse must have a flat top. The range of cathode voltages over which a magnetron oscillates in the desired frequency spectrum is relatively small.

When a low voltage is applied to a magnetron, the magnetron produces a noise voltage output instead of oscillations. If this noise enters the receiver, it can completely mask the returning echoes. If a modulator pulse builds up and decays slowly, noise is produced at both the beginning and end of the pulse. Therefore, for efficient radar operation, a magnetron requires a modulator pulse that has a flat top and steep leading and trailing edges. An effective modulator pulse must perform in the following manner: • Rise from zero to its maximum value almost instantaneously • Remain at its maximum value for the duration of the transmitted rf pulse • Fall from its maximum value to zero almost instantaneously In radars that require accurate range measurement, the transmitted rf pulse must have a steep leading edge. The leading edge of the echo is used for range measurement. If the leading edge of the echo is not steep and clearly defined, accurate range measurement is not possible. The leading and trailing edges of echoes have the same shape as the leading and trailing edges of the transmitted rf pulse.

A transmitted rf pulse with a steep trailing edge is essential for the detection of objects at short ranges. If the magnetron output voltage drops gradually from its maximum value to zero, it contributes very little to the usable energy of the transmitted rf pulse. Furthermore, part of the magnetron output voltage enters the receiver and obscures nearby object echoes. 2.2.1.1 Types of Modulators The two types of modulators are the LINE-PULSING MODULATOR and the HARD- TUBE MODULATOR. (A hard tube is a high-vacuum electron tube.) The line-pulsing modulator stores energy and forms pulses in the same circuit element. This element is usually the pulse-forming network. The hard-tube modulator forms the pulse in the driver; the pulse is then amplified and applied to the modulator. The hard tube modulator has been replaced by the line-pulsed modulator in most cases. This is because the hard- tube modulator has lower efficiency, its circuits are more complex, a higher power supply voltage is required, and it is more sensitive to voltage changes.

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The line-pulsing modulator is easier to maintain because of its less complex circuitry. Also, for a given amount of power output, it is lighter and more compact. Because it is the principally used modulator in modern radar, it is the only type that will be discussed. Figure 2-4 shows the basic sections of a radar modulator. They are as follows: • The power supply. • The storage element (a circuit element or network used to store energy). • The charging impedance (used to control the charge time of the storage element and to prevent short-circuiting of the power supply during the modulator pulse).

• The modulator switch (used to discharge the energy stored by the storage element through the transmitter oscillator during the modulator pulse). Figure 2-4 Basic line-pulsing modulator block diagram NEETS MODULE 18-Radar Principles

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View A of figure 2-4 shows the modulator switch open and the storage element charging. With the modulator switch open, the transmitter produces no power output, but the storage element stores a large amount of energy. View B shows the modulator switch closed and the storage element discharging through the transmitter. The energy stored by the storage element is released in the form of a high-power, dc modulator pulse. The transmitter converts the dc modulator pulse to an rf pulse, which is radiated into space by the radar antenna. Thus, the modulator switch is closed for the duration of a transmitted rf pulse, but open between pulses.

Many different kinds of components are used in radar modulators. The power supply generally produces a high-voltage output, either alternating or direct current. The charging impedance may be a resistor or an inductor. The storage element is generally a capacitor, an artificial transmission line, or a pulse-forming network. The modulator switch is usually a thyratron. 2.2.1.2 Modulator Storage Element Capacitor storage elements are used only in modulators that have a dc power supply and an electron tube modulator switch.

The capacitor storage element is charged to a high voltage by the dc power supply. It releases only a small part of its stored energy to the transmitter. The electron-tube modulator switch controls the charging and discharging of the capacitor storage element. The artificial transmission line storage element, shown in view A of figure 2-5, consists of identical capacitors (C) and inductors (L) arranged to simulate sections of a transmission line. The artificial transmission line serves two purposes: (1) to store energy when the modulator switch is open (between transmitted rf pulses) and (2) to discharge and form a rectangular dc pulse (modulator pulse) of the required duration when the modulator switch is closed.

Figure 2-5A Modulator storage elements NEETS MODULE 18-Radar Principles

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The duration of the modulator pulse depends on the values of inductance and capacitance in each LC section of the artificial transmission line in view A and the number of LC sections used. Other arrangements of capacitors and inductors (such as the pulse-forming network shown in view B) are very similar in operation to artificial transmission lines. ARTIFICIAL TRANSMISSION LINES and PULSE-FORMING NETWORKS (pfn) are used more often than the capacitor-type storage elements. ARTIFICIAL TRANSMISSION LINE.—Figure 2 -6 shows a radar modulator that uses an artificial transmission line as its storage element. A modulator switch controls the pulse-repetition rate. When the modulator switch is open (between transmitted rf pulses), the transmission line charges.

Figure 2-5B.— Modulator storage elements Figure 2-6 Modulator with an artificial transmission line for the storage element NEETS MODULE 18-Radar Principles

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The charge path includes the primary of the pulse transformer, the dc power supply, and the charging impedance. When the modulator switch is closed, the transmission line discharges through the series circuit. This circuit consists of the modulator switch and the primary of the pulse transformer. The artificial transmission line is effectively an open circuit at its output end. Therefore, when the voltage wave reaches the output end of the line, it is reflected. As the reflected wave propagates from the output end back toward the input end of the line, it completely discharges each section of the line. When the reflected wave reaches the input end of the line, the line is completely discharged, and the modulator pulse ceases abruptly. If the oscillator and pulse transformer circuit impedance is properly matched to the line impedance, the voltage pulse that appears across the transformer primary equals one-half the voltage to which the line was initially charged.

The width of the pulse generated by an artificial transmission line depends on the time required for a voltage wave to travel from the input end to the output end of the line and back. Therefore, we can say the pulse width depends on the velocity of propagation along the line (determined by the inductances and capacitances of each section of the line) and the number of line sections (the length of the line). PULSE-FORMING NETWORKS.—A pulse -forming network is similar to an artificial transmission line in that it stores energy between pulses and produces a nearly rectangular pulse. The pulse-forming network in view B of figure 2-5 consists of inductors and capacitors so arranged that they approximate the behavior of an artificial transmission line.

Each capacitor in the artificial transmission line, shown in view A, must carry the high voltage required for the modulator pulse. Because each capacitor must be insulated for this high voltage, an artificial transmission line consisting of many sections would be bulky and cumbersome. The pulse-forming network, shown in view B of figure 2-5, can carry high voltage but does not require bulky insulation on all of its capacitors. Only series capacitor C1 must have high-voltage insulation. Because the other capacitors are in parallel with the corresponding inductors, the modulator pulse voltage divides nearly equally among them.

Thus, except for C1, the elements of the pulse-forming network are relatively small. Pulse-forming networks are often insulated by immersing each circuit element in oil. The network is usually enclosed in a metal box on which the pulse width, characteristic impedance, and safe operating voltage of the network are marked. If one element in such a network fails, the entire network must be replaced. NEETS MODULE 18-Radar Principles

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Q10. What are the two basic types of transmitters? Q11. What controls transmitter pulse width? Q12. In addition to a flat top, what characteristics must a modulator pulse have? Q13. What type of modulator is most commonly used in modern radar systems? Q14. What three types of storage elements most often are used in modulators? Q15. What characteristic is determined by the time required for a voltage wave to travel from the input end of an artificial transmission line to the output end and back again? 2.2.1.3 Modulator Switching Devices The voltage stored in a storage-element capacitor, artificial transmission line, or pulse- forming network must be discharged through a MODULATOR SWITCHING DEVICE.

The modulator switching device conducts for the duration of the modulator pulse and is an open circuit between pulses. Thus, the modulator switch must perform the following four functions: 1. Close very quickly and then reach full conduction in a small fraction of a microsecond 2. Conduct large currents (tens or hundreds of amperes) and withstand large voltages (thousands of volts) 3. Cease conducting (become an open circuit) with the same speed that it starts to conduct 4. Consume only a very small fraction of the power that passes through it NEETS MODULE 18-Radar Principles

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These switching and conducting requirements are met best by the THYRATRON tube. The thyratron tube is normally held below cutoff by a negative grid voltage and conducts when a positive trigger pulse is applied to its grid. Once fired, the thyratron tube continues to conduct as long as the storage element (artificial transmission line or pulse- forming network) is discharging. During discharge of the storage element, the gas in the thyratron tube is highly ionized. While the storage element discharges, the plate-to-cathode resistance of the thyratron is practically zero. When the storage element is completely discharged, current ceases to flow through the thyratron and the gases become deionized; the negative grid bias regains control, and the thyratron is cut off (the modulator switch opens).

Most radar modulators use a high-voltage, dc power supply. Typical dc power supplies for radar modulators use a half-wave rectifier, a full-wave rectifier, or a bridge rectifier. The modulator charging impedance, shown in figure 2-7, prevents the dc power supply from becoming short-circuited when the modulator switch closes. When the modulator switch is open, the charging impedance also controls the rate at which the storage element charges. When the charging impedance is small, the storage element charges rapidly.

Figure 2-7 Modulator charging impedance NEETS MODULE 18-Radar Principles

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Many different kinds of charging impedance and charging circuits are used in radar modulators. The type of charging impedance and charging circuit used depends on the following five elements: 1. The type of power supply (ac or dc) 2. The type of storage element 3. The amount of modulator pulse voltage required 4. The pulse-repetition rate 5. The frequency of the available ac supply voltage Q16. What type of tube best meets the requirements of a modulator switching element? Q17. What modulator element controls the rate at which the storage element charges?

2.2.2 KEYED-OSCILLATOR TRANSMITTER The KEYED-OSCILLATOR TRANSMITTER most often uses a MAGNETRON as the power oscillator. The following discussion is a description of a magnetron used as a keyed-oscillator radar transmitter. Figure 2-8 shows the typical transmitter system that uses a magnetron oscillator, waveguide transmission line, and microwave antenna. The magnetron at the bottom of the figure is connected to the waveguide by a coaxial connector. High-power magnetrons, however, are usually coupled directly to the waveguide. A cutaway view of a typical waveguide-coupled magnetron is shown in figure 2-9.

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Figure 2-8 Keyed oscillator transmitter physical layout NEETS MODULE 18-Radar Principles

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The magnetron is an electron tube in which a magnetic (H) field between the cathode and plate is perpendicular to an electric (E) field. Tuned circuits, in the form of cylindrical cavities in the plate, produce rf electric fields. Electrons interact with these fields in the space between the cathode and plate to produce an ac power output. Magnetrons function as self-excited microwave oscillators. These multicavity devices may be used in radar transmitters as either pulsed or cw oscillators at frequencies ranging from approximately 600 to 30,000 megahertz. (If you wish to review magnetron operation in more detail, refer to NEETS, Module 11, Microwave Principles.) Let’s examine the following characteristics of a magnetron used as a pulse radar transmitter oscillator stage: • Stability • Pulse characteristics • The magnet • Output coupling Figure 2-9 Typical magnetron NEETS MODULE 18-Radar Principles

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2.2.2.1 Stability In speaking of a magnetron oscillator, STABILITY usually refers to the stability of the mode of operation of the magnetron. The two main types of mode instability are MODE SKIPPING and MODE SHIFTING. Mode skipping (or misfiring) is a condition in which the magnetron fires randomly in an undesired, interfering mode during some pulse times, but not in others. Pulse characteristics and tube noises are factors in mode skipping. Mode shifting is a condition in which the magnetron changes from one mode to another during pulse time. This is highly undesirable and does not occur if the modulator pulse is of the proper shape, unless the cathode of the magnetron is in very poor condition.

2.2.2.2 Pulse Characteristics PULSE CHARACTERISTICS are the make up of the high-voltage modulator pulse that is applied to the magnetron. The pulse should have a steep leading edge, a flat top, and a steep trailing edge. If the leading edge is not steep, the magnetron may begin to oscillate before the pulse reaches its maximum level. Since these low-power oscillations will occur in a different mode, the mode of the magnetron will be shifted as the pulse reaches maximum power. This mode shifting will result in an undesirable magnetron output. For the same reason (to prevent mode shifting), the top of the modulator pulse should be as flat as possible. Variations in the applied operating power will cause variations in the mode of operation. The trailing edge of the pulse should also be steep for the same reason--to prevent mode shifting.

2.2.2.3 Magnet The purpose of the MAGNET is to produce a fairly uniform magnetic field of the desired value over the interaction space between the cathode and plate of the magnetron. The strength of the magnet is critical for proper operation. If the magnetic field strength is too high, the magnetron will not oscillate. If the magnetic field strength is too low, the plate current will be excessive and power output will be low. Frequency of operation will also be affected. Since the strength of the magnet is critical, you should be careful when handling the magnet. Striking the magnet, especially with a ferromagnetic object, will misalign the molecular structure of the magnet and decrease the field strength.

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2.2.2.4 Output Coupling The OUTPUT COUPLING transfers the rf energy from the magnetron to the output transmission line (coaxial line or waveguide). A number of considerations impose restrictions upon the output circuit. The wavelength (frequency) and the power level of the magnetron output energy determine whether the transmission line to the antenna will be waveguide or coaxial line. The coaxial output circuit consists of a length of coaxial line in which the center conductor is shaped into a loop and inserted into one of the magnetron cavities for magnetic coupling. The load side of the coupling line may feed either an external coaxial line or a waveguide. If the external line is coaxial, the connection may be direct or by means of choke joints. If the external line is a waveguide, the output circuit must include a satisfactory junction from the coaxial line to the waveguide. One type of junction used quite often is the PROBE COUPLER. The probe coupler acts as an antenna radiating into the waveguide.

The waveguide output may be fed directly by an opening (slot) into one of the magnetron cavities, as shown in figure 2-9. This opening must be covered by an iris window to maintain the vacuum seal. The peak power ratings of magnetrons range from a few thousand watts (kilowatts) to several million watts (megawatts). The average power ratings are much lower, however, and vary from a few watts to several kilowatts. Additionally, many of the magnetrons used in modern radar systems are tunable in frequency. Typically, a tunable magnetron can vary the output frequency ±5 percent about the center of its frequency band. Thus the carrier frequency of radar can be changed to obtain the best operation or avoid electronic jamming on a particular frequency.

Modulator signals of many thousands of volts are applied to the magnetron cathode during operation. These high voltage levels require large glass posts to insulate the cathode and filaments from the anode block. In some high-power magnetrons, the cathode is completely enclosed in a container filled with insulating oil. WARNING All radar transmitters contain lethal voltages. Extreme care and strict observance of all posted safety precautions are essential when working on a radar transmitter. NEETS MODULE 18-Radar Principles

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Q18. What is the frequency range of magnetron oscillators? Q19. What two forms of instability are common in magnetrons? Q20. What is the effect on magnetron operation if the magnetic field strength is too high? Q21. What is the typical frequency range about the center frequency of a tunable magnetron? 2.2.3 Power-Amplifier Transmitter POWER-AMPLIFIER TRANSMITTERS are used in many recently developed radar sets. This type of transmitter was developed because of the need for more stable operation of the moving target indicator (mti). In a magnetron transmitting system, the high-power magnetron oscillator has a tendency to drift in frequency because of temperature variations, changes in the modulating pulse, and various other effects.

Frequency drift is compensated for, in part, by the use of automatic frequency control (afc) circuits designed to control the frequency of the local oscillator in the receiver system. This, however, does not completely eliminate the undesirable effects of frequency drift on mti operation. The power-amplifier transmitter system does the same thing as the keyed-oscillator transmitter but with fewer stability problems. It generates, shapes, and amplifies pulses of rf energy for transmission. Figure 2-10 is a block diagram of a typical power-amplifier transmitter system. In this transmitter system a multicavity klystron tube amplifies lower-powered rf pulses that have been generated and shaped in other stages. CROSSED-FIELD AMPLIFIERS (AMPLITRONS) are used in radar systems with a wide band of transmitted frequencies because they are stable over a wider frequency range. A crossed-field amplifier transmitter is discussed later in this section.

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In figure 2-10, the power-amplifier chain input signals are generated by heterodyning (mixing) two frequencies. That is, two different frequencies are fed to a mixer stage (mixer amplifier) and the resultant, either the sum or difference frequency, may be selected as the output. (The operation of mixer circuits is explained in more detail in the section on receivers.) The low-power pulse is then amplified by intermediate power amplifier stages and applied to the klystron power-amplifier. The klystron power amplifier concentrates the rf output energy into a very narrow frequency spectrum. This concentration makes the system more sensitive to smaller targets. In addition the detection range of all targets is increased.

Figure 2-10 Power amplifier transmitter block diagram NEETS MODULE 18-Radar Principles

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To examine the operation of the transmitter, we will trace the signal through the entire circuit. The local oscillator shown at the left of figure 2-10 is a very stable rf oscillator that produces two cw rf outputs. As shown, the cw output is sent to the receiver system; the cw output is also one of the two rf signals fed to the mixer amplifier by way of the two BUFFER AMPLIFIER STAGES. The buffer amplifiers raise the power level of the signal and also isolate the local oscillator. The COHERENT OSCILLATOR (COHO) is triggered by the system trigger and produces as its output an rf pulse. This rf pulse is fed directly to the mixer amplifier.

The mixer-amplifier stage receives three signals: the coherent rf pulse, the local oscillator cw rf signal, and a dc modulating pulse from the low-voltage modulator. The coherent and local oscillator signals are mixed to produce sum and difference frequency signals. Either of these may be selected as the output. The modulator pulse serves the same purpose as in the keyed-oscillator transmitter, because it determines the pulse width and power level. The mixer stage functions only during the modulator pulse time. Thus the mixer amplifier produces an output of rf pulses in which the frequency may be either the sum or difference of the coherent and local oscillator signals.

The mixer-amplifier feeds the pulses of rf energy to an intermediate power amplifier. This amplifier stage is similar to the buffer-amplifier stage except that it is a pulsed amplifier. That is, the pulsed amplifier has operating power only during the time the modulator pulse from the low-voltage modulator is applied to the stage. The amplified output signal is fed to a second intermediate power amplifier that operates in the same manner as the first. From the second intermediate power amplifier, the signal is fed to the KLYSTRON POWER AMPLIFIER. This stage is a multicavity power klystron. The input rf signal is used as the exciter signal for the first cavity. High-voltage modulating pulses from the high-voltage modulator are also applied to the klystron power amplifier. These high- voltage modulating pulses are stepped up across a pulse transformer before being applied to the klystron. All cavities of the klystron are tunable and are tuned for maximum output at the desired frequency.

Provisions are made in this type of transmitter to adjust the starting time of the modulating pulses applied to the coherent oscillator, mixer amplifier, intermediate power amplifiers, and klystron power amplifier. By this means the various modulator pulses are made to occur at the same time. This transmitter produces output rf pulses of constant power and minimum frequency modulation and ensures good performance. NEETS MODULE 18-Radar Principles

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Q22. What is the primary advantage of power-amplifier transmitters over keyed- oscillator transmitters? Q23. In the power amplifier shown in figure 2-10, what two signals are mixed to produce the output signal? Q24. What type of klystron is used as the final stage of a power-amplifier transmitter? Figure 2-11 is a block diagram of a power-amplifier transmitter that uses a FREQUENCY SYNTHESIZER to produce the transmitted frequency rather than the heterodyning mixer. The frequency synthesizer allows the transmitter to radiate a large number of discrete frequencies over a relatively wide band. Such a system is commonly used with frequency-scan search radars that must transmit many different frequencies to achieve elevation coverage and to compensate for the roll and pitch of a ship.

Figure 2-11 Power amplifier transmitter using crossed-field amplifiers NEETS MODULE 18-Radar Principles

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A typical frequency synthesizer consists of a bank of oscillators producing different fixed frequencies. The outputs of a relatively few fixed oscillators can be mixed in various combinations to produce a wide range of frequencies. In mti systems the selected oscillator frequencies are mixed with a coherent oscillator frequency to provide a stable reference for the mti circuits. The frequency synthesizer also produces the local oscillator signals for the receiver system. Because the transmitted pulse changes frequency on each transmission, the local oscillator signal to the receiver must also change and be included in the transmitted frequency. A system of this type is frequency-programmed by select gates from the synchronizer.

The detailed operation of frequency synthesizers is beyond the scope of this manual but may be found in the technical manuals for most frequency scan radar systems. The first rf amplifier receives the pulses of the selected frequency from the synthesizer and a modulator pulse (from the first stage modulator) at the same time. The rf pulse is usually slightly wider than the modulator pulse which prevents the amplifier tube from pulsing when no rf energy is present. Most pulsed rf amplifiers will oscillate at an undesired frequency if pulsed without an rf input. The output of the first rf amplifier is an amplified rf pulse that is the same width as the first stage modulator pulse. The second stage modulator is designed to produce a pulse slightly narrower than the first stage modulator pulse; this also prevents the amplifier from pulsing when no rf is present.

Therefore, the second stage amplifier receives a modulator pulse a short time after the first stage rf arrives at the input. As shown in figure 2-11, the same procedure is repeated in the third and final stage. The amplifiers in this type of power-amplifier transmitter must be broad-band microwave amplifiers that amplify the input signals without frequency distortion. Typically, the first stage and the second stage are traveling-wave tubes (twt) and the final stage is a crossed- field amplifier. Recent technological advances in the field of solid-state microwave amplifiers have produced solid-state amplifiers with enough output power to be used as the first stage in some systems. Transmitters with more than three stages usually use crossed-field amplifiers in the third and any additional stages. Both traveling-wave tubes and crossed-field amplifiers have a very flat amplification response over a relatively wide frequency range.

Crossed-field amplifiers have another advantage when used as the final stages of a transmitter; that is, the design of the crossed-field amplifier allows rf energy to pass through the tube virtually unaffected when the tube is not pulsed. When no pulse is present, the tube acts as a section of waveguide. Therefore, if less than maximum output power is desired, the final and preceding cross-field amplifier stages can be shut off as needed. This feature also allows a transmitter to operate at reduced power, even when the final crossed-field amplifier is defective.

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Q25. What transmitter component allows the radiation of a large number of discrete frequencies over a wide band? Q26. What is the result of pulsing a pulsed rf amplifier when no rf is present? 2.3 DUPLEXERS Whenever a single antenna is used for both transmitting and receiving, as in a radar system, problems arise. Switching the antenna between the transmit and receive modes presents one problem; ensuring that maximum use is made of the available energy is another. The simplest solution is to use a switch to transfer the antenna connection from the receiver to the transmitter during the transmitted pulse and back to the receiver during the return (echo) pulse. No practical mechanical switches are available that can open and close in a few microseconds. Therefore, ELECTRONIC SWITCHES must be used.

Switching systems of this type are called DUPLEXERS. 2.3.1 Basic Duplexer Operation In selecting a switch for this task, you must remember that protection of the receiver input circuit is as important as are output power considerations. At frequencies where amplifiers may be used, amplifier tubes can be chosen to withstand large input powers without damage. However, the input circuit of the receiver is easily damaged by large applied signals and must be carefully protected. An effective radar duplexing system must meet the following four requirements: 1. During the period of transmission, the switch must connect the antenna to the transmitter and disconnect it from the receiver.

2. The receiver must be thoroughly isolated from the transmitter during the transmission of the high power pulse to avoid damage to sensitive receiver components. 3. After transmission, the switch must rapidly disconnect the transmitter and connect the receiver to the antenna. For targets close to the radar to be seen, the action of the switch must be extremely rapid. 4. The switch should absorb an absolute minimum of power both during transmission and reception. NEETS MODULE 18-Radar Principles

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Therefore, a radar duplexer is the microwave equivalent of a fast, low-loss, single-pole, double throw switch. The devices developed for this purpose are similar to spark gaps in which high-current microwave discharges furnish low-impedance paths. A duplexer usually contains two switching tubes (spark gaps) connected in a microwave circuit with three terminal transmission lines, one each for the transmitter, receiver, and antenna. As shown in views A and B of figure 2-12, these circuits may be connected in parallel or in series. Both systems will be discussed in detail in this section. One tube is called the TRANSMIT-RECEIVER TUBE, or TR TUBE; the other is called the ANTITRANSMITRECEIVE TUBE, or ATR TUBE. The tr tube has the primary function of disconnecting the receiver, and the atr tube of disconnecting the transmitter.

Figure 2-12 Duplexer systems NEETS MODULE 18-Radar Principles

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The overall action of the tr and atr circuits depends upon the impedance characteristics of the quarter-wavelength section of transmission line. A quarter-wavelength, or an odd multiple of the quarter wavelength, transmission line presents opposite impedance values at the ends; one end of the line appears as a short and the other end appears as an open. 2.3.1.1 TR Tube The type of spark gap used as a tr tube may vary. It may be one that is simply formed by two electrodes placed across the transmission line; or it may be one enclosed in an evacuated glass envelope with special features to improve operation. The requirements of the spark gap are (1) high impedance prior to the arc and (2) very low impedance during arc time. At the end of the transmitted pulse the arc should be extinguished as rapidly as possible. Extinguishing the arc stops any loss caused by the arc and permits signals from nearby targets to reach the receiver.

The simple gap formed in air has a resistance during conduction of from 30 to 50 ohms. This is usually too high for use with any but an open-wire transmission line. The time required for the air surrounding the gap to completely deionize after the pulse voltage has been removed is about 10 microseconds. During this time the gap acts as an increasing resistance across the transmission line to which it is connected. However, in a tr system using an air gap, the echo signals reaching the receiver beyond the gap will be permitted to increase to half their proper magnitude 3 microseconds after the pulse voltage has been removed. This interval is known as RECOVERY TIME.

Tr tubes are usually conventional spark gaps enclosed in partially evacuated, sealed glass envelopes, as shown in figure 2-13. The arc is formed as electrons are conducted through the ionized gas or vapor. You may lower the magnitude of voltage necessary to break down a gap by reducing the pressure of the gas that surrounds the electrodes. Optimum pressure achieves the most efficient tr operation. You can reduce the recovery time, or DEIONIZATION TIME, of the gap by introducing water vapor into the tr tube. A tr tube containing water vapor at a pressure of 1 millimeter of mercury will recover in 0.5 microseconds. It is important for a tr tube to have a short recovery time to reduce the range at which targets near the radar can be detected. If, for example, echo signals reflected from nearby objects return to the radar before the tr tube has recovered, those signals will be unable to enter the receiver.

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Tr tubes used at microwave frequencies are built to fit into, and become a part of, a resonant cavity. You may increase the speed with which the gap breaks down after the transmitter fires by placing a voltage across the gap electrodes. This potential is known as KEEP-ALIVE VOLTAGE and ranges from 100 volts to 1,000 volts. A glow discharge is maintained between the electrodes. (The term GLOW DISCHARGE refers to the discharge of electricity through a gas-filled electron tube. This is distinguished by a cathode glow and voltage drop much higher than the gas-ionization voltage in the cathode vicinity.) This action provides for rapid ionization when the transmitter pulse arrives.

Failure of the tr tube is primarily caused by two factors. The first and most common cause of failure is the gradual buildup of metal particles that have been dislodged from the electrodes. Such metal bits become spattered on the inside of the glass envelope. These particles act as small, conducting areas and tend to lower the Q of the resonant cavity and dissipate power. If the tube continues in use for too long a period in this condition, the particles will form a detuning wall within the cavity and eventually prevent the tube from functioning. A second cause of failure is the absorption of gas within the enclosure by the metal electrodes. This results in a gradual reduction of pressure within the tube to a point where gap breakdown becomes very difficult. The final result is that extremely strong signals (from the transmitter) are coupled to the receiver. Because both types of failures develop gradually, the tr tube periodically must be checked carefully to determine performance level.

Figure 2-13 Tr tube with keep-alive electrode NEETS MODULE 18-Radar Principles

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Q27. What type of switches are used as duplexers? Q28. What tube in a duplexer has the primary function of disconnecting the receiver? Q29. How may the tr tube ionization speed be increased? 2.3.1.2 ATR Tube The atr tube is usually a simpler device than a tr tube. An atr tube might use a pure inert gas, such as argon, because recovery time generally is not a vital factor. Furthermore, a priming agent, such as keep alive voltage, is not needed. The absence of either a chemically active gas or a keep-alive voltage results in atr tubes having longer useful lives than tr tubes.

WARNING Tr and atr tubes may contain radioactive material. Handle with care to avoid breakage and possible contamination. There are two basic tr-atr duplexer configurations. They are the parallel-connected and the series connected duplexer systems. The following paragraphs describe the operation of both systems. 2.3.1.3 Parallel Connected Duplexer Operation First, let’s consider a PARALLEL-CONNECTED DUPLEXER system, as shown in figure 2-14. The tr spark gap shown in figure 2-14 is located in the receiver coupling line one-quarter wavelength from the T-junction. A half-wavelength, closed-end section of transmission line, called a STUB, is shunted across the main transmission line. An atr spark gap is located in this line one-quarter wavelength from the main transmission line and one-quarter wavelength from the closed end of the stub. As shown in the figure, antenna impedance, line impedance, and transmitter output impedance, when transmitting, are all equal. The action of the circuit during transmission is shown in figure 2-15.

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Figure 2-14 Parallel-connected duplexer showing distance and impedance Figure 2-15 Parallel-connected duplexer during transmission NEETS MODULE 18-Radar Principles

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During the transmitting pulse, an arc appears across both spark gaps and causes the tr and atr circuits to act as shorted (closed-end) quarter-wave stubs. The circuits then reflect an open circuit to the tr and atr circuit connections to the main transmission line. None of the transmitted energy can pass through these reflected opens into the atr stub or into the receiver. Therefore, all of the transmitted energy is directed to the antenna. During reception, as shown in figure 2-16, the amplitude of the received echo is not sufficient to cause an arc across either spark gap. Under this condition, the atr circuit now acts as a half-wave transmission line terminated in a short-circuit. This is reflected as an open circuit at the receiver T-junction, three-quarter wavelengths away. The received echo sees an open circuit in the direction of the transmitter. However, the receiver input impedance is matched to the transmission line impedance so that the entire received signal will go to the receiver with a minimum amount of loss.

Figure 2-16 Parallel-connected duplexer during reception NEETS MODULE 18-Radar Principles

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2.3.1.4 Series-Connected Duplexer Operation In the SERIES-CONNECTED DUPLEXER SYSTEM, shown in figure 2-17, the tr spark gap is located one-half wavelength from the receiver T-junction. The atr spark gap is located one-half wavelength from the transmission line and three-quarters wavelength from the receiver T-junction. During transmission, the tr and atr gaps fire in the series- connected duplexer system, as shown in figure 2-18. This causes a short-circuit to be reflected at the series connection to the main transmission line one half wavelength away.

The transmitted pulse "sees" a low impedance path in the direction of the antenna and does not go into the atr stub or the receiver. Figure 2-17 Series-connected duplexer showing distance and impedance NEETS MODULE 18-Radar Principles

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During reception, neither spark gap is fired, as shown in figure 2-19. The atr acts as a half-wave stub terminated in an open. This open is reflected as a short-circuit at the T- junction three-quarters of a wavelength away. Consequently, the received signal sees a low impedance path to the receiver, and none of the received signal is lost in the transmitting circuit. Figure 2-18 Series-connected duplexer during transmission Figure 2-19 Series-connected duplexer during reception NEETS MODULE 18-Radar Principles

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2.3.2 Duplexer Types Duplexers are constructed in many forms, such as RESONANT-CAVITY COAXIAL SYSTEMS, WAVEGUIDE SYSTEMS, and HYBRID RINGS. Since waveguide and hybrid-ring duplexers are most common in radar systems, those will be discussed in this section. 2.3.2.1 Waveguide Duplexer WAVEGUIDE DUPLEXERS usually consist of tr tubes and atr tubes housed in a resonant cavity and attached to a waveguide system in some manner. Resonant-cavity tr tubes may be applied to waveguides, either directly or indirectly, to obtain switching action. The indirect method uses a coaxial line system, and then couples the coaxial line into the waveguide that feeds the antenna. If large losses are incurred by the use of a coaxial line, the resonant cavity can be coupled directly to the waveguide. Figure 2-20 shows a direct method of cavity tr switching in a waveguide system. The waveguide terminates in the antenna at one end and in a shorting plate at the other. The magnetron uses a voltage probe to excite the waveguide. The transmitted pulse travels up the guide and moves into the tr box through a slot. The cavity builds up a strong electric field across the gap, breaks it down, and detunes the cavity. This action effectively seals the opening and passes the pulse energy to the antenna.

Figure 2-20 Waveguide duplexer with cavity tr tube NEETS MODULE 18-Radar Principles

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The signals received during the resting time travel down the guide to the magnetron and the shorting end plate where they are reflected. The slot coupling the waveguide to the cavity is located at a point where the standing-wave magnetic field produced by reflections in the waveguide is maximum. The maximum magnetic field, therefore, energizes the cavity. The echo signals are not strong enough to cause an arc, and the cavity field is undisturbed by the gap. Therefore, the cavity field couples rf energy into the receiver coaxial line and provides maximum energy transfer.

The cavity tr switch can also be applied to branch lines of the waveguide, as shown in figure 2-21. The magnetron is coupled to the guide by a voltage probe to produce proper excitation. Figure 2-21 Branched waveguide duplexer NEETS MODULE 18-Radar Principles

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Maximum use of the received signals is ensured by an atr tube. The transmitted pulse travels from the magnetron to the atr branch where part of the energy is diverted into the gap. A slot (S) is placed across the waveguide one-half wavelength from the main guide, and passes the rf energy through it and into the cavity. The cavity builds up the electric field that breaks down the gap, detunes the cavity, and, as a result, effectively closes the slot. One-half wavelength away, this action effectively closes the entrance to the atr branch and limits the amount of energy entering the atr branch to a small value.

Most of the energy is, therefore, directed down the guide to the antenna. Upon reaching the receiver branch, the same effect is produced by the tr tube in the receiver line. Because the energy entering both openings is effectively limited by the gaps, maximum energy is transferred between the magnetron and the antenna. During the resting time, the atr spark gap is not broken down by the received signals. The received signal sets up standing waves within the cavity that cause it to resonate. At resonance, the low impedance of the atr cavity is reflected as a high impedance at the entrance to the transmitter waveguide (three quarter wavelength away). This ensures that the maximum received signal will enter the receiver branch.

2.3.2.2 Hybrid Ring Duplexer The HYBRID RING is used as a duplexer in high-power radar systems. It is very effective in isolating the receiver during transmission. A simplified version of the hybrid- ring duplexer is shown in views A and B of figure 2-22. The operation of the duplexer, in terms of the E field distribution during transmission and reception, is illustrated in views C and D. The H lines, though present, have been omitted to simplify the explanation. NEETS MODULE 18-Radar Principles

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During transmission the E field from the transmitter enters arm 3 and divides into two fields 180 degrees out of phase. One field moves clockwise around the ring and the other moves counterclockwise. The two fields must be 180 degrees out of phase at the entrance of an arm to propagate any energy down the arm. The field moving clockwise from arm 3 ionizes the tr tube in arm 4, and the energy is blocked from the receiver. The tr tube reflects a high impedance equivalent to an open circuit. This high impedance prevents any energy from entering the receiver - even though the two fields are out of phase at the entrance to arm 4. The field moving counterclockwise from arm 3 ionizes the tr tube in arm 2, which reflects a short circuit back to the ring junction. No energy is sent to the receiver, however, because the fields arriving at arm 2 are in phase. The clockwise and counterclockwise fields arrive at arm 1 out of phase by 180 degrees. They are then propagated through the arm to the antenna.

Figure 2-22 Hybrid-ring duplexer NEETS MODULE 18-Radar Principles

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During reception, the relatively weak field from the antenna enters arm 1 and divides at the junction into two out-of-phase components. Neither field is sufficient to fire the tr tubes in arms 2 and 4; since the fields arrive at these arms out of phase, energy is propagated to the receiver. The energy arriving at arm 3 is in phase and will not be coupled to the transmitter. Since the operation of the arms of a hybrid ring is the same as the operation of E-type waveguide T-junctions, you may find it helpful to review NEETS, Module 11, Microwave Principles.

Q30. The actions of the tr and atr circuits depend on the impedance characteristics of what length of transmission line? Q31. During which of the transmit or receive cycles are both the tr and atr tubes of a parallel connected duplexer ionized (arcing)? Q32. In a series-connected duplexer, what tube (tr or atr), if any, fires during the receive cycle? Q33. To propagate energy down an arm of a hybrid ring duplexer, the two fields at the junction of the arm and the ring must have what phase relationship? 2.4 RECEIVERS The energy that a distant object reflects back to the antenna in a radar system is a very small fraction of the original transmitted energy. The echoes return as pulses of rf energy of the same nature as those sent out by the transmitter. However, the power of a return pulse is measured in fractions of microwatts instead of in kilowatts, and the voltage arriving at the antenna is in the range of microvolts instead of kilovolts. The radar receiver collects those pulses and provides a visual display of object information.

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Information about the position of the object is present visually when the reception of an echo causes the movement or appearance of a spot of light on a cathode-ray tube (crt). The crt requires a signal on the order of at least several volts for proper operation and will not respond to the high frequencies within a return pulse. Therefore, a receiver amplifier and detector must be used that are capable of producing a visible indication on the cathode-ray tube under the following conditions: (1) when the input signal to the amplifier is in the form of pulses of extremely high-frequency, (2) the amplitude of the pulses is in the microvolt range, and (3) the pulses last for only a few microseconds.

The radar receiver evolved directly from the simple radio receiver. The radar receiver operates on exactly the same principles as the radio receiver. However, the overall requirements and limitations of a radar receiver differ somewhat from those of a radio receiver because of the higher frequencies involved and the greater sensitivity desired. In studying the radar receiver, we will first examine the overall requirements of a radar receiver. Second, we will examine a typical radar receiver that satisfies these requirements. Finally, we will discuss the individual components of the receiver.

2.4.1 Radar Receiver Requirements The following characteristics determine the design requirements of an effective radar receiver: • Noise • Gain • Tuning • Distortion • Blocking 2.4.1.1 Noise The word NOISE is a carryover from sound-communications equipment terminology. Noise voltages in sound equipment produce actual noise in the loudspeaker output. In radar, noise voltages result in erratic, random deflection or intensity of the indicator sweep that can mask small return signals. NEETS MODULE 18-Radar Principles

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Were it not for noise, the maximum range at which an object would be detectable by radar could be extended almost infinitely. Objects at great range return exceedingly small echoes. However, without noise, almost any signal could be amplified to a usable level if enough stages were added to the receiver. Because of noise, the signal detection limit or sensitivity level of a receiver is reached when the signal level falls below the noise level to such an extent as to be obscured. A simple increase of amplification is of no help because both signal and noise are amplified at the same rate.

In the radar portion of the rf spectrum, external sources of noise interference are usually negligible; consequently, the sensitivity that can be achieved in a radar receiver is usually determined by the noise produced in the receiver. Not only must noise be kept down, but everything possible must be done to minimize attenuation of the video signal (echo) before it is amplified. 2.4.1.2 Gain The GAIN of a radar receiver must be very high. This is because the strength of the signal at the antenna is at a level of microvolts and the required output to the indicator is several volts. The gain of a radar receiver is roughly in the range of 106 to 108.

FEEDBACK, or REGENERATION, is one of the most serious difficulties in the design of an amplifier with such high gain. Special precautions must be taken to avoid feedback. Such precautions include careful shielding, decoupling (isolation) between voltage supplies for the different tubes, and amplification at different frequencies in separate groups of stages. 2.4.1.3 Tuning The radar receiver requires a limited tuning range to compensate for transmitter and local oscillator frequency changes because of variations in temperature and loading.

Microwave radar receivers usually use automatic frequency control (afc) for this purpose. 2.4.1.4 Distortion If distortion occurs in the receiver, the time interval between the transmitted pulse and the received pulse changes, thereby affecting range accuracy. NEETS MODULE 18-Radar Principles

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2.4.1.5 Blocking BLOCKING refers to a condition of the receiver in which the voltage pulse at the receiver input is too large. As a result, for a short time after the pulse, the receiver is insensitive or blocked to signals below a certain level. This condition results from one or more of the amplifier stages in the receiver being overdriven. After a strong pulse, the receiver may be biased to a point at which it will not amplify small signals. Recovery after blocking may be only a fraction of a microsecond, or it may take several hundred microseconds, depending upon the point in the receiver at which blocking occurs. To detect a weak echo immediately following a strong one, the receiver must have a short BLOCKING RECOVERY TIME. The blocking itself must be minimized as much as possible. If a portion of the transmitted pulse leaks into the receiver input, then the receiver may be blocked and not show small, nearby objects. In most receivers, blocking is minimized from this cause by a duplexer. The duplexer protects the receiver by isolating it during the transmitted pulse.

2.4.2 Receiver Block Diagram The SUPERHETERODYNE receiver is almost always used in microwave radar systems. A typical superheterodyne radar receiver is shown in figure 2-23. A receiver of this type meets all the requirements listed above. Signals from the antenna enter the receiver via the duplexer. A low-noise rf amplifier is usually the first stage of modern radar receivers. Some receivers, however, send the antenna signal directly to the mixer, as shown by the dashed path. The low-noise amplifiers used in modern systems are usually solid-state devices, such as tunnel-diode, parametric, or microwave transistor amplifiers.

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The MIXER stage is often called the FIRST DETECTOR. The function of this stage is to convert the received rf energy to a lower, intermediate frequency (IF) that is easier to amplify and manipulate electronically. The intermediate frequency is usually 30 or 60 megahertz. It is obtained by heterodyning the received signal with a local-oscillator signal in the mixer stage. The mixer stage converts the received signal to the lower IF signal without distorting the data on the received signal. After conversion to the intermediate frequency, the signal is amplified in several IF AMPLIFIER stages. Most of the gain of the receiver is developed in the IF amplifier stages. The overall bandwidth of the receiver is often determined by the bandwidth of the IF stages.

The output of the IF amplifiers is applied to the SECOND DETECTOR. It is then rectified and passed through one or more stages of amplification in the video amplifier(s). The output stage of the receiver is normally an emitter follower. The low-impedance output of the emitter follower matches the impedance of the cable. The video pulses are coupled through the cable to the indicator for video display on the crt. Figure 2-23 Typical superheterodyne radar receiver NEETS MODULE 18-Radar Principles

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As in all superheterodyne receivers, controlling the frequency of the local oscillator keeps the receiver tuned. Since this tuning is critical, some form of automatic frequency control (afc) is essential to avoid constant manual tuning. Automatic frequency control circuits mix an attenuated portion of the transmitted signal with the local oscillator signal to form an IF signal. This signal is applied to a frequency-sensitive discriminator that produces an output voltage proportional in amplitude and polarity to any change in IF frequency. If the IF signal is at the discriminator center frequency, no discriminator output occurs. The center frequency of the discriminator is essentially a reference frequency for the IF signal. The output of the DISCRIMINATOR provides a control voltage to maintain the local oscillator at the correct frequency.

Different receiving systems may vary in the type of coupling between stages, the type of mixer, the detector, the local oscillator, and the number of stages of amplification at the different frequencies. However, the receiver is always designed to have as little noise as possible. It is also designed to have sufficient gain so that noise, rather than lack of gain, limits the smallest visible signal. 2.4.3 Receiver Components This section will analyze in more detail the operation of the receiver circuits mentioned above. The circuits discussed are usually found in some form in all radar superheterodyne receivers.

2.4.3.1 Low-Noise Amplifier LOW-NOISE AMPLIFIERS, sometimes called PREAMPS, are found in most modern radar receivers. As previously mentioned, these amplifiers are usually solid-state microwave amplifiers. The most common types are tunnel diode and parametric amplifiers. These amplifiers are discussed in detail in NEETS, Module 11, Microwave Principles. Some older systems may still use a traveling-wave tube (twt) as a low-noise first stage amplifier. However, the solid-state amplifiers produce lower noise levels and more gain.

2.4.3.2 Local Oscillator Most radar receivers use a 30 or 60 megahertz intermediate frequency. The IF is produced by mixing a local oscillator signal with the incoming signal. The local oscillator is, therefore, essential to efficient operation and must be both tunable and very stable. For example, if the local oscillator frequency is 3,000 megahertz, a frequency change of 0.1 percent will produce a frequency shift of 3 megahertz. This is equal to the bandwidth of most receivers and would greatly decrease receiver gain.

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The power output requirement for most local oscillators is small (20 to 50 milliwatts) because most receivers use crystal mixers that require very little power. The local oscillator output frequency must be tunable over a range of several megahertz in the 4,000-megahertz region. The local oscillator must compensate for any changes in the transmitted frequency and maintain a constant 30 or 60 megahertz difference between the oscillator and the transmitter frequency. A local oscillator that can be tuned by varying the applied voltage is most desirable.

The REFLEX KLYSTRON is often used as a local oscillator because it meets all the requirements mentioned above. The reflex klystron is a very stable microwave oscillator that can be tuned by changing the repeller voltage. Most radar systems use an automatic frequency control (afc) circuit to control the output of the local oscillator. A block diagram of a typical afc circuit is included in figure 2-23. Note that the afc circuits form a closed loop. This circuit is, in fact, often called the afc loop. A sample of the transmitter energy is fed through the afc mixer and an IF amplifier to a discriminator. The output of the discriminator is a dc error voltage that indicates the degree of mistuning between the transmitter and the local oscillator. In this particular example let’s assume that the IF is 30 megahertz. If the output of the mixer is correct, the discriminator will have no output. If the mixer output is above 30 megahertz, the output of the discriminator will be positive dc pulses; if the mixer output is below 30 megahertz, the discriminator output will be negative dc pulses. In either case, this output is fed through an amplifier to the control circuit. The control circuit adjusts the operating frequency of the local oscillator so that no mistuning exists and the IF is 30 megahertz. In this example the local oscillator is a reflex klystron and the control circuit provides he repeller plate voltage for the klystron; thus, the klystron directly controls the local oscillator frequency. In this manner the local oscillator is maintained exactly 30 megahertz below the transmitter frequency.

Q34. What is the greatest limiting factor in a receiver’s detectable range? Q35. What type of receiver is most often used in radar systems? Q36. What IF frequencies are normally used in radar receivers? Q37. Which component of the receiver produces the signal that is mixed with the received signal to produce the IF signal? NEETS MODULE 18-Radar Principles

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2.4.3.3 Mixer Many older radar receivers do not use a low-noise amplifier as the receiver front end; they simply send the echo signal directly to a crystal mixer stage. A crystal is used rather than an electron-tube diode because, at microwave frequencies, the tube would generate excessive noise. Electron tubes are also limited by the effects of transit time at microwave frequencies. The crystal most commonly used is the point-contact crystal diode; however, recent developments in the field of solid-state microwave devices may soon replace the point-contact diode with devices that produce even less noise. The Schottky-barrier diode is an example of a relatively recent development that produces less noise than the point-contact crystal.

The simplest type of radar mixer is the SINGLE ENDED or UNBALANCED CRYSTAL MIXER, shown in figure 2-24. The mixer illustrated uses a tuned section of coaxial transmission line one-half wavelength long. This section matches the crystal to the signal echo and the local oscillator inputs. Local oscillator injection is accomplished by means of a probe. In the coaxial assembly, the signal is injected by means of a slot. This slot would normally be inserted in the duplexer waveguide assembly and be properly oriented to provide coupling of the returned signal. In this application, the unwanted signals at the output of the mixer (carrier frequency, the local oscillator frequency, and sum of these two signals) are effectively eliminated by a resonant circuit tuned to the intermediate, or difference frequency. One advantage of the unbalanced crystal mixer is its simplicity. It has one major disadvantage; its inability to cancel local oscillator noise. Difficulty in detecting weak signals will exist if noise is allowed to pass through the mixer along with the signal.

Figure 2-24 Single-ended crystal mixer NEETS MODULE 18-Radar Principles

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One type of mixer which cancels local oscillator noise is the BALANCED, OR HYBRID, MIXER (sometimes called the MAGIC T). Figure 2-25 shows this type of mixer. In hybrid mixers, crystals are inserted directly into the waveguide. The crystals are located one-quarter wavelength from their respective short-circuited waveguide ends (a point of maximum voltage along a tuned line). The crystals are also connected to a balanced transformer, the secondary of which is tuned to the desired IF. The local oscillator signal is introduced into the waveguide local oscillator arm and distributes itself as shown in view A of figure 2-26. Observe that the local oscillator signal is in phase across the crystals. In view B the echo signal is introduced into the echo signal arm of the waveguide and is out of phase across the crystals. The resulting fields are shown in view C.

Figure 2-25 Balanced (hybrid) crystal mixer NEETS MODULE 18-Radar Principles

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Figure 2-26A Balanced mixer fields. WAVEGUIDE AND LOCAL OSCILLATOR ARM Figure 2-26B.— Balanced mixer fields. WAVEGUIDE AND ECHO SIGNAL ARM NEETS MODULE 18-Radar Principles

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A difference in phase exists between echo signals applied across the two crystals. The signal applied to the crystals from the local oscillator is in phase. Therefore, at some point both signals applied to crystal #1 will be in phase, and the signals applied to crystal #2 will be out of phase. This means that an IF signal of one polarity will be produced across crystal #1 and an IF signal of the opposite polarity will be produced across crystal #2. When these two signals are applied to the balanced output transformer (figure 2-25), they will add. Outputs of the same polarity will cancel across the balanced transformer.

This action eliminates the noise of the local oscillator. Noise components introduced from the local oscillator are in phase across the crystals and are, therefore, cancelled in the balanced transformer. The rf characteristics of the crystals must be nearly equal, or the noise of the local oscillator will not completely cancel. Note that only the noise produced by the local oscillator is canceled. Noise arriving with the echo signal is not affected. 2.4.3.4 IF Amplifier Stage The IF AMPLIFIER SECTION of a radar receiver determines the gain, signal-to-noise ratio, and effective bandwidth of the receiver. The typical IF amplifier (commonly called an IF strip) usually contains from three to ten amplifier stages. The IF amplifier has the capability to vary both the bandpass and the gain of a receiver. Normally, the bandpass is as narrow as possible without affecting the actual signal energy. When a selection of pulse widths is available, such as short and long pulses, the bandpass must be able to match the bandwidth of the two different signals. Gain must be variable to provide a constant voltage output for input signals of different amplitudes. Figure 2-27 is a block diagram of an IF amplifier that meets these requirements.

Figure 2-26C.— Balanced mixer fields. WAVEGUIDE NEETS MODULE 18-Radar Principles

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The most critical stage of the IF section is the input (first stage). The quality of this stage determines the noise figure of the receiver and the performance of the entire receiving system with respect to detection of small objects at long ranges. Gain and bandwidth are not the only considerations in the design of the first IF stage. A consideration perhaps of more importance is noise generation. Noise generation in this stage must be low. Noise generated in the input IF stage will be amplified by succeeding stages and may exceed the echo signal in strength.

2.4.3.5 Detectors The detector in a microwave receiver serves to convert the IF pulses into video pulses. After amplification, these are applied to the indicator. The simplest form of detector, and the one most commonly used in microwave receivers, is the DIODE DETECTOR. Figure 2-27 IF amplifier block diagram NEETS MODULE 18-Radar Principles

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A diode detector circuit is shown in view A of figure 2-28. The secondary of T1 and C1 form a tuned circuit that is resonant at the intermediate frequency. Should an echo pulse of sufficient amplitude be received, the voltage (ei) developed across the tuned circuit is an IF pulse. Its shape is indicated by the dashed line in view B. Positive excursions of e i cause no current to flow through the diode. However, negative excursions result in a flow of diode current and a subsequent negative voltage (eo) to be developed across R1 and C2. Between peak negative voltage excursions of the ei wave, capacitor C2 discharges through R1. Thus, the eo waveform is a negative video pulse with sloping edges and superimposed IF ripple, as indicated by the solid line in view B. A negative polarity of the output pulse is ordinarily preferred, but a positive pulse may be obtained by reversing the connections of the diode. In view A, inductance L1, in combination with wiring capacitance and C2, forms a low-pass filter. This filter attenuates the IF components in the eo waveform but results in a minimum loss of video high-frequency components.

Figure 2-28 Diode detector NEETS MODULE 18-Radar Principles

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2.4.3.6 Video Amplifiers The video amplifier receives pulses from the detector and amplifies these pulses for application to the indicating device. A video amplifier is fundamentally an RC coupled amplifier that uses high-gain transistors or pentodes. However, a video amplifier must be capable of a relatively wide frequency response. Stray and interelectrode capacitances reduce the high-frequency response of an amplifier, and the reactance of the coupling capacitor diminishes the low-frequency response. These problems are overcome by the use of FREQUENCY COMPENSATION NETWORKS in the video amplifier. The types of frequency compensation networks that may be used in a video amplifier stage are discussed in detail in NEETS, Module 8, Amplifiers.

Q38. What receiver circuit actually produces the IF frequency? Q39. The IF amplifiers are connected in what amplifier configuration? Q40. Which receiver component converts the IF pulses to video pulses? 2.4.4 Receiver Special Circuits The performance efficiency of radar receivers is often greatly decreased by interference from one or more of several possible sources. Weather and sea return are the most common of these interference sources, especially for radar systems that operate above 3,000 megahertz. Unfavorable weather conditions can completely mask all radar returns and render the system useless. Electromagnetic interference from external sources, such as the deliberate interference by an enemy, called jamming or electronic counter measures (ECM), can also render a radar system useless. Many special circuits have been designed to help the radar receiver counteract the effects of external interference. These circuits are called VIDEO ENHANCEMENT FEATURES, ANTIJAMMING CIRCUITS, or ELECTRONIC COUNTERCOUNTERMEASURES (ECCM) CIRCUITS. This section will discuss, in general terms, some of the more common video enhancement features associated with radar receivers.

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2.4.4.1 Automatic Gain Control (AGC) Most radar receivers use some means to control the overall gain. This usually involves the gain of one or more IF amplifier stages. Manual gain control by the operator is the simplest method. Usually, some more complex form of automatic gain control (agc) or instantaneous automatic gain control (iagc) is used during normal operation. Gain control is necessary to adjust the receiver sensitivity for the best reception of signals of widely varying amplitudes. Agc and iagc circuits are designed with, a shut-off feature so that receiver gain may be adjusted manually. In this way, manual gain control can be used to adjust for best reception of a particular signal.

The simplest type of agc adjusts the IF amplifier bias (and gain) according to the average level of the received signal. Agc is not used as frequently as other types of gain control because of the widely varying amplitudes of radar return signals. With agc, gain is controlled by the largest received signals. When several radar signals are being received simultaneously, the weakest signal may be of greatest interest. Iagc is used more frequently because it adjusts receiver gain for each signal. The iagc circuit is essentially a wide-band, dc amplifier. It instantaneously controls the gain of the IF amplifier as the radar return signal changes in amplitude. The effect of iagc is to allow full amplification of weak signals and to decrease the amplification of strong signals. The range of iagc is limited, however, by the number of IF stages in which gain is controlled. When only one IF stage is controlled, the range of iagc is limited to approximately 20 dB. When more than one IF stage is controlled, iagc range can be increased to approximately 40 dB.

2.4.4.2 Sensitivity Time Control (STC) In radar receivers, the wide variation in return signal amplitudes make adjustment of the gain difficult. The adjustment of receiver gain for best visibility of nearby target return signals is not the best adjustment for distant target return signals. Circuits used to adjust amplifier gain with time, during a single pulse-repetition period, are called stc circuits. NEETS MODULE 18-Radar Principles

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Sensitivity time-control circuits apply a bias voltage that varies with time to the IF amplifiers to control receiver gain. Figure 2-29 shows a typical stc waveform in relation to the transmitted pulse. When the transmitter fires, the stc circuit decreases the receiver gain to zero to prevent the amplification of any leakage energy from the transmitted pulse. At the end of the transmitted pulse, the stc voltage begins to rise, gradually increasing the receiver gain to maximum. The stc voltage effect on receiver gain is usually limited to approximately 50 miles. This is because close-in targets are most likely to saturate the receiver; beyond 50 miles, stc has no affect and the receiver operates normally.

The combination of stc and iagc circuits results in better overall performance than with either type of gain control alone. Stc decreases the amplitude of nearby target return signals, while iagc decreases the amplitude of larger-than-average return signals. Thus, normal changes of signal amplitudes are adequately compensated for by the combination of iagc and stc. Figure 2-29 Stc voltage waveform NEETS MODULE 18-Radar Principles

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2.4.4.3 Antijamming Circuits Among the many circuits used to overcome the effects of jamming, two important ones are GATED AGC CIRCUITS and FAST-TIME-CONSTANT CIRCUITS. A gated agc circuit permits signals that occur only in a very short time interval to develop the agc. If large-amplitude pulses from a jamming transmitter arrive at the radar receiver at any time other than during the gating period, the agc does not respond to these jamming pulses. Without gated agc, a large jamming signal would cause the automatic gain control to follow the interfering signal. This would decrease the target return signal amplitude to an unusable value. Gated agc produces an output signal for only short time periods; therefore, the agc output voltage must be averaged over several cycles to keep the automatic gain control from becoming unstable.

Gated agc does not respond to signals that arrive at times other than during the time of a target return signal. However, it cannot prevent interference that occurs during the gating period. Neither can gating the agc prevent the receiver from overloading because of jamming signal amplitudes far in excess of the target return signal. This is because the desired target is gated to set the receiver gain for a signal of that particular amplitude. As an aid in preventing radar receiver circuits from overloading during the reception of jamming signals, fast-time-constant coupling circuits are used. These circuits connect the video detector output to the video amplifier input circuit.

A fast-time-constant (ftc) circuit is a differentiator circuit located at the input of the first video amplifier. When a large block of video is applied to the ftc circuit, only the leading edge will pass. This is because of the short time constant of the differentiator. A small target will produce the same length of signal on the indicator as a large target because only the leading edge is displayed. The ftc circuit has no effect on receiver gain; and, although it does not eliminate jamming signals, ftc greatly reduces the effect of jamming.

Q41. Which of the two types of automatic gain control, agc or iagc, is most effective in radar use for the Navy? Q42. Immediately after the transmitter fires, stc reduces the receiver gain to what level? Q43. How does ftc affect receiver gain, if at all? NEETS MODULE 18-Radar Principles

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2.4.5 Special Receivers The basic receiver of a radar system often does not meet all the requirements of the radar system, nor does it always function very well in unfavorable environments. Several special receivers have been developed to enhance target detection in unfavorable environments or to meet the requirements of special transmission or scanning methods. A radar system with a moving target indicator (mti) system or a monopulse scanning system requires a special type of receiver. Other types of special receivers, such as the logarithmic receiver, have been developed to enhance reception during unfavorable conditions. These receivers will be discussed in general terms in this section.

2.4.5.1 Moving Target Indicator (mti) System The MOVING TARGET INDICATOR (mti) system effectively cancels CLUTTER (caused by fixed unwanted echoes) and displays only moving target signals. Clutter is the appearance on a radar indicator of confusing, unwanted echoes which interfere with the clear display of desired echoes. Clutter is the result of echoes from land, water, weather, and so forth. The unwanted echoes can consist of GROUND CLUTTER (echoes from surrounding land masses), SEA CLUTTER (echoes from the irregular surface of the sea), or echoes from the clouds and rain. The problem is to find the desired echo in the midst of the clutter. To do this, the mti system must be able to distinguish between fixed and moving targets and then must eliminate only the fixed targets. This is accomplished by phase detection and pulse-to-pulse comparison.

Target echo signals from stationary objects have the same phase relationship from one receiving period to the next. Moving objects produce echo signals that have a different phase relationship from one receiving period to the next. This principle allows the mti system to discriminate between fixed and moving targets. Signals received from each transmitted pulse are delayed for a period of time exactly equal to the pulse-repetition time. The delayed signals are then combined with the signals received from the next transmitted pulse. This is accomplished in such a manner that the amplitudes subtract from each other as shown in figure 2-30, views A and B. Since the fixed targets have approximately the same amplitude on each successive pulse, they will be eliminated. The moving target signals, however, are of different amplitudes on each successive pulse and, therefore, do not cancel. The resulting signal is then amplified and presented on the indicators.

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In figure 2-31, 30-megahertz signals from the signal mixer are applied to the 30- megahertz amplifier. The signals are then amplified, limited, and fed to the phase detector. Another 30-megahertz signal, obtained from the coherent oscillator (coho) mixer, is applied as a lock pulse to the coho. The coho lock pulse is originated by the transmitted pulse. It is used to synchronize the coho to a fixed phase relationship with the transmitted frequency at each transmitted pulse. The 30-megahertz, cw reference signal output of the coho is applied, together with the 30-megahertz echo signal, to the phase detector.

Figure 2-30A Fixed target cancellation Figure 2-30B.— Fixed target cancellation. NEETS MODULE 18-Radar Principles

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The phase detector produces a video signal. The amplitude of the video signal is determined by the phase difference between the coho reference signal and the IF echo signals. This phase difference is the same as that between the actual transmitted pulse and its echo. The resultant video signal may be either positive or negative. This video output, called coherent video, is applied to the 14-megahertz cw carrier oscillator. The 14-megahertz cw carrier frequency is amplitude modulated by the phase-detected coherent video. The modulated signal is amplified and applied to two channels. One channel delays the 14-megahertz signal for a period equal to the time between transmitted pulses. The signal is then amplified and detected. The delay required (the period between transmitted pulses) is obtained by using a mercury delay line or a fused-quartz delay line, which operates ultrasonically at 14 megahertz.

Figure 2-31 Mti block diagram NEETS MODULE 18-Radar Principles

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The signal to the other channel is amplified and detected with no delay introduced. This channel includes an attenuating network that introduces the same amount of attenuation as does the delay line in the delayed video channel. The resulting nondelayed video signal is combined in opposite polarity with the delayed signal. The amplitude difference, if any, at the comparison point between the two video signals is amplified; because the signal is bipolar, it is made unipolar. The resultant video signal, which represents only moving targets, is sent to the indicator system for display.

An analysis of the mti system operation just described shows that signals from fixed targets produce in the phase detector recurring video signals of the same amplitude and polarity. (Fixed targets have an unchanging phase relationship to their respective transmitted pulses.) Thus, when one video pulse is combined with the preceding pulse of opposite polarity, the video signals cancel and are not passed on to the indicator system. Signals from moving targets, however, will have a varying phase relationship with the transmitted pulse. As a result, the signals from successive receiving periods produce signals of different amplitudes in the phase detector. When such signals are combined, the difference in signal amplitude provides a video signal that is sent to the indicator system for display.

The timing circuits, shown in figure 2-31, are used to accurately control the transmitter pulse repetition frequency to ensure that the pulse-repetition time remains constant from pulse to pulse. This is necessary, of course, for the pulses arriving at the comparison point to coincide in time and achieve cancellation of fixed targets. As shown in figure 2-31, a feedback loop is used from the output of the delay channel, through the pickoff amplifier, to the trigger generator and gating multivibrator circuits. The leading edge of the square wave produced by the detected carrier wave in the delayed video channel is differentiated at the pickoff amplifier. It is used to activate the trigger generator and gating multivibrator. The trigger generator sends an amplified trigger pulse to the modulator, causing the radar set to transmit.

The gating multivibrator is also triggered by the negative spike from the differentiated square wave. This stage applies a 2,000-microsecond negative gate to the 14-megahertz oscillator. The oscillator operates for 2,400 microseconds and is then cut off. Because the delay line time is 2,500 microseconds, the 14-megahertz oscillations stop before the initial waves reach the end of the delay line. This wave train, when detected and differentiated, turns the gating multivibrator on, producing another 2,400- microsecond wave train. The 100 microseconds of the delay line is necessary to ensure that the mechanical waves within the line have time to damp out before the next pulse-repetition time. In this manner the pulse-repetition time of the radar set is controlled by the delay of the mercury, or quartz delay line. Because this delay line is also common to the video pulses going to the comparison point, the delayed and the undelayed video pulses will arrive at exactly the same time.

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Q44. What type of target has a fixed phase relationship from one receiving period to the next? Q45. What signal is used to synchronize the coherent oscillator to a fixed phase relationship with the transmitted pulse? Q46. What is the phase relationship between the delayed and undelayed video? 2.4.5.2 Logarithmic Receiver The LOGARITHMIC RECEIVER uses a linear logarithmic amplifier, commonly called a LIN-LOG AMPLIFIER, instead of a normal IF amplifier. The lin-log amplifier is a nonsaturating amplifier that does not ordinarily use any special gain-control circuits. The output voltage of the lin-log amplifier is a linear function of the input voltage for low- amplitude signals. It is a logarithmic function for high-amplitude signals. In other words, the range of linear amplification does not end at a definite saturation point, as is the case in normal IF amplifiers. The comparison of the response curves for normal IF and lin-log amplifiers is shown in figure 2-32. The curves show that a continued increase in the input to the lin-log amplifier causes a continued increase in the output, but at a reduced rate.

Therefore, a large signal does not saturate the lin-log amplifier; rather, it merely reduces the amplification of a simultaneously applied small signal. A small echo signal can often be detected by the lin-log receiver when a normal receiver would be saturated. Figure 2-32 Lin-Log amplifier versus normal IF amplifier NEETS MODULE 18-Radar Principles

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A typical circuit for obtaining a lin-log response is shown in figure 2-33. If detectors 2 and 3 were not present, the output voltage would be limited by the saturation point of the final IF stage, as it is in a normal IF section. However, when the final stage of the lin-log is saturated, larger signals cause an increase in the output of the next to last stage. This increase is detected by detector 2 and summed with the output of detector 1. This sum produces an increase in the output even though the final stage is saturated. Detector 3 causes the output to continue to increase after the second stage saturates. The overall gain becomes less and less as each stage saturates, but some degree of amplification is still available. The proper choice of IF stage gains and saturation points produces an approximately logarithmic response curve.

Figure 2-34, shows the response curves of the three IF stages in the lin-log amplifier shown in figure 2-33. The responses of the individual stages produce a segmented overall response curve for the receiver. Figure 2-33 Lin-Log receiver block diagram Figure 2-34 Lin-Log amplifier stage response curves NEETS MODULE 18-Radar Principles

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2.4.5.3 Monopulse Receiver The most common of the automatic tracking radars is the MONOPULSE RADAR. The monopulse radar obtains the three target position coordinates of range, bearing, and elevation from a single pulse. The receiver for a monopulse radar must have three separate channels to process range, bearing, and elevation information. The block diagram of a simplified monopulse receiver is shown in figure 2-35. As in a conventional receiver, each channel of the monopulse receiver converts the return echo to an IF frequency by mixing the returned signal with a common local oscillator signal. The sum of the energy from all four return signals is mixed with the local oscillator signal to produce range IF information. Bearing information is obtained by subtracting the energy from horns B and D from the energy from horns A and C: (A + C) - (B + D) and mixing the difference with the local oscillator signal. The result is a bearing IF signal. Elevation information is obtained in the same way, except the energy from horns C and D is subtracted from the energy from horns A and B: (A + B) - (C + D) Figure 2-35 Monopulse receiver block diagram NEETS MODULE 18-Radar Principles

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If the target is on the elevation and bearing axis, the summations will both be zero; therefore, neither the bearing nor elevation channels will receive an input signal. If either of the bearing or elevation signals is off the axis, an input to the IF channel is produced. This input is subsequently converted to an IF signal in the appropriate channel. The major difference between the monopulse receiver and the conventional receiver is the requirement for a dc error voltage output from the bearing and elevation channels.

The range channel of a monopulse receiver is sent to a conventional ranging circuit for presentation are on an indicator or for use by a range-tracking circuit. However, since most monopulse radars are automatic tracking radars, the outputs of the bearing and elevation channels must be converted to dc error signals for use by automatic bearing and elevation tracking systems. The dc error voltages are applied to the antenna bearing and elevation positioning servos. These servos reposition the antenna until the errors are nulled.

The phase detectors compare the phase of the bearing and elevation IF with a reference IF from the range channels. This comparison produces the dc error pulses needed to drive the antenna servos. The signals from both the bearing and elevation channels are the result of a summation process. They can be either positive (in-phase) or negative (180- degrees out of phase) when compared to the reference IF signal. For example, if the output of horns A and C is smaller than the output of horns B and D, a negative or 180- degree-out-of-phase signal is produced by the bearing channel (A + C) - (B + D). If output A + C is greater than output B + D, a positive or in-phase signal is produced by the bearing channel. The phase of the bearing and elevation output signals determines the direction in which the antenna moves; the magnitude of the signal determines the amount of movement. Since two signals must be present at the phase detector to produce an output, an error signal occurs only when a return echo is not on the antenna beam axis.

This technique produces an error signal when the target moves off the radiated beam axis in either bearing or elevation. The error signal causes the antenna to move in the proper direction and for the proper duration to cancel the error signal. This method of automatic tracking is commonly used by weapons-control tracking radar systems. Q47. When a large signal and a small signal are applied to a lin-log amplifier at the same time, what is the effect on the small signal? Q48. What happens to the overall gain of a lin-log amplifier as each stage saturates?

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Q49. A monopulse receiver has how many separate channels? Q50. If a target is on the bearing axis of the radiated beam, what is the input to the bearing IF channel? Q51. What characteristic of the bearing and elevation output signals determines the direction of antenna movement? 2.5 SUMMARY The following paragraphs summarize the important points of this chapter. The SYNCHRONIZER is essential to any radar because it controls and times the operation of the entire system. Radar systems may be self-synchronized by triggers from the transmitter or externally synchronized from a master oscillator.

Most modern systems are synchronized by a MASTER OSCILLATOR, which may be a SINEWAVE OSCILLATOR, an ASTABLE MULTIVIBRATOR, or a BLOCKING

Oscillator.

Each of these oscillators fulfills the basic requirements of a synchronizer, which must be: • free running • stable in frequency • frequency variable (in steps) The TRANSMITTER produces the short-duration, high-power, rf pulses of energy that are radiated into space by the antenna. The MODULATOR controls the radar pulse, width and amplitude. KEYED-OSCILLATOR TRANSMITTERS produce a high-power output pulse by keying a high power oscillator, such as a MAGNETRON. POWER-AMPLIFIER TRANSMITTERS amplify a low-level pulse to the desired power level using a series of microwave amplifiers such as TRAVELING-WAVE TUBES or KLYSTRONS.

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The DUPLEXER is a device that allows the same antenna to both transmit and receive. Most duplexers use the impedance characteristics of transmission lines and waveguides in conjunction with TR and ATR tubes to route the energy to the correct place. One of the most important functions of the duplexer is isolation of the receiver during transmission. The RECEIVER detects the very small target return echo and amplifies it to a usable level for display on the indicator. A typical SUPERHETERODYNE RECEIVER consists of a low-noise amplifier, a mixer, a local oscillator, an IF amplifier, a detector, and a video amplifier.

Some special purpose receivers are the MOVING TARGET INDICATOR and

Monopulse Receivers.

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Answers To Questions Q1. Through Q51.

A1. Controls system operation and timing. A2. Timing and control. A3. Transmitter. A4. Free-running. A5. The master oscillator. A6. Leakage from the duplexer. A7. Sine-wave oscillator, single-swing blocking oscillator, and master-trigger (astable) multivibrator. A8. It requires additional shaping circuits. A9. Blocking oscillators. A10. Keyed oscillator and power-amplifier chain. A11. The modulator. A12. Steep leading and trailing edges. A13. Line-pulsed. A14. Capacitor, artificial transmission line, or pulse-forming network.

A15. Pulse width. A16. Thyratron. A17. The charging impedance. A18. 600-30,000 megahertz. A19. Mode skipping and mode shifting. A20. The magnetron will not oscillate. NEETS MODULE 18-Radar Principles

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A21. ±5 percent. A22. Frequency stability. A23. Local oscillator and coherent oscillator. A24. Multicavity klystron. A25. Frequency synthesizer. A26. Oscillations at an undesired frequency. A27. Electronic. A28. Tr tube. A29. Apply keep-alive voltage. A30. Quarter-wavelength section. A31. Transmit. A32. Neither fires. A33. 180 degrees out of phase. A34. Noise. A35. Superheterodyne. A36. Thirty or sixty megahertz. A37. Local oscillator. A38. Mixer. A39. Cascade. A40. Detector.

A41. Iagc.

A42. Zero. NEETS MODULE 18-Radar Principles

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A43. FTC has no effect on receiver gain. A44. Stationary. A45. Coho lock pulse. A46. Opposite. A47. Amplification is reduced. A48. Decreases. A49. Three. A50. Zero. A51. Phase.