Waveform Interpretation

NEETS Module 21, NAVEDTRA 14193A

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5 WA VEFORM INTERPRETATION

Learning Objectives

Upon completion of this chapter, you will be able to do the following: 1. Explain the use of waveform interpretation in testing applications. 2. Identify the different types of modulation and methods of measuring modulation. 3. Explain the various uses of spectrum analyzers. 4. Explain the various uses of time-domain reflectometers. 5. Identify the various tests that can be performed with the swept-frequency technique. 5.1 INTRODUCTION TO WAVEFORM INTERPRETATION Measurements performed with oscilloscopes, time-domain reflectometers, and spectrum analyzers enable you to view the signal produced by the equipment or circuit under test.

However, a visual display is of no value unless you are able to interpret the signal characteristics. A displayed waveform is a representation of a varying signal related to time. You can graphically plot an unknown waveform by using a system of coordinates in which the amplitude of the unknown signal is plotted linearly against time. An analysis of the resultant waveform provides you with valuable information in determining the characteristics of many electronic (and some mechanical) devices. For example, the waveform of a signal may indicate the presence of harmonics or parasitic oscillations, or it may indicate how closely a device is following a desired cycle of operation. As the parts in an amplifier begin to shift in value or deteriorate, waveform distortion often occurs and indicates abnormal operation of a circuit and often precedes circuit breakdown. Malfunctioning of electrical or electronic circuits within equipment can usually be traced, by waveform inspection, to a specific part or parts of the circuit responsible for the distorted signal. On the basis of these facts, it is apparent that there is an important need for test equipment that can provide a visual presentation of a waveform at the instant of its occurrence in a circuit.

DISTORTION is a term used by technicians and engineers alike that generally signifies dissatisfaction with the shape of the wave processed by an amplifier. Distortion of a waveform is the undesired change or deviation in the shape of the observed signal with respect to a reference waveform. Classifying any waveform as a distorted wave without reference to the electronic circuitry involved is meaningless. A waveform that can be validly termed distorted with respect to a specific amplifier circuit may be the normal waveform to be expected from another amplifier circuit. One of the most important steps in waveform analysis, the one that usually proves the most difficult for the maintenance personnel, is the interpretation of patterns viewed on the test equipment.

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This chapter will cover some of the basic test methods and practices associated with waveform interpretation. 5.2 MODULATION MEASUREMENTS Modulation measurements are sometimes required during tuning procedures to adjust transmitting equipment for the proper amount of modulation. During maintenance tests of modulated transmitter equipment, you should determine the amount of distortion in the output signal and the modulation level or index. The modulation level in multiplexing equipment is usually set at the factory or during corrective maintenance procedures.

Proper adjustment of the input signal level and automatic signal-level regulation circuits provides the correct amount of modulation. Defects in modulation circuits of a transmitter can be detected by measurements of the quality of the received signals at the receiver. Corrective maintenance analysis of multiplex equipment modulation circuits can usually be made by signal-level measurements. Some radio transmitters, when operating in the AM mode, must be adjusted for correct modulation during normal tuning procedures. If the modulation level is low, the transmitter is not operating at its maximum efficiency. On the other hand, modulation in excess of 100% produces serious distortion. Since neither of these conditions is desirable, amplitude modulation should be maintained between 60% and 95% when possible. The modulation level or index of AM and fm radio transmitters that operate in the vhf range is initially adjusted by the manufacturer or during corrective maintenance. The amplifier gain of the modulator can be initially adjusted by reference to the modulation meter provided on the front panel of the equipment.

Pulse modulation of radar and radio beacon signals can be measured by waveform displays presented on a standard oscilloscope. The amount of usable energy in a pulsed waveform, as measured by a spectrum analyzer, is also an indication of the pulse modulation quality. Attaining 100% amplitude modulation of an rf carrier with a sine wave requires a modulating power equal to one-half of the rf carrier power. Under this condition, the average power of the modulated carrier is equal to 1.5 times the average unmodulated carrier power. The added power is divided equally between the upper and lower sidebands. During the peaks of 100% modulation, the amplitude of the carrier is doubled.

This will cause the instantaneous peak power to be four times the instantaneous unmodulated peak power P = E2/R. When voice modulation is employed, only the highest amplitude peaks can be allowed to modulate the carrier 100%. Since many speech components do not modulate the carrier 100%, the average power required for voice modulation is less than that required for modulation with a sine wave. Voice peaks usually modulate a carrier 100% when the modulation increases the average carrier output power 25% over its normal value.

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Q-1. What is the result of over modulating an AM signal? Q-2. For AM transmissions, the carrier is normally modulated within what range? 5.2.1 Amplitude-Modulation Measurements An increase in the power output of an AM transmitter is indicated by an increase in antenna current. The increase can be taken as a measure of the degree of modulation and can be expressed as a percentage, as shown in figure 5-1. The graph for this figure was developed from the relationship existing between the carrier power and the increased power resulting from the added modulation power.

Figure 5-1 Antenna current increase with amplitude modulation NEETS Module 21, NAVEDTRA 14193A

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The formula for calculating the PERCENTAGE of MODULATION is as follows: Percentage of modulation = 100 (Emax − E0) E0 Where: Emax = the highest peak E0 = the unmodulated carrier voltage The use of this formula is based on the assumption that the modulating voltage is a pure sine wave. Normal broadcasting, however, is characterized by complex envelope patterns, as illustrated in figure 5-2. In this light, the previous formula is not so clear. Consequently, the preceding formula should be viewed more correctly as the PERCENTAGE OF POSITIVE PEAK MODULATION. When the minimum voltage (E min) rather than the peak voltage (Emax) is used to compute percentage of modulation, the computed percentage (shown below) is the PERCENTAGE OF NEGATIVE PEAK MODULATION: negative peak percentage of modulation = 100 (E0 − Emin) E0 Figure 5-2 Rf carrier amplitude-modulated by a complex wave envelope NEETS Module 21, NAVEDTRA 14193A

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Since the preceding two modulation percentages often differ, you should define the AVERAGE PERCENTAGE OF MODULATION, as shown below (refer to fig. 5-3): average percentage of modulation = 100 (Emax − Emin) 2 × E0 From the preceding definitions of percentage of modulation, you should note that methods of measuring all three types of modulation percentages must be devised. When differing values are obtained, however, the cause may not necessarily be directly related to unequal positive and negative peaks of a complex modulation wave. Another possibility is distortion caused by carrier shift. Distortion may also be produced by effects other than the modulation process - for example, parasitic oscillation, nonlinear radio- frequency amplification of modulated signals, and distortion present in the audio amplifiers.

Unfortunately, continuous variations in the percentage of modulation create a number of additional problems. For example, damping is necessary so that a meter can provide an average reading despite fluctuations. An average reading, on the other hand, will not disclose the presence of transient overmodulation. This shortcoming is serious because of the large number of sideband frequencies produced in addition to the normal ones whenever overmodulation occurs. Not only do these extra frequencies interfere drastically with other transmissions, but they also may significantly distort the modulation signal. These considerations account for the importance of using a meter that responds to modulation peak; specifically, both positive-peak and negative-peak overmodulation must be indicated. Positive-peak overmodulation occurs when the positive modulation exceeds 100%; negative-peak overmodulation occurs when the negative modulation exceeds 100%.

Figure 5-3 Rf amplitude percentage modulation wave envelope. NEETS Module 21, NAVEDTRA 14193A

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5.2.1.1 Oscilloscope Measurement Methods The oscilloscope is widely used as an amplitude-modulation monitor and measuring instrument. Since it is capable of presenting visual indications of the modulated output of AM transmitters, the oscilloscope is reliable for detecting overmodulation and determining the percentage of modulation. For example, the relative error of most measurements taken with a 5-inch crt is about 10%. Although such accuracy is adequate for many maintenance checks, the oscilloscope is usually considered more valuable as a monitor of general modulation conditions. It is also used to monitor the amplitude- modulated output of a radio transmitter when photographic records are desired.

5.2.1.2 Types of Modulation Display Two types of modulation patterns are provided by the oscilloscope, depending upon the hookup used. These patterns are the WAVE-ENVELOPE PATTERNS, as shown in figures 5-2 and 5-3, and the TRAPEZOIDAL PATTERN, as shown in figure 5-4. Figure 5-4 Trapezoidal modulation patterns NEETS Module 21, NAVEDTRA 14193A

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Figure 5-2 shows an oscilloscope presentation of an rf carrier that is amplitude-modulated by a complex wave, such as that of speech. Figures 5-4 and 5-5 show the effects of over 100% modulation on the carrier wave. The carrier wave envelope pattern (as shown in fig. 5-3) is obtained by applying the rf modulated wave to the vertical input of the oscilloscope. The trapezoidal pattern is obtained in a similar manner except that the modulation signal from the transmitter is used to horizontally sweep the oscilloscope (instead of having the sweep signal generated internally by the oscilloscope). Both methods are limited by the frequency response of the oscilloscope; therefore, these methods find greater applicability in the lf to hf ranges.

5.2.2 VHF And UHF Measurements In the vhf and uhf ranges, modulation is normally measured by applying a specific-level, 1-kilohertz tone to the input of the modulator. This, in turn, produces a significant drop in the plate voltage of the final output stage of the modulator. The correct setting of output plate voltage ensures that overmodulation will not occur. Figure 5-5 Overmodulated rf carrier NEETS Module 21, NAVEDTRA 14193A

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5.2.3 Single-Sideband Measurements Single-sideband modulation is a form of amplitude modulation in which only one sideband is transmitted with a suppressed carrier. Since balanced modulators are used to provide carrier cancellation, the exact balancing of the carriers to provide cancellation requires a null adjustment. The null can be observed and adjusted by using either a detector and an indicator, such as a voltmeter, or an oscilloscope for observation of the output while tuning the transmitter. Measurements peculiar to sideband technology also include special modulation- amplitude and modulation-distortion checks. If the sideband modulator is overdriven or mistuned or the associated linear amplifiers are improperly loaded or overdriven, spurious output frequencies are produced. These are harmonically related to the driving signals and can cause splatter over a large range of frequencies, thus causing interference to other transmitting stations.

To determine the proper amplitude so that the modulation will not cause distortion or splatter, you use the audio two-tone modulation test. The resulting signals are shown in views A, B, and C of figure 5-6. The two-tone test is used for initial adjustment and for precise checking because it will indicate distortion. The two-tone test corresponds to the wave envelope method of AM modulation checking. Two signals of equal amplitude but of slightly different frequencies beating together are applied to the sideband modulator input to produce a single tone of approximately 1,000 hertz. On an oscilloscope, the output appears as a series of fully modulated sine waves and is similar to a 100-percent- amplitudemodulated waveform, as shown in view A. A spectrum analyzer presentation is shown in view B.

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Figure 5-6A Examples of ideal two-tone test waveforms Figure 5-6B Examples of ideal two-tone test waveforms NEETS Module 21, NAVEDTRA 14193A

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When the trapezoidal method is used, two opposed triangles appear on the oscilloscope, as shown in figure 5-6, view C. When equally balanced modulators are used, the triangles are mirror images. Elliptical or straight-line patterns appear when the phase-distortion check is used. It is also possible to make a rough operating adjustment by varying the audio drive from the microphone so that on peak swings a definite value of final plate current is not exceeded. This check depends upon the initial accuracy of calibration and response characteristics of the ammeter in the final stage, as well as other factors.

Figure 5-6C Examples of ideal two-tone test waveforms NEETS Module 21, NAVEDTRA 14193A

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5.2.4 Frequency Modulation In frequency modulation, the carrier amplitude remains constant, and the output frequency of the transmitter is varied about the carrier (or mean) frequency at a rate corresponding to the audio frequencies. The extent to which the frequency changes in one direction from the unmodulated (carrier) frequency is called the FREQUENCY

Deviation.

Deviation in frequency is usually expressed in kilohertz. It is equal to the difference between the carrier frequency and either the highest or lowest frequency reached by the carrier in its excursions with modulation. There is no modulation percentage in the usual sense. With suitable circuit design, the frequency deviation may be made as large as desired without encountering any adverse effects that are equivalent to the overmodulation in amplitude-modulation transmissions. However, the maximum permissible frequency deviation is determined by the width of the band assigned for station operation.

In frequency modulation, the equivalent of 100% modulation occurs when the frequency deviation is equal to a predetermined maximum value. There are several methods of measuring the modulation in frequency-modulated transmissions. The frequency-deviation measurement of a frequency-modulated signal is normally performed with either a spectrum analyzer or with a modulation analyzer. The modulation analyzer method is more commonly used because of its accuracy. Typical accuracies for a modulation analyzer are within ±1%. Figure 5-7 shows a typical modulation analyzer.

Figure 5-7 Typical modulation analyzer NEETS Module 21, NAVEDTRA 14193A

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Q-3. What is meant by frequency deviation? 5.3 SPECTRUM WAVEFORM ANALYSIS AND MEASUREMENTS An analysis of a complex waveform, prepared in terms of a graphic plot of the amplitude versus frequency, is known as SPECTRUM ANALYSIS. Spectrum analysis recognizes the fact that waveforms are composed of the summation of a group of sinusoidal waves, each of an exact frequency and all existing together simultaneously. Three axes of degree (amplitude, time, and frequency) are important when considering varying frequency. The time-domain (amplitude versus time) plot is used to consider phase relationships and basic timing of the signal and is normally observed with an oscilloscope. The frequency-domain (amplitude versus frequency) plot is used to observe frequency response - the spectrum analyzer is used for this purpose. Figure 5-8 illustrates the differences between frequency- and time-domain plots. View A illustrates a three- dimensional coordinate of a fundamental frequency (f1) and its second harmonic (2f1) with respect to time, frequency, and amplitude. View B shows the time-domain display as it would be seen on an oscilloscope. The solid line, f1 + 2f1 is the actual display. The dashed lines, f and 2f1 are drawn to illustrate the fundamental and second harmonic frequency relationship used to formulate the composite signal f1 + 2f1. View C is the frequency-domain display as it would be seen on a spectrum analyzer. Note in view C that the components of the composite signal are clearly seen.

Q-4. A spectrum analyzer is designed to display what signal characteristic? Figure 5-8A Time versus frequencies NEETS Module 21, NAVEDTRA 14193A

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Figure 5-8B Time versus frequencies Figure 5-8C Time versus frequencies NEETS Module 21, NAVEDTRA 14193A

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5.3.1 Frequency-Domain Display Capabilities The frequency domain contains information not found in the time domain. The spectrum analyzer can display signals composed of more than one frequency (complex signals). It can also discriminate between the components of the signal and measure the power level at each one. It is more sensitive to low-level distortion than an oscilloscope. Its sensitivity and wide, dynamic range are also useful for measuring low-level modulation, as illustrated in views A and B of figure 5-9. The spectrum analyzer is useful in the measurement of long- and short-term stability such as noise sidebands of an oscillator, residual fm of a signal generator, or frequency drift of a device during warm-up, as shown in views A, B, and C of figure 5-10.

Figure 5-9A Examples of time-domain (left) and frequency-domain (right) low-level signals NEETS Module 21, NAVEDTRA 14193A

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Figure 5-9B Examples of time-domain (left) and frequency-domain (right) low-level signals Figure 5-10A Spectrum analyzer stability measurements NEETS Module 21, NAVEDTRA 14193A

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Figure 5-10B Spectrum analyzer stability measurements Figure 5-10C Spectrum analyzer stability measurements NEETS Module 21, NAVEDTRA 14193A

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The swept-frequency response of a filter or amplifier and the swept-distortion measurement of a tuned oscillator are also measurable with the aid of a spectrum analyzer. However, in the course of these measurements, a variable persistence display or an X-Y recorder should be used to simplify readability. Examples of tuned-oscillator harmonics and filter response are illustrated in figure 5-11. Frequency-conversion devices such as mixers and harmonic generators are easily characterized by such parameters as conversion loss, isolation, and distortion.

Figure 5-11 Swept-distortion and response characteristics NEETS Module 21, NAVEDTRA 14193A

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These parameters can be displayed, as shown in figure 5-12, with the aid of a spectrum analyzer. Present-day spectrum analyzers can measure segments of the frequency spectra from 0 hertz to as high as 300 gigahertz when used with waveguide mixers. Figure 5-12 Frequency-conversion characteristics NEETS Module 21, NAVEDTRA 14193A

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5.3.2 Spectrum Analyzer Applications Figure 5-13 shows a typical spectrum analyzer. The previously mentioned measurement capabilities can be seen with a spectrum analyzer. However, you will find that the spectrum analyzer generally is used to measure spectral purity of multiplex signals, percentage of modulation of AM signals, and modulation characteristics of fm and pulse- modulated signals. The spectrum analyzer is also used to interpret the displayed spectra of pulsed rf emitted from a radar transmitter. 5.3.3 Complex Waveforms Complex waveforms are divided into two groups, PERIODIC WAVES and NONPERIODIC WAVES. Periodic waves contain the fundamental frequency and its related harmonics. Nonperiodic waves contain a continuous band of frequencies resulting from the repetition period of the fundamental frequency approaching infinity and thereby creating a continuous frequency spectrum.

Figure 5-13 Typical spectrum analyzer NEETS Module 21, NAVEDTRA 14193A

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5.3.4 Modulation Measurements In all types of modulation, the carrier is varied in proportion to the instantaneous variations of the modulating waveform. The two basic properties of the carrier available for modulation are the AMPLITUDE CHARACTERISTIC and ANGULAR (frequency or phase) CHARACTERISTIC. 5.3.4.1 Amplitude Modulation The modulation energy in an amplitude-modulated wave is contained entirely within the sidebands. Amplitude modulation of a sinusoidal carrier by another sine wave would be displayed as shown in figure 5-14. For 100% modulation, the total sideband power would be one-half of the carrier power; therefore, each sideband would be 6 dB less than the carrier, or one-fourth of the power of the carrier. Since the carrier component is not changed with AM transmission, the total power in the 100-percent-modulated wave is 50% higher than in the unmodulated carrier. The primary advantage of the log display that is provided by the spectrum analyzer over the linear display provided by the oscilloscopes for percentage of modulation measurements is that the high dynamic range of the spectrum analyzer (up to 70 dB) allows accurate measurements of values as low as 0.06%. It also allows the measurements of low-level distortion of AM signals.

Figure 5-14 Spectrum analyzer display of an AM signal NEETS Module 21, NAVEDTRA 14193A

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Both capabilities are illustrated in figure 5-15, view A, view B, and view C. The chart in figure 5-16 provides an easy conversion of dB down from carrier into percentage of modulation. Figure 5-15A Spectrum analyzer displays of AM signals Figure 5-15B Spectrum analyzer displays of AM signals NEETS Module 21, NAVEDTRA 14193A

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Figure 5-15C Spectrum analyzer displays of AM signals Figure 5-16 Modulation percentage versus sideband levels NEETS Module 21, NAVEDTRA 14193A

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NOTE: Anything greater than -6 dB exceeds 100% modulation and produces distortion, as shown in figure 5-16. In modern, long-range hf communications, the most important form of amplitude modulation is ssb (single-sideband). In ssb either the upper or lower sideband is transmitted, and the carrier is suppressed. Ssb requires only one-sixth of the output power required by AM to transmit an equal amount of intelligence power and less than half the bandwidth. Figure 5-17 shows the effects of balancing the carrier of an AM signal. The most common distortion experienced in ssb is intermodulation distortion, which is caused by nonlinear mixing of intelligence signals. The two-tone test is used to determine if any intermodulation distortion exists. Figure 5-18 illustrates the spectrum analyzer display of the two-tone test with the modulation applied to the upper sideband input.

Figure 5-17 Double sideband carrier suppressed NEETS Module 21, NAVEDTRA 14193A

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Q-5. What is the advantage of single-sideband (ssb) transmission over AM transmission? 5.3.4.2 Frequency Modulation In frequency modulation, the instantaneous frequency of the radio-frequency wave varies with the modulation signal. As mentioned in NEETS, module 12, the amplitude is kept constant. The number of times per second that the instantaneous frequency varies from the average (carrier frequency) is controlled by the frequency of the modulating signal. The amount by which the frequency departs from the average is controlled by the amplitude of the modulating signal. This variation is referred to as the FREQUENCY DEVIATION of the frequency-modulated wave. We can now establish two clear-cut rules for frequency deviation rate and amplitude in frequency modulation: • Amount of frequency shift is proportional to the amplitude of the modulating signal.

(This rule simply means that if a 10-volt signal causes a frequency shift of 20 kilohertz, then a 20-volt signal will cause a frequency shift of 40 kilohertz.) • Rate of frequency shift is proportional to the frequency of the modulating signal. (This second rule means that if the carrier is modulated with a 1-kilohertz tone, then the carrier is changing frequency 1,000 times each second.) Figure 5-18 Two-tone test NEETS Module 21, NAVEDTRA 14193A

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The amplitude and frequency of the signal used to modulate the carrier will determine both the number of significant sidebands (shown in fig. 5-19) and the amplitude of the sidebands (shown in fig. 5-20). Both the number of significant sidebands and the bandwidth increase as the frequency of the modulating signal increases. Figure 5-19 Distribution of sidebands NEETS Module 21, NAVEDTRA 14193A

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NEETS, module 12, should be consulted for an in-depth discussion of frequency- modulation principles. Q-6. What happens to an fm signal as you increase the frequency of the modulating signal? Figure 5-20 Spectrum distribution for a modulation index of 2 NEETS Module 21, NAVEDTRA 14193A

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5.3.5 Pulsed Waves An ideal pulsed radar signal is made up of a train of rf pulses with a constant repetition rate, constant pulse width and shape, and constant amplitude. To receive the energy reflected from a target, the radar receiver requires almost ideal pulse radar emission characteristics. By observing the spectra of a pulsed radar signal, you can easily and accurately measure such characteristics as pulse width, duty cycle, and peak and average power. The principles of radar are covered in NEETS, Module 18, Radar Principles, which can be consulted for an explanation of pulsed waves.

5.3.5.1 Rectangular Pulse A rectangular wave is used to pulse-modulate the constant frequency rf carrier to produce the pulse radar output. The rectangular wave is made up of a fundamental frequency and its combined odd and even harmonics. Figure 5-21 shows the development of a rectangular wave. Figure 5-21 Rectangular pulse NEETS Module 21, NAVEDTRA 14193A

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5.3.5.2 Pulsed Wave Analysis In amplitude modulation, sidebands are produced above and below the carrier frequency. A pulse is also produced above and below the carrier frequency, but the pulse is made up of many tones. These tones produce multiple sidebands that are commonly referred to as SPECTRAL LINES, or RAILS, on the spectrum analyzer display. Twice as many rails will be in the pulse-modulated output of the radar as there are harmonics contained in the modulating pulse (upper and lower sidebands), as shown in figure 5-22. In the figure, the pulse repetition frequency (prf) is equal to the pulse interval of 1/T. The actual spectrum analyzer display would show the lower lobes (shown below the reference line in the figure) on top because the spectrum analyzer does not retain any polarity information.

Changing the pulse interval, or pulse width, of the modulation signal will change the amount of rails (prf), or number of lobe minima, as illustrated in figure 5-23. Figure 5-22 Pulsed radar output NEETS Module 21, NAVEDTRA 14193A

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5.3.6 Analyzing the Spectrum Pattern The leading and trailing edges of the radiated pulse-modulated signal must have a sharp rise time and decay time and a constant amplitude between them. Incorrect pulse shape will cause frequency spread and pulling, which results in less available energy at the frequency to which the receiver is tuned. The primary reason for analyzing the spectrum is to determine the exact amount of amplitude and frequency modulation present. The amount of amplitude modulation determines the increase in the number of sidebands within the applied pulse spectrum; an increase in frequency modulation increases the amplitude of the side-lobe frequencies. In either case, the energy available to the main spectrum lobe is decreased.

Figure 5-23 Pulsed radar changes caused by modulating signal changes NEETS Module 21, NAVEDTRA 14193A

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5.3.7 Spectrum Analyzer Operation The information desired from the spectra to be analyzed determines the SPECTRUM ANALYZER requirements. Real-time analysis is used if a particular point in the frequency spectrum is to be analyzed, such as a line spectra display. Continuous- or swept-frequency analysis, which is the most common mode of observation, is used to display a wider portion of the frequency spectrum or (in some cases) the entire range of the spectrum analyzer in use. Changing the spectrum analyzer setting from one mode to another is accomplished by varying the scan time and the bandwidth of the spectrum analyzer or a combination of the two. Most real-time spectrum analyzers, however, are preceded by mechanical filters, which limit the input bandwidth of the spectrum analyzer to the desired spectra to be analyzed. Tunable- or swept-spectrum analyzers function basically the same as heterodyne receivers, the difference being that the local oscillator is not used but is replaced by a voltage-controlled oscillator (vco). The vco is swept electronically by a ramp input from a sawtooth generator. The output of the receiver is applied to a crt, which has its horizontal sweep in synchronization with the vco. The lower frequency appears at the left of the crt display. As the trace sweeps to the right, the oscillator increases in frequency. Figure 5-24 is a block diagram of a heterodyne spectrum analyzer.

Figure 5-24 Block diagram of a heterodyne spectrum analyzer NEETS Module 21, NAVEDTRA 14193A

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Before the frequency of a signal can be measured on a spectrum analyzer, it must be RESOLVED. Resolving a signal means distinguishing it from other signals near it. Resolution is limited by the narrowest bandwidth of the spectrum analyzer because the analyzer traces out its own IF bandwidth shape as it sweeps through a signal. If the narrowest bandwidth is 1 kilohertz, the nearest any two signals can be, and still be resolved, is 1 kilohertz. Reducing the IF bandwidth indefinitely would obtain infinite resolution except that the usable IF bandwidth is limited by the stability of the spectrum analyzer. The smaller the IF bandwidth, the greater the capability of the analyzer to resolve closely spaced signals of unequal amplitudes. Modern spectrum analyzers have been refined to the degree that IF bandwidths are less than 1 hertz.

It is important that the spectrum analyzer be more stable in frequency than the signals being measured. The stability of the analyzer depends on the frequency stability of its vco. Scan time of the spectrum analyzer must be long enough, with respect to the amplitude of the signal to be measured, to allow the IF circuitry of the spectrum analyzer to charge and recover. This will prevent amplitude and frequency distortion. Q-7. When referring to spectrum analyzers, what is meant by the term resolving signals? 5.4 TIME-DOMAIN REFLECTOMETRY TIME-DOMAIN REFLECTOMETRY is a testing and measurement technique that has found increasing usefulness in testing transmission lines (both metallic and fiber-optic), cables, strip lines, connectors, and other wideband systems or components. Basically, time-domain reflectometry is an extension of an earlier technique in which reflections from an electrical pulse were monitored to locate faults and to determine the characteristics of power transmission lines. You can compare time-domain reflectometry to a closed-loop radar system in which the transmitted signal, a very fast step pulse, is fed into the system and the reflections resulting from discontinuities or impedance deviations in the system are monitored on a crt.

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The technique used in time-domain reflectometry consists of feeding an impulse of energy into the system and then observing that energy as it is reflected by the system at the point of insertion. When the fast-rise input pulse meets with a discontinuity or impedance mismatch, the resultant reflections appearing at the feed point are compared in phase, time, and amplitude with the original pulse. By analyzing the magnitude, deviation, and shape of the reflected waveform, you can determine the nature of the impedance variation in the transmission system. Also, since distance is related to time and the amplitude of the reflected step is directly related to impedance, the comparison indicates the distance to the fault as well as the nature of the fault. Figure 5-25, view A, view B, view C, and view D, illustrates typical transmission line problems that can easily be identified by using a time-domain reflectometer (tdr). In addition to showing both the distance to and the nature (resistive, inductive, or capacitive) of each line discontinuity, time-domain reflectometry also reveals the characteristic impedance of the line and indicates whether losses are shunt or series. They are also used to locate and analyze connectors and splices.

Figure 5-25A Time-domain reflectometer display of transmission line problems NEETS Module 21, NAVEDTRA 14193A

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Figure 5-25B Time-domain reflectometer display of transmission line problems Figure 5-25B Time-domain reflectometer display of transmission line problems NEETS Module 21, NAVEDTRA 14193A

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A conventional method of evaluating high-frequency transmission systems and components has been through the use of standing wave ratio (swr) measurements to obtain an overall indication of transmission line performance. This method involves feeding a sine-wave signal into the system and measuring the maximum and minimum amplitudes of the standing waves that result from system discontinuities or load mismatches. The ratio between the minimum and maximum swr values is then taken as the system FIGURE OF MERIT. The swr measurement, however, does not isolate individual discontinuities or mismatches when multiple reflections are present; it only indicates their total effect. Time-domain reflectometry measurements, on the other hand, isolate the line characteristics in time (location). As a result, multiple reflections resulting from more than one discontinuity or impedance variation that are separated in distance on the line are also separated in time at the monitoring point and can be individually analyzed.

Prior to the advent of time-domain reflectometers, time-domain reflectometry was performed with the aid of sampling oscilloscopes and pulse generators with very fast rise times. Figure 5-25D Time-domain reflectometer display of transmission line problems NEETS Module 21, NAVEDTRA 14193A

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Figure 5-26 shows the earlier type of test setup, which is still an option. However, today's time-domain reflectometers have several advantages over the old pulse-generator and oscilloscope methods. Modern time-domain reflectometers are compact, lightweight, are often supplied with battery pack options for field use, and provide a direct readout of distances instead of time. Some equipment provides a paper-tape recording for a permanent record. Figure 5-27 shows a typical time-domain reflectometer. Figure 5-26 Time-domain reflectometry, basic equipment setup Figure 5-27 Typical time-domain reflectometer NEETS Module 21, NAVEDTRA 14193A

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Q-8. Why are time-domain reflectometers often compared to a radar system? Q-9. What is the main advantage of using a time-domain reflectometer (tdr) to test a transmission line? 5.5 SWEPT-FREQUENCY TESTING EQUIPMENT SWEPT-FREQUENCY testing is used to determine the bandwidth, alignment, frequency response, impedance matching, and attenuation in various circuits, systems, and components. Swept-frequency testing can be used to quickly determine the broadband response of a device that otherwise would require a number of separate measurements and manual plotting of the response curve. Swept-frequency techniques are applicable over the entire electronic spectrum from vlf to ehf and are generally limited only by your resourcefulness and the basic limitation of the equipment employed. The basic swept- frequency arrangement is shown in figure 5-28.

Figure 5-28 Frequency-response test NEETS Module 21, NAVEDTRA 14193A

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The swept-frequency technique can effectively determine the frequency response of an amplifier or filter and is useful in the alignment or bandwidth determination of an IF or rf stage. The test equipment permits direct visual readout on the crt of the spectrum analyzer. The spectrum analyzer can also be connected to an X-Y chart recorder if a permanent record or print is desired. Figure 5-29 shows a spectrum analyzer crt display of the frequency response of a multicoupler. The tracking generator used must be capable of sweeping the desired frequency range of the device under test.

Q-10. What is the purpose of swept-frequency testing? Figure 5-29 Typical spectrum analyzer frequency-response display NEETS Module 21, NAVEDTRA 14193A

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5.5.1 Tracking Generator Figure 5-30 shows a typical tracking generator used with the Hewlett-Packard 141 T spectrum analyzer. A TRACKING GENERATOR is basically a sweep generator in which the sweep rate is matched to that of the spectrum analyzer. The output circuitry of the tracking generator contains a network that ensures a constant output amplitude over the entire range being swept. When the fm signal produced by the tracking generator is applied to a device or circuit under test, the instantaneous output amplitude is always proportional to the response of the circuit to the frequency at that instant. Thus, the original fm input signal is changed in passing through the circuit under test. The output signal, therefore, would consist of an fm signal that is also amplitude-modulated. For equal deviations, the positive and negative portions of this envelope are symmetrical, making it necessary to observe only one side of the envelope.

Figure 5-30 Tracking generator used with a spectrum analyzer NEETS Module 21, NAVEDTRA 14193A

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After the detection stage in the spectrum analyzer, only the modulation remains to appear on the face of the crt. This presentation will appear as a continuous curve because of the persistence of vision and the phosphor characteristic of the crt. The polarity of the detector determines whether a positive or a negative output is displayed. The frequency at any point on the crt display can be analyzed by arresting the scan of the spectrum analyzer either electronically or manually at the point of interest. For greater accuracy in frequency determination, a frequency counter may be attached to the output of the tracking generator at the point of the arrested scan.

5.5.2 Impedance Matching Conventional tuners cannot be used successfully to cancel source or load reflections in swept-frequency measurements. This is because the tuning is effective only at single frequencies; therefore, pads or isolators are required. However, by the use of automatic- level control, the power output of the sweep generator can be maintained relatively constant at the point of measurement. The source impedance may thus be maintained very close to the nominal value. With this arrangement, any impedance variation in the connecting cables, connectors, and adapters is effectively cancelled since these components are within the leveling loop. The attenuation of a device under test will be displayed on the associated crt as a continuous response curve as it is scanned. This will result in attenuation versus frequency plot of the device under test only.

5.5.3 Impedance Circuit impedance is measured conveniently by using the reflectometer principle. The individual values of the incident and reflected signals (swr) in a transmission line feeding an unknown impedance are measured. The ratio between these signals indicates how closely the load impedance matches that of the transmission line. Another method is the use of an auto-mechanical load control to hold the forward power at a constant level while the return load of a specific load is measured. A short is then placed in the circuit, and 100% reflected power is measured. The loss detected is then calculated to obtain swr figures.

5.5.4 Noise Figure By using a frequency-sweeping receiver and an automatic noise-figure meter, you can make noise-figure measurements on broadband microwave devices, such as a traveling- wave-tube amplifier. To conduct such a test properly, you must first check the receiver noise figure. Q-11. In swept-frequency testing the impedance of a transmission line, what electrical characteristic is actually being measured? NEETS Module 21, NAVEDTRA 14193A

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5.6 SWEEPING ANTENNAS Antenna system testing is one of the more common and useful applications for using the swept-frequency technique. The main parameters that an antenna system is tested for are vswr, frequency response, and impedance. Figure 5-31 shows a typical test setup for testing a transmitting antenna for vswr. Remember that any transmitting antenna can also act as a receiving antenna and send induced power from adjacent antennas back to the test equipment. You should make an initial power check on the antenna to prevent damage to your test equipment. Figure 5-32 shows a typical hf transmitting antenna vswr display as measured using the swept-frequency technique. The setup for testing a receiving antenna vswr, shown in figure 5-33, is similar, with the exception of the attenuators. The measured vswr (within the operating frequency range) of any broadband antenna should not exceed a vswr of 2.5 to 1. The vswr for any single-tuned antenna should not exceed 1.5 to 1 at the tuned frequency.

Figure 5-31 Vswr test for transmitting antennas NEETS Module 21, NAVEDTRA 14193A

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Figure 5-32 Typical spectrum analyzer vswr display Figure 5-33 Vswr test for receiving antennas NEETS Module 21, NAVEDTRA 14193A

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Q-12. What precautions must be taken when sweeping a transmitting antenna? 5.7 SUMMARY This chapter has presented information on waveform interpretation. The information that follows summarizes the important points of this chapter. Interpretation of a waveform is best accomplished with test equipment that gives you a visual indication of the waveform. The most common devices used in systems applications are OSCILLOSCOPES and SPECTRUM ANALYZERS. An amplitude-modulated signal can be tested with either an oscilloscope or a spectrum analyzer to determine its percentage of modulation, sideband characteristics, and carrier frequency. Frequency-modulated signals are normally tested with a spectrum analyzer or a modulation analyzer.

Oscilloscopes are designed to view a time-domain waveform (amplitude versus time). Spectrum analyzers are designed to view a frequency-domain waveform (amplitude versus frequency). One advantage of using a spectrum analyzer is its ability to graphically display the composition of COMPLEX WAVEFORMS. TIME-DOMAIN REFLECTOMETERS work on a principle similar to that used in radar. A precise signal is produced by the time-domain reflectometer and injected into the device under test (usually a transmission line); and the resulting reflections are displayed to discover such things as impedance mismatches, opens, and shorts. The display sections of time-domain reflectometers are calibrated to give you a graphical display of amplitude versus distance.

SWEPT-FREQUENCY TESTING is performed by using a TRACKING GENERATOR to inject a signal into a device and then monitoring the output of the device with a spectrum analyzer. The tracking generator is designed to sweep or scan through the entire frequency range of the device being tested. Its sweep rate must be matched with the sweep rate of the spectrum analyzer. NEETS Module 21, NAVEDTRA 14193A

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Answers To Questions Q1. Through Q12.

A-1. Distortion. A-2. 60% to 95%. A-3. The difference between the carrier frequency of an fm signal and its maximum frequency excursion when modulated. A-4. Amplitude versus frequency (the frequency domain of the signals). A-5. The same amount of intelligence can be transmitted with one-sixth of the output power with less than one-half the bandwidth. A-6. Both the bandwidth and the number of significant sidebands increase. A-7. The ability of the analyzer to discriminate between display signals of slightly different frequencies.

A-8. Both transmit a pulse and analyze the signal reflection. A-9. A Tdr will indicate the nature of and the distance to or location of any faults. A-10. To determine various characteristics of a component, piece of equipment, or system over its operational frequency range. A-11. Swr on the transmission line. A-12. You must ensure that power induced from any adjacent transmitting antennas does not damage your test equipment.