Module 21 − Test Methods and Practices
Pages i ,
5−31, AI−1 to AI−3, Index
Figure 5-8C. - Time versus frequencies.
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.
Figure 5-9B. - Examples of time-domain (left) and frequency-domain (right) low-level signals.
Figure 5-10A. - Spectrum analyzer stability measurements.
Figure 5-10B. - Spectrum analyzer stability measurements.
Figure 5-10C. - Spectrum analyzer stability measurements.
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. These parameters can be displayed, as shown in figure 5-12, with the aid of a spectrum analyzer.
Figure 5-11. - Swept-distortion and response characteristics.
Figure 5-12. - Frequency-conversion characteristics.
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.
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.
Figure 5-13. - Typical spectrum analyzer.
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.
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.
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. 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 5-14. - Spectrum analyzer display of an AM signal.
Figure 5-15A. - Spectrum analyzer displays of AM signals.
Figure 5-15B. - Spectrum analyzer displays of AM signals.
Figure 5-15C. - Spectrum analyzer displays of AM signals.
Figure 5-16. - Modulation percentage versus sideband levels.
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.
Figure 5-18. - Two-tone test.
Q-5. What is the advantage of single-sideband (SSB) transmission over AM transmission?
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
(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.)
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.
Figure 5-20. - Spectrum distribution for a modulation index of 2.
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?
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
and Direct Current
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