Fig. 2-18. Spectrum of an amplitude modulated signal. Sideband
Fig. 2-19. Formation of a spectrum. F is the fundamental or carrier
frequency, F1 and F2 are the modulating frequencies.
amplitude is ½ the percentage of modulation. This spectrum shows
Spectrum of Amplitude Modulation
3. Decrease the dispersion to open the screen. Keep the
signal centered on screen with the IF CENTER FREQ controls.
When a single frequency (CW) signal is amplitude-modu-
If the local oscillator should lose its lock condition when the
lated by a single frequency, two additional frequencies will
dispersion settings are 100 kHz or less, the signal will dis-
be generated; the carrier plus the two side bands. See Fig.
appear from the screen. A slight adjustment of the FINE RF
2-18. The amplitude of either sideband with respect to the
CENTER FREQ control will usually return the signal to the
carrier voltage is ½ the percentage of modulation. The
frequency difference between the carrier and either sideband
equals the modulating frequency.
4. If two or more high frequency (upper scale) signals are
to be resolved, they can be moved on the display without
Figure 2-19 illustrates how a spectrum is generated when
losing phase lock by adjusting the INT REF FREQ control.
a fundamental carrier frequency F is modulated by two
frequencies F1 and F2.
Recorder Out Connector
The sideband spectrum af multiple frequency amplitude-
Signals on the display may be recorded by connecting to
modulated signal spectrum is determined by the modulating
the RECORDER output connector on the rear panel. A linear
frequencies. To resolve this complex spectrum, the analyzer
output is provided when the VERTICAL DISPLAY switch is in
resolution bandwidth must be less than the lowest modulating
the LOG and LIN positions. With the DISPLAY switch in
frequency, or the bandwidth must be less than the difference
the SQ LAW position, the output to the RECORDER con-
between any two modulating frequencies, whichever is the
nector is square law.
Control Setup Chart
In wideband amplitude-modulation such as television pic-
ture information, the spectrum analyzer may be used to
Fig. 2-17 is
a control setup chart for the front panel of the
measure the sideband energy distribution and modulation
Type 491. This
figure may be reproduced and used as a test
for special applications or procedures. It may
also serve as
a training aid to facilitate control operation.
The amplitude modulated signal spectrum will therefore
furnish the following information: 1) Fundamental or carrier
frequency, 2) modulation frequency or frequencies, 3) modu-
SPECTRUM ANALYZER DISPLAYS
lation percentage, 4) sideband energy distribution and 5)
The Spectrum Analyzer displays a plot of signal amplitude
modulation bandwidth. Other characteristics which may
be evaluated are; degree of incidental FM (evidenced by
as a function of frequency. With this type of display, in
signal jitter), nonlinear modulation, and over-modulation.
the frequency domain, individual frequency. components in
These characteristics will be described in more detail with
the signal can be displayed and readily analyzed. This
other types of spectrum display patterns.
section describes some basic spectrum analyzer displays.