Over modulation will produce extra sideband frequencies.
The spectrum is very similar to multi-frequency modulation.
Over modulation, however, is usually distinguished from the
multi-frequency modulation by: 1) The spacing between over-
modulated sidebands is equal, while, multi-frequency side-
bands may be arbitrarily spaced, unless the modulating fre-
quencies are harmonically related; 2) The amplitude of the
sidebands decreases progressively out from the carrier, but.
the amplitude multifrequency modulated signals is determined
by the modulation percentage of each frequency and can be
arbitrary.
Frequency Modulated Spectrum
FM measurements are generally measurements that deter-
mine; the modulating frequency, amplitude of the modulating
signal or frequency deviation, and index of modulation.
A typical FM spectrum is shown in Fig. 2-26. The exterior
modulation envelope resembles a cOS2 curve, and identifies
the signal as frequency modulation.
Frequency Deviation Measurement
There is no clear relationship between spectral width and
deviation, because in theory the FM spectrum approaches
infinity. In practice however the spectral level falls quite
rapidly as shown in Fig. 2-26B.
Accurate deviation measurements can be made if the
modulating frequency and the modulation index (where the
carrier goes to zero) are known.
Carrier deviation
Modulation Index =
Modulating frequency
Values of modulation index corresponding to zero carrier
amplitude are listed in Table 2-1.
Accurate carrier null is essential for accurate measurement.
Pulse Modulated RF Spectral Measurements
A visual examination of the pulse modulated spectrum can
check a number of the characteristics about a transmitting
and modulating device. Some of these characteristics are:
1, The transmitting oscillator stability can be checked by
illustrating pulse width and
Fig. 2-27. Pulse modulation display,
noting the degree of frequency shift as described previously.
PRF measurements.
2-23
