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Figure 2-3. Example of distortion as a result of overdriving an amplifier stage
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TM-11-490-5 Army Communications Facilities Operational Electromagnetic Compatability Manual
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Figure 2-4. Intermodulation products occurring at a receiver.


TM 11-490-5
f1 = frequency of interference closest to the
1.
Cross  modulation,  in  contrast  with
receiver tuned frequency
intermodulation, does not involve the mixing of two RF
f2 = frequency of interference farthest from the
signals to result in intermodulation products at different
receiver tuned frequency
frequencies, although it does occur as a result of circuit
m and n = integers (1,2,3 and so forth) Any
nonlinearities.  If a nonlinearity exists, such as an
frequencies produced which fall within the RF bandwidth
overdriven RF amplifier stage, an amplitude modulated
of the filter following the RF amplifier will be presented
adjacent channel signal will cause the gain of the
as false desired signals to the mixer stage.  The
amplifier to vary with the modulation.  In effect, this
resulting demodulated signals will appear at the audio
modulation is transferred to the desired signal as it
output.
As an example of the production of an
progresses through the amplifier.  The possibility of
intermodulation  frequency,  consider  the  following
experiencing cross modulation problems as well as
example from experiments on the AN/GRR-23 receiver:
intermodulation problems can be decreased by following
In this experiment, a squelch break occurred with no
good maintenance practices including proper receiver
desired signal present when two signals at 139.8 and
alignment to maintain designed selectivity and linearity
140.2 MHz were transmitted in the vicinity of the victim
characteristics, maintaining good electrical connections,
receiver. The intermodulation frequency responsible for
and replacing components as needed to maintain
this squelch break was found to be 139.0 MHz, the
optimum operational performance.
tuned frequency of the receiver.
Although both
2.
Intermodulation  frequencies  may  be
frequencies, 139.8 and 140.2 MHz, were outside the RF
produced  at  a  receiver  when  two  signals  are
bandwidth of the receiver, their combined effect was to
simultaneously received which, although they may be
cause a squelch break to occur. Analysis revealed that
outside the RF bandwidth of the preselector stage,
the intermodulation frequency responsible was of the 5th
override the preselection and are presented to the RF
order, calculated as follows:
amplifier (fig 2-4). If a nonlinearity exists within the RF
amplifier which allows the mixing of these two signals,
fI = 3fI - 2f2
intermodulation frequencies will be produced according
= 3(139.8) - 2(140.2) MHz
to the following equation:
= 419.4 - 280.4 MHz
= 139.0 MHz
fI= mf1 nf2
Therefore, fI = f0 = 139.0 MHz.
where
fI= intermodulation frequency
2-6


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