Intermodulation and cross modulation-continued

TM 11-490-5

f1 = frequency of interference closest to the

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

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