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TM 11-490-5
In other words, the second harmonic of the spurious
(c) Spurious response interference. In addition
signal frequency mixes with the second harmonic of the
to the susceptibility of receivers to cochannel. adjacent
local oscillator frequency in the mixer stage to produce a
channel, and IF interference, most receivers may also
potential interfering signal at 70 MHz which is identical
be susceptible to interference at other frequencies
to the IF of the receiver. The IF amplifier stages will
because of their "spurious" response characteristics; that
treat this signal in the same manner as the desired
is, their response to frequencies other than the tuned
frequency of the receiver.  Most spurious response
(d ) Communications receivers are susceptible
characteristics are the result of nonlinear elements of
to intermodulation and cross modulation interference
the RF circuitry (front end) as well as in the mixer stage.
resulting from simultaneous reception of two or more
which is designed to be nonlinear. Although a signal
signals combined through circuit nonlinearities in the
may be relatively free of harmonic content, harmonics
receiver front end. The result of this combining (mixing)
of the signal may be generated in the receiver front end
may  be  the  production  of  sum  and  difference
whenever a nonlinearity is encountered. Such nonlinear
frequencies within the receiver pass band which are
elements include mixer stages and, frequently, antenna
converted to the intermediate frequency in the mixer,
grounds or other signal connections (oxide films formed
demodulated, amplified, and presented as noise in the
on the metal by corrosion), cold solder joints, and
audio output. The interfering signals do not have to be
overdriven RF stages. An input signal will mix with local
within the receiver's preselection pass band for this to
oscillator harmonics. harmonics of an input signal will
occur. They must merely be of such sufficient strength
mix with the local oscillator signal, or combinations of
as to override the selectivity capabilities of the receiver.
both may occur.  If any combination of harmonics
Cross modulation may be apparent to the operator as an
produces a signal at the same frequency as the IF, at
unclear signal, or as the receiving of two signals of
the mixer stage output, interference may occur. Most
different intensity.
spurious responses (fsp ) can be identified by the
1.  Interference due to intermodulation may
following relationships:
For a single-conversion receiver:
occur at a receiver when the frequencies of two signals
are separated by an amount equal to the frequency to
which the receiver is tuned.  If two such signals are
passed through a nonlinear element in the receiver front
end, their sum and difference frequencies are generated
For a dual-conversion receiver:
by heterodyne action.  These new frequencies are
termed the A-B and A + B intermodulation products.
When these frequencies are of sufficient amplitude and
are within the receiver pass band, they will be detected
in the same way as a desired signal.
Where p is an integer or zero denoting the harmonic
2. Interference due to intermodulation may also
order of the local oscillator: q is an integer denoting the
result when the harmonic of one signal and the
harmonic order of the spurious response frequency; fu
fundamental of any other signal combine to produce a
and fIF denote the local oscillator and intermediate
sum or difference frequency equal either to the receiver
frequencies, respectively.
the numerical subscripts
tuned frequency or to a spurious-response frequency.
indicate the number of conversions preceding the
Any multiple of the tuned frequency, such as 3A2B,
section of the receiver of concern, and the positive and
3B2A, 4A3B, and so on, may cause interference of
negative signs identify different spurious responses. As
this type if it is of sufficient amplitude.
an example of spurious response identification, consider
3. Interference due to intermodulation may also
the AN/FRC109(V) tuned to 7680 MHz.  The local
occur when three signals combine to generate the
oscillator frequency is 7750 MHz and the intermediate
frequency to which a receiver is tuned.  Two signal
frequency is 70 MHz.
frequencies, when combined with a third frequency in a
For p = 2, q = 2 and sign of(-- ):
nonlinear element, will produce sum and difference
frequencies. If these frequencies are equal to either the
tuned frequency or a spurious-response frequency of a
receiver, detection will take place and interference will
This frequency combination is termed the
ABC intermodulation products.

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