Spurious response interference

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

signal.

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

occur.

This frequency combination is termed the

ABC intermodulation products.

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