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Figure 2-2. Transmitter spurious emissions for an AN/GRT-22.
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TM-11-490-5 Army Communications Facilities Operational Electromagnetic Compatability Manual
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Intermodulation and cross modulation-continued


TM 11-490-5
distorts an applied signal.  An example of nonlinear
operation of a transistor is shown in figure 2-3. This
figure shows the manner in which the output voltage
waveform becomes distorted as a result of operation
over the nonlinear portion of a transistor's base current
versus collector current operating curve. This distorted
output contains frequencies which were not present in
the sine wave input signal.
These are actual
frequencies which comprise the distorted output signal
in that they respond to such actions as filtering,
coupling, and frequency selection as though they had
been input to the circuit. In the case of two or more RF
signals being presented to an RF amplifier under such
nonlinear
conditions,
sum
and
difference
intermodulation product frequencies can be produced.
Many nonlinear devices are used to perform useful
operations, such as within a mixer stage where the
nonlinearity is used to heterodyne or beat the incoming
signal frequency with the local oscillator frequency to
produce an intermediate frequency.  A diode is a
nonlinear device which may be placed in a circuit so as
to pass only the positive going voltage peaks, while
shunting the negative going peaks to ground. However,
when the RF amplifier of a receiver is nonlinear, or is
operated over a nonlinear portion of its characteristic
curve (overdriven), it has the potential for producing
cross modulation and interlmodulation products. Either
of these types of distortion is objectionable.
Figure 2-3. Example of distortion as a result of
overdriving an amplifier stage.
2-5


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