Figure 2-5. Intermodulation products occurring at a transmitter.
2fa - fb, 2fb - fa, 3fa - 2fb, and fa + fb - fc.
4. Coupling of transmitter signals which result
Higher order frequency intermodulation products are
in cross modulation and intermodulation may occur
usually not detected, even though they are present,
because they are of a very low power level.
another and conduction by means of the normal
(2) Broadband interference.
transmission line; through conduction from one
interference is characterized by noise emission which
transmitter to another due to improper grounding or
extends over a wide frequency range. One means by
which wideband interference is produced is the periodic
or improperly shielded, bonded, or grounded transmitter
opening and closing of a current-carrying circuit, which
to another transmitter which is also improperly shielded.
causes sudden voltage changes (signal transients
Frequently, it has been found that poor or rusting
containing steep wavefronts). These transients are
connections on guy wires for antenna supports or power
composed of many frequencies that are even multiples
lines can cause serious interference problems. As has
(harmonically related) of the occurrence rate of closing
been shown, intermodulation products may be produced
and opening the circuit. Undesirable responses appear
at many sum and difference frequencies when the
in a receiver throughout a broad tuning range when
fundamental frequencies of two transmitters are present
signals at these frequencies are radiated and detected.
in the same nonlinear element. Interference may also
Interference sources of this type are discussed in (a)
occur when three signals mix in a nonlinear element.
through (h ) below.
The sum frequency of two of the signals may produce a
(a) Powerline. Powerline interference is a result
difference frequency with a third signal. Of importance
are the intermodulation products which fall in the same
of any sudden change of voltage which may accompany
surges, corona, sparking, or the interruption of an
or adjacent channel of the band in which the desired link
electric power circuit. Typical sources of powerline
equipment is operating.
These are the products
resulting from such combinations as:
interference are: insulators