Technical Topics
Kevin Arber, W3DAD
IMD
IMD or intermodulation distortion is the unwanted mixing of two or more strong signals
that results in the production of unwanted signal components. In a transmitter these
signals are referred to as spurious signals and are regulated by FCC part 97.307. In a
receiver these spurious signals restrict dynamic range and produce unwanted signals in
the receiver passband. Obviously, in both cases, IMD is to be avoided or minimized.
IMD is created when a non-linear device (amplifier) is used somewhere in the signal
chain. This could be in either an audio, IF or RF stage. Sometimes non-linear devices
are required for operation of the radio, such as in a mixer. In this case the desired output
is one of the "spurious" IMD products, and filters are used to reject all other signals.
IMD is often referred to as a product of two signals; the third order product is often the
strongest and most offensive. A third order IMD product is produced by two signals
called f1 and f2 mixing to produce spurious signals fs:
fs = 2f1 +/- f2 and fs2 = 2f2 +/- f1
For example, a strong repeater on 147 MHz and another strong repeater on 148 MHz,
both of which are in the passband of a 2 meter receiver are operating simultaneously.
Say you are tuned to a station on 146 MHz. When the receiver hears the strong 147 and
148 MHz signals, its rf-amplifier or other amplifier stages are driven into the non-linear
portion of their operating curve. This produces spurious signals one of which will be fs =
2(147) -148, equals 146 MHz. The cure to this problem lies in the ability of the radio
manufacturer to construct receiver rf stages that have very high dynamic range that can
handle strong signals. Other signals, outside the amateur bands, such as paging and
public service signals, can also result in non-linear operation of the amateur receiver. For
signals outside the amateur band, the cure is to restrict the bandpass of the amateur
receiver to the amateur band by using an input filter.
Transmitters in the VHF range (30 - 225 MHz in this case) are restricted by FCC part
97.307 rules to spurious emissions at least 60 dB below the mean power of the
fundamental (40 dB for transmitters of 25 watts or less). Transmitters in the HF range
(below 30 MHz) are restricted to a spurious mean power of 50 mW and must be 40 dB
below the mean power of the fundamental. Amateurs get in trouble when they attach a
"linear" amplifier to their VHF or HF transmitter. The amplifier has a restricted linear
range of operation, the top end of which is usually specified. This means that an
amplifier that claims 10 dB of gain with 10 watts in for 100 watts output, is pushed to its
maximum at 10 watts input. The danger is in driving this amplifier with a greater than 10
watt transmitter and forgetting to turn down the drive power. The output may actually
increase under these circumstances, however, spurious signals are being produced and
amplifier gain is actually reduced. For instance, one could conceivably get 140 watts
output with 20 watts input, but this is a gain of only 8 dB.
Another source of IMD is overdriving a transmitter with the modulating signal. Most HF
transmitters have a built automatic level control (ALC) circuit that allows the modulation
(microphone or modem tones) to be set correctly. If the transmitter is overdriven by the
modulating signal, the bandwidth of the transmitted signal will expand due to he
production of unwanted sidebands. This is the cause of the very wide SSB signals that
are sometimes heard on the HF bands. In the case of an FM transmitter, the deviation is
increased and the signal width sometimes exceeds the bandwidth of the receiver. The
result is that the modulation sounds very weak or nonexistent at the receiver. This can be
a perplexing problem with external modems (TNC or soundcard) that take careful
adjustment. The FCC part 97.307 requires that no angle-modulation (FM) have a
modulation index greater than 1. In any case, no amateur transmission should occupy
more bandwidth than is necessary.
IMD can be prevented by proper operation of amateur equipment. When field day comes
along, resist the urge to crank up the microphone and processor gain. Your fellow
operators on the band will appreciate it!