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HOW TO DEAL WITH INTERMODULATION AND KEEP YOUR FREQUENCIES STRAIGHT
SETTING UP MULTICHANNEL SYSTEMS
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Ideal and real gain curves
of ideal and real amplifiers
High audio input levels may overload the amplifier, so the
peaks of the amplified signal are clipped as a result of
saturation. The compression characteristic may be des-
cribed by a polynomial (i.e., the sum of multiples of powers
of a variable X). This polynomial includes all powers, with
the odd powers (3, 5, 7, ...) responsible for intermodulation
in multichannel systems. Because of its high coefficient,
the third power term is especially important which is why
third-order intermodulation products are dominant. The
reciprocal value of the third-order coefficient defines the
IP 3 Intercept (see below), which is the most important
parameter for the intermodulation resistance of an RF
amplifier. A smaller third-order coefficient of the trans-
mission polynomial means a higher IP 3, which implies
greater linearity of the RF amplifier and thus better resi-
stance to intermodulation distortion.
IP 3 Intercept
The Intercept marks the intersection of the theoretical
linear transfer curve for the wanted signal’s amplifier res-
ponse curve and the theoretical linear transfer curve for
the third-order intermodulation product. It is never actual-
ly reached because the amplifier will compress the wanted
signal before it reaches the IP 3 Intercept level.
The higher the Intercept of a radio transmission system,
the lower the IM risk, and the more channels may be used
within a given frequency band.
Whenever two or more signals are transmit-
ted by a non-ideal system, undesired inter-
modulation products will be created, cau-
sing distortions (see also WMS 400, p. 31).
An ideal system would deliver an output
signal that is identical to the input signal
over the whole frequency range even at lar-
ger amplitudes, and no problems would
arise.
In practice, however, ideal systems do not
exist, as transistors in particular have only
a relatively narrow linear gain range. This is
why the transmission of several signals via
nonlinear systems, such as transmitters
and receivers, will result in unwanted arti-
facts generated by intermodulation. These
intermodulation products have to be dealt
with somehow in practice.
The order of intermodulation products
depends on the nonlinearity of the system
response curve; the amplitudes of intermo-
dulation products will always grow in pro-
portion to the product of the mathematical
powers of the fundamental signals genera-
ting a given intermodulation product. In
reality, third-order intermodulation pro-
ducts tend to be particularly troublesome
because they rise much more rapidly than
the fundamental signal, thus turning into
real, i.e., audible noise.
Whenever the frequency of the desired sig-
nal coincides with that of an intermodula-
tion product the signal will be distorted.
Moreover, the intermodulation product may
activate the receiver's squelch function if
the amplitude of the intermodulation fre-
quencies exceeds the squelch threshold.
Obviously, the effective impact of intermo-
dulation distortion also depends on the
distance between transmitter and receiving
antenna. In the case of wireless micropho-
nes transmitting on an intermodulation fre-
quency, the desired signal is often ruined by
intermodulation distortion if you move the
transmitter too far away from the receiver.
AKG WMS MULTICHANNEL TECHNOLOGY