Page 107 - Complete Wireless Design

Amplifier Design

106 Chapter Three

Intermodulation distortion (Fig. 3.11), quite similar to amplitude distortion
above, is produced when frequencies that are not harmonically related to the
fundamental are created through nonlinearities in a linear Class A or a non-
linear Class C amplifier, or in a nonlinear mixer’s output. IMD is formed by
this mixing together of the carrier, any harmonics, the sidebands, IMD from
other stages, etc., to produce various spurious responses—both in and out of
band. Since these IMD products can fall in band, or cause other signals to fall
in band, they can possibly swamp out the desired baseband signal, creating
interference, which also causes additional noise which will degrade system
performance and BER. In addition, IMD can be manufactured in the power
output amplifier of a transmitter when another neighboring transmitter’s sig-
nal (and/or its harmonics) arrives at its output stage and mixes. This can be
particularly problematic in dense urban environments, as there are many sig-
nals present that will modulate each other within the nonlinearities of a nor-
mal power amplifier, producing a multitude of sum-and-difference frequencies.
In these transmitter-to-transmitter cases, the IMD can be attenuated by
employing a wavetrap that is tuned to the interfering transmitter’s frequency,
and/or by shielding and proper grounding to prevent mixing within the other
internal stages of the transmitter. However, within a receiver this effect can
be much worse: The desired signal and a close transmitter’s undesired signal,
and/or its harmonics, can be allowed into the receiver’s front end, creating
reception of unwanted signals and the obliteration of the desired frequency by
the IMD products generated by the nonlinear mixing of the two signals. This
can be somewhat mitigated by using, at the receiver, an input notch filter,
tighter bandpass filtering, amplifiers that are biased for maximum linearity,
and confirming that the RF amplifiers are not functioning in a nonlinear
region as a result of being overdriven by an input signal.
A more in-depth explanation of “intermod” is warranted because of its vital
importance in the design of any linear amplifier. Since intermodulation dis-
tortion is produced when two or more frequencies mix in any nonlinear device,
this causes not only numerous sum and difference combinations of the origi-
nal fundamental frequencies (second-order products: f f and f f ), but
1 2 1 2
also intermodulation products of mf nf and mf nf , in which m and n are
1 2 1 2
whole numbers. In fact, third-order intermodulation distortion products,
which would be 2f f , 2f f , 2f f , and 2f f , can be the most damag-
1 2 1 2 2 1 2 1
ing intermodulation products of any of the higher or lower IMD. This is
because the second-order IMD products would usually be too far from the
receiver’s or transmitter’s pass band to create many problems, and would be
strongly attenuated by an amplifier’s tuned circuits, the system’s filters, and
the selectivity of the antenna. As an example: Two desired input signals to a
receiver, one at 10.7 MHz and the other at 10.9 MHz, would produce sum and
difference second-order frequencies at both 21.6 MHz and 0.2 MHz. These fre-
quencies would be far from the actual passband of the receiver, and will be
rejected by the receiver’s selectivity. But the third-order IMD formed from
these same two signals would be at 10.5 MHz, 11.1 MHz, 32.3 MHz, and 32.5

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