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For determining compatibility of three frequencies
(200 MHz, 195 MHz and 187 MHz in this example)
the significant combinations become:
In this example, the third system frequency (187 MHz)
has been chosen to avoid the first two frequencies and
their respective IM products. A third system that coincided
with an IM product may experience interference when its
transmitter is far from its receiver while the first two trans-
mitters are close to each other and to the third receiver.
Note that the addition of the third frequency creates
four new third-order, two-transmitter products as well as
three third-order, three transmitter products. In general, N
transmitters create N x (N-1) third-order, two-transmitter IM
products, as well as a number of third-order, three-
transmitter products. Thus, the number of available
frequencies for additional systems decreases exponentially
as the number of systems increases. For this reason,
computer programs are used to generate and evaluate
compatible sets of frequencies.
5th order two-transmitter IM products are not usually
strong enough to cause problems, but may be a factor in
cases of extreme transmitter or receiver proximity. 5th
order three-transmitter IM products such as (3 x f1) - f2 - f3
and (2 x f1) - (2 x f2) + f3 are generally too weak to be of
concern.
Maintaining adequate physical distance between
transmitters and between transmitters and receivers
will minimize the creation of IM products. The figure
below indicates the effect of distance on the amplitude
of 3rd order IM products created by two transmitters.
Selection
and Operation
of W
ireless Microphone Systems
56
R
E F E R E N C E
I
N F O R M A T I O N
Appendix A
three-transmitter intermodulation two-transmitter IMD strength vs. transmitter separation
two-transmitter intermodulation
Three-Transmitter Intermodulation Calculation
Product Order Frequency
f1 (original frequency) 1 200
f2 (original frequency) 1 195
f3 (original frequency) 1 187
f1 + f2 - f3 3 208
f1 - f2 + f3 3 192
f2 + f3 - f3 3 182
(2 x f1) - f2 3 205
(2 x f2) - f1 3 190
(2 x f1) - f3 3 213
(2 x f3) - f1 3 174
(2 x f2) - f3 3 203
(2 x f3) - f2 3 179
(3 x f1) - (2 x f2) 5 210
(3 x f2) - (2 x f1) 5 185
(3 x f1) - (2 x f3) 5 226
(3 x f3) - (2 x f1) 5 161
(3 x f2) - (2 x f3) 5 211
(3 x f3) - (2 x f2) 5 171