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operating frequency. This further restricts available frequency
choices as the number of simultaneous systems increases.
It should be apparent from this discussion that the
prediction of potential compatibility problems due to
IM products is best left to computer programs.
The complexity increases exponentially for additional
systems: a group of 10 wireless microphone systems
involves thousands of calculations. See Appendix 1 for
details on calculation of IMD products.
INTERNAL FREQUENCY
INTERACTIONS:
LO, IF, CRYSTAL MULTIPLIERS
In addition to frequency conflicts due to intermodulation
between operating frequencies there are certain other sources
of potential conflicts due to the various "internal" frequencies
present in the normal operation of transmitters and receivers.
These differ from manufacturer to manufacturer and even from
system to system from the same manufacturer.
One such source is the local oscillator (LO) of the receiver
itself. Although this is a low-level signal which is generally
confined within the receiver, it is possible for the local
oscillator of one receiver to be picked up by another receiver
tuned to that LO frequency if they or their antennas are in close
proximity to each other (stacked, for instance). For example,
assuming a typical intermediate frequency (IF) of 10.7 MHz, a
receiver tuned to 200.7 MHz would have its LO operating at
190.0 MHz. Another receiver tuned to 190 MHz should not be
used close to the first receiver because the second unit could
pick up the LO of the first, especially if the 190 MHz transmitter
is turned off or is operating at a great distance. (See Figure
3-8.) Good design and shielding in receivers and physical
separation of receivers will minimize the possibility of LO
interference. For multiple units, active antenna splitters will
effectively isolate antenna inputs from each other. However, it
is still recommended that operating frequencies be chosen to
avoid LO frequencies by at least 250 KHz.
An "image" frequency is another source of possible
interference. In a receiver, recall that the frequency of the
local oscillator (LO) always differs from the frequency of the
Selection
and Operation
of W
ireless Microphone Systems
26
C
H A P T E R
3
Wireless System Operation
Figure 3-6: three-transmitter intermodulation
Figure 3-7: two-transmitter IMD strength vs.
transmitter separation
Figure 3-8: local oscillator (LO) interference
Figure 3-5: two-transmitter intermodulation