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SYSTEM COMPATIBILITY
The two main areas of concern are: interaction
between transmitters and receivers related to their operating
frequencies, and interactions between transmitters and
receivers related to their internal frequencies. The first
class of interactions is the more important one and may
occur in any group of wireless microphone systems. It is
also the one more cumbersome to calculate. The second
class of interactions is less problematic and is also
relatively easy to predict. However, it is determined by
specific system characteristics.
OPERATING FREQUENCY
INTERACTIONS: INTERMODULATION
A single wireless microphone system can theoretically
be used on any open operating frequency. When a
second system is added it must be on a different
operating frequency in order to be used at the same time
as the first. This limitation arises from the nature of radio
receivers: they cannot properly demodulate more than
one signal on the same frequency. In other words, it is not
possible for a receiver to "mix" the signals from multiple
transmitters. If one signal is substantially stronger than the
others it will "capture" the receiver and block out the other
signals. If the signals are of comparable strength none of
them will be received clearly.
The effect of this is often heard in automobile radios
when travelling out of range of one station and into the
range of another station at the same frequency. The
receiver will switch back and forth between the two
stations as their relative signal strength changes, often with
considerable noise and distortion. The result is that neither
station is listenable when the signals are nearly equal.
If the wireless microphone systems must be on
different frequencies, how "different" should they be? The
limiting characteristic of the receiver in this regard is its
"selectivity" or its ability to differentiate between adjacent
frequencies. The greater the selectivity the closer
together the operating frequencies can be. Most
manufacturers recommend a minimum frequency
difference of 400 kHz (0.4 MHz) between any two systems.
When a third system is added to the group it must of
course be at least 400 kHz away from each of the existing
systems. However, it is now necessary to consider other
potential interactions between the transmitters to insure
that all three systems will be compatible with each other.
The most important type of interaction is called
intermodulation (IM), and it arises when signals are
applied to "non-linear" circuits. (See Figure 3-4.)
A characteristic of a non-linear circuit is that its output
contains "new" signals in addition to the original signals
that were applied to the circuit. These additional signals
are called IM products and are produced within the circuit
components themselves. The frequencies of IM products
are mathematically related to the original transmitter
frequencies. Specifically, they consist of sums and
differences of the original frequencies, multiples of the
original frequencies, and sums and differences of the
multiples. Non-linear circuits are intrinsic to the design of
wireless components and include the output stages of
transmitters and the input stages of receivers. The "mixer"
stage at the receiver input is an example of a non-linear
circuit: recall that it is designed to produce a "difference"
frequency that becomes the intermediate frequency (IF)
for subsequent stages.
IM can occur when transmitters are in close proximity
to each other. The signal from each transmitter generates
IM products in the output stage of the other. These new
signals are transmitted along with the original signals and
can be picked up by receivers operating at the
corresponding IM frequencies. (See Figure 3-7.)
IM can also occur when transmitters are operated very
close to receivers. In this case IM products are generated
in the receiver input stage which can interfere with the
desired signal or be detected by the receiver if the desired
signal (transmitter) is not present.
The strongest IM products are the two so-called 3rd
order products produced by two adjacent transmitters
operating at frequency f1 and frequency f2, where f1 is lower
than f2. The resulting IM products may be calculated as:
IM1 = (2 x f1) – f2
IM2 = (2 x f2) –f1
If the interval between f1 and f2 is F, then IM1 = f1 – F and
IM2 = f2 + F. That is, one IM will appear exactly at interval F
above the upper frequency f2 while the other IM will appear
exactly at interval F below the lower frequency. For example, if
f1 = 180MHz and f2 = 190MHz, then F = 10MHz. Thus,
IM1 = 170MHz and IM2 = 200MHz. (See Figure 3-5.)
In addition to IM products generated by interaction
between two transmitters, other IM products are generated by
interaction between three transmitters in a similar fashion.
(See Figure 3-6.) In order to avoid potential IM problems most
manufacturers recommend a minimum margin of 250 kHz
(0.25 MHz) between any 3rd order IM product and any
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C
H A P T E R
3
Wireless System Operation
Selection
and Operation
of W
ireless Microphone Systems
Figure 3-4: linear vs. non-linear circuits