Frequency Coordination; Multi-Channel System Checkout - Lectrosonics R400A Instruction Manual

Frequency Coordination

IM (intermodulation) is a process of two or more
RF signals mixing in any stage in the transmitter or
receiver that generates another RF signal. If this new
signal happens to land on a carrier, IF or oscillator fre-
quency you may have interference problems that affect
range or audio quality. The possible combinations also
include odd and even order harmonics of the carriers.
Feel free to contact the factory if you need help in
coordinating frequencies. A specialized computer pro-
gram is used to perform thousands of calculations and
identify various interfering signals. Potential problems
and trouble areas can be identified in advance, and
proposed new frequencies or other solutions can be
suggested. This service is offered to authorized Lec-
trosonics dealers and other customers who are using
Lectrosonics wireless microphone and wireless IFB
®
systems.
Even with thorough analysis, interference can still be
present from local sources that cannot be predicted in
advance. This makes it mandatory to check out a multi-
channel system before the production or use begins.
Multi-channel System
Checkout
Intermodulation (IM) and crosstalk increases as the
distance between transmitters and receiver decreas-
es. In order to conduct a valid checkout of multi-chan-
nel compatibility using the procedure shown here, it is
best to adhere to the following guidelines:
• 4 to 5 feet between transmitters
• 20 to 25 feet between transmitters and receiver
antennas
• Receiver antennas not touching one another
These distances are valid for a general checkout of
the system. If the distances are less than this, IM will
be exaggerated and not likely to be realistic except
for special situations where shorter distances will
occur during production. If the distances are greater
than listed above, IM products that could occur dur-
ing actual use that may not show up in the checkout
procedure.
Interference can result from a wide variety of sources
including TV station signals, other wireless equipment
in use nearby, or from intermodulation within a multi-
channel wireless system itself.
The pre-coordinated frequencies in the tables on the
previous pages address in-system compatibility, but
obviously do not take into account RF signals from
external sources that may be present in the location
where the system will be operating.
The scanning process will identify external RF sig-
nals, but it does not address the compatibility of the
selected frequencies. Always go through the following
steps to make sure the frequencies that are chosen
Rio Rancho, NM
UHF Wireless Diversity Receiver
are compatible within themselves and also free from
external interference.
1. Set up the system for testing. Place antennas
in the position they will be used and connect to
the receivers. Place transmitters about 4 to 5 feet
apart and about 20 to 25 feet from the receiver
antennas. If possible, have all other equipment
on the set, stage or location turned on as well,
especially any mixing or recording equipment
that will be used with the wireless system.
2. Turn on all receivers. Leave transmitters off.
Look at the RF level display on each receiver. If
an indication is present, change the frequency
to a clear channel where no signal is indicated. If
a completely clear channel cannot be found, set
it for the one with the lowest RF level indication.
Once all receivers are on clear channels, go to
the next step.
3. Start with all transmitters turned off. Then
turn on one transmitter at a time. Look at the
matching receiver to verify a strong RF signal is
received. Then, look at the other receivers and
see if one of them is also picking up the signal.
Only the matching receiver should indicate a sig-
nal. Change frequencies on either system slightly
until it will pass this test, then check again to see
that all receivers are still on clear channels as in
Step 2. Repeat this procedure for each transmit-
ter, one at a time.
4. With all transmitters and receivers turned
on, turn each transmitter OFF one at a time.
Look at the RF level indicator on the receiver
that matches the transmitter that is turned off.
It should "fall silent" and the RF level should
disappear or drop to a very low level. If it does
not, change the frequency on that receiver and
transmitter and try it again.
IMPORTANT: Any time a frequency is changed
on any of the systems in use, you must start at
the beginning and go through this procedure
again for all systems. With a little practice, you will
be able to do this quickly and save yourself some
grief.
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