User's Manual

UHF Digital Hybrid Wireless
®
Rio Rancho, NM
27
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 frequency
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 coor-
dinating frequencies. A specialized computer program is
used to perform thousands of calculations and identify
various interfering signals. Potential problems and trou-
ble areas can be identified in advance, and proposed
new frequencies or other solutions can be suggested.
This service is offered to authorized Lectrosonics deal-
ers 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
IM (intermodulation) and crosstalk increases as the
distance between transmitters and receiver decreases.
In order to conduct a valid checkout of multi-channel
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
If the distances are less than this, IM will be exagger-
ated and not likely to be realistic. If the distances are
greater than this, IM products that could occur during
actual use that may not show up in the checkout proce-
dure.
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 signals,
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 are com-
patible 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 match-
ing receiver should indicate a signal. Change fre-
quencies 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 transmitter, 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.