Lectrosonics SRa Series - Manual (Page 22)

SRa5P and SRa Dual Receivers
LECTROSONICS, INC. 22
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
Intermodulation (IM) 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:
• 2 to 3 feet between transmitters
• 15 to 20 feet between transmitters and receiver
antennas
• Receiver antennas not touching one another
If the distances are less than this, IM will be exaggerat-
ed and likely be unrealistic. If the distances are greater
than this, IM products could occur during actual use
that do not show up in the checkout procedure.
If you know in advance that the transmitters will be
closer than 2 to 3 feet during the actual production,
place them at that distance instead.
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.
Using the scanning process with the RF spectrum
analyzer will identify external RF signals, but it does not
address the compatibility of the selected frequencies.
It is always good practice to go through the following
steps to make sure the frequencies that are chosen are
compatible within themselves and also free from exter-
nal 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 3 to 5 feet apart,
about 25 to 30 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. 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 signal. 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 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 “off” transmitter. 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.