Frequency coordination, Multi-channel system checkout – Lectrosonics SRa Series User Manual

Page 22

SRa5P and SRa Dual Receivers

LECTROSONICS, INC.

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.

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.

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.

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.

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.