Frequency coordination, Multi-channel system checkout – Lectrosonics SRb_SRb5P User Manual

Page 21

UHF Digital Hybrid Wireless

Rio Rancho, NM

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:

• 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.

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.

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 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.

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.