4 Nov 13 print-friendly format | back to article index

Finding a good frequency - a refresher course

In a congested RF environment Lectrosonics receivers have a scanner to aid in finding usable frequencies (except the UCR100). Other useful tools for frequency finding and coordination are the RF Explorer (available ex stock), FreqFinder app on iTunes or Android Play or IAS software 15-day free evaluation.

Frequency Coordination (adapted from the Lectrosonics manual)
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. Sound Techniques can assist clients if you need help in coordinating frequencies.  A specialized computer program (such as IAS or FreqFinder) 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. 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 multichannel 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 exaggerated 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 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 multichannel 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 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 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 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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