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Adjusting FM Deviation

The proper adjustment of the deviation control is important for good FM communications, If it is too low. the audio at the receiver is also low. If it is too high the overdeviation will bring you reports that though your signal is strong, it breaks up when you talk.

Test instruments for measuring transmitter deviation cost upwards of $250, There is, fortunately, a shortcut method for deviation measurement using just an FM receiver and an ac voltmeter (or oscilloscope).

To adjust deviation using this method you must use a receiver with the appropriate bandwidth. The economy priced monitor receivers are of no use here. If you want to use a narrow band — 5 khz system, you must use a receiver with this bandwidth. If your system contains both wide and narrow band units, adjust all transmitters for narrow band operation. This will cause slightly reduced audio in the wide band receivers, but will provide much better overall performance,

Most commercial units such as Motorola, GE, etc., have power supplies which will allow the transmitter and receiver to be used simultaneously for short periods of time. Refer to the schematic to see how this can be accomplished. In this way you can use your receiver to check the deviation of your associated transmitter.

Connect the ac voltmeter or scope across the speaker terminals. Substitute a five watt resistor lor the speaker if you can t stand the noise. Apply a 1 kHz tone to the transmitter or whistle steadily into the mike if an audio oscillator is not available. Slowly advance the deviation control from its lowest position (with the transmitter turned on) and watch the ac meter.

As you increase the deviation you 11 see a fairly linear increase in the receiver audio level, followed by a flattening out, and then, as you go outside the passband of Ihe receiver, the audio level will fall off and the noise level will increase. This is just what happens when an over-deviated signal is received by another FM mobile.

Repeat the adjustment several times, paying particular attention to the point at which the linear rise just starts to flatten. This is the point at which the deviation control is properly set,

IVe used this system to set deviation on many occasions and have been amazed at its accuracy when compared against properly calibrated instruments.

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