﻿ FM Deviation Measurements

FM Deviation Measurements

1. Bessel Zero Method

This method makes use of the fact that the carrier and sideband amplitudes of an FM transmission vary and with the modulation index and are zero at certain precise modulation indices. The modulation index is defined as the peak deviation divided by the modulation frequency and the values in the table are derived from a Bessel function.

In order to use this method the carrier of the FM transmission must be monitored using a separate, narrow band SSB/CW receiver, preferably with a bandwidth which is less than the proposed modulation frequency so that the carrier beat note is easily heard. Set the modulation frequency to 1KHz and zero deviation. Tune the receiver to the transmitter frequency so that the carrier beat note is clearly audible. Now slowly increase the deviation and you will hear the carrier decrease to zero and then reappear - when the carrier is inaudible the deviation will be 2.405KHz. Continue increasing the deviation and the next carrier null will occur at 5.52KHz deviation. The nulls are very sharp so they are easy to miss.

The following table gives the sequence number and value of the modulation index for zero carrier amplitude (null) commencing from zero deviation. In order to obtain the correct deviation we calculate the required audio frequency using the formula:

Audio Frequency = Frequency Deviation / Modulation Index

or by rearranging this formula

Frequency Deviation = Modulation Index * Audio Frequency

 Null Number Modulation Index Null Number Modulation Index 1 2.405 6 18.071 2 5.520 7 21.212 3 8.654 8 24.353 4 11.792 9 27.494 5 14.931 10 30.635

For 2.5KHz deviation look for the first null with a modulation frequency of 1039.5Hz and for 5KHz deviation look for the first null with a modulation frequency of 2079Hz.

The maximum deviation for each channel spacing as is as follows:

 Channel Spacing Maximum Peak Deviation 12.5KHz +/- 2.5KHz 20KHz +/- 4KHz 25KHz +/- 5KHz