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 -
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 |
+/- |
20KHz |
+/- |
25KHz |
+/- |