An Inexpensive And Accurate FM Deviation Meter For Hams
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Written by: Charles A. Bliley, Rochester, New York, 2/4/83
BACKGROUND
This article was written as the result of a complaint. I was working in a radio-electronic company and was an active ham radio operator at the time operating on the two-meter (146 MHz) FM band. The equipment was relative simple and most transceivers had internal transmit deviation adjustment. That was great, but to measure the deviation level was the domain of professionals who owned very expensive deviation meters costing thousands of dollars. The measurement of your transmitter's deviation was impractical for most of us.
One day, I complained to a fellow radio/electronics instructor about my need to measure narrow-band FM deviation. That man was Tom Wise, an electronics engineer turned technical trainer. With abundant help from Tom, we came up with a scheme that would allow an accurate measurement with minimal equipment. Tom explained the principal, I supplied the equipment and off we went on a journey to test the theoretical and make it practical. Below is the article I wrote in 1983 to pass out to a few friends. I sat in my files until March of 2011 when I decided it merited wider circulation via the Internet.
Thank you Tom for your joyful assistance in solving my problem and expanding my knowledge of radio-electronics.
I hope you find it interesting and useful.
Chuck Bliley Rochester, New York
INTRODUCTION
The two biggest problems with FM and repeater operation are: 1. staying, or getting on, frequency, and 2. too little or too much deviation. With the development of inexpensive frequency counters with fairly good accuracy, only the second problem is left to be tackled.
Because most amateurs can get away without a FM deviation meter, few ever spend the $150 or more to buy one. Nevertheless, safely modulating your FM transmitter to its fullest can give you a bit of a boost when the going gets tough, and signals are weak.
Knowing most amateurs are interested in bargains and care little for high powered mathematics, I will attempt to explain how you can build a setup to accurately measure transmitter deviation with less than a 5% error and spend under $7.00 for the critical parts. In this case, the only critical part is a crystal whose frequency matches your receiver's second IF center frequency. First the boring mathematical part. There exist some mathematical formulas called Bessel Functions " which confuse and frustrate most people, but what they do is show a predictable relationship between the FM signal's modulating frequency and the amount of change in carrier frequency (deviation). These formulas predict for example, with a 1 KHz modulating frequency, at precisely +2.404 KHz deviation, the carrier frequency signal will disappear and only the upper and lower sideband signals will remain.
Now if we can come up with a scheme to find out and measure when the carrier disappears, we can create a deviation meter.
What follows is an inexpensive and fairly easy way to come up with an accurate deviation meter. Be prepared to take on the added responsibility that will come along with being the keeper of the measurement keys. You will soon be tempted to tell others they are under or over deviating with certainty. The power is awesome! Treat it with respect. .
Figure 1: FM Signal with 1kHz Modulation
Figure 1 shows a typical FM signal as observed on a spectrum analyzer test set. Of course an unmodulated carrier would appear only as a single pulse on the screen of the spectrum analyzer's oscilloscope. As we increase the deviation of our 1 KHz modulating tone, sidebands will be created with some carrier level present most of the time. At certain combinations of the modulating frequency and deviation, the carrier will disappear completely from view as shown on the right. With 1 KHz modulation this occurs at 2.404 KHz, 5.520 KHz and at an "infinite" number of other points until the deviation swing equals the carrier frequency.
Table 1: Deviation for FM Carrier Null