A Deviation Meter

Build a Service Monitor? Well sort of...
A circuit that will allow your scanner to measure deviation and allow you to 'see' the modulation.
By Tom Alldread VA7TA

I have not been able to find a service monitor at a hobby price level. We usually end up borrowing test equipment from industry sources when we need it. The need for having a simple deviation meter conveniently available to balance levels motivated me to make my own based on a scanner receiver.

I built a simple circuit on a proto-type board which is driven from the discriminator output of my Radio Shack Realistic PRO-2021 scanner. It provides me with an AC coupled deviation meter and a DC coupled oscilloscope output. The scanner covers the 6M, 2M and 70 cm bands and has a specified IF acceptance bandwidth of +/- 9 kHz. I decided it could be suitable to form the basis for an inexpensive frequency agile deviation meter with an oscilloscope output.

The deviation meter circuit is AC coupled with a sufficiently low frequency response to pass the CTCSS (PL) tones. The output of the AC amplifier drives a peak detector with a fast attack slow decay characteristic. A buffer stage is provided for driving the meter movement. I had the good fortune of having access to an industrial grade digital Marconi signal generator to debug the circuit and check it for accuracy. I used a first Bessel function zero to confirm the signal generator was in a good state of calibration. After making several modifications, I found my circuit was surprising linear with accuracy's within about 100 Hz for variations of deviation from 500 to 5000 Hz and modulation tone variations from 100 Hz to 3 kHz. All the testing was done at room temperature thus I have not established confidence in calibration with wide variations in ambient temperature.

The DC coupled oscilloscope output provides a 150 mV/kHz deviation signal for viewing the audio wave form and checking for frequency offset.

73 & Enjoy!
Tom, VA7TA

There are 2 different files that contain printable schematics:
One is deviation-meter.doc
It will open and print properly from Microsoft Word.
The other is a PDF file: deviation-meter.pdf.