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.

Copyright April 2001,  Thomas Marshall Alldread VA7TA
HTML Copyright,  April 22 2001 by  Kevin K. Custer W3KKC, All Rights Reserved.