FM deviation - UK Vintage Radio Repair and Restoration Discussion Forum

Chris 1155 · 26th Apr 2018, 9:38 am

Hello,
I want to check and possibly adjust the deviation on my FT221r.
Does anyone have any advice on how to do this without a dedicated deviation meter.
Thanks and 73 Chris G4BYZ

G6Tanuki · 26th Apr 2018, 10:19 am

There is a trick you can do simply using a calibrated audio-frequency sinewave oscillator and another (SSB/CW) receiver.

This makes use of the mathematical "Bessel equation" analysis of FM, one property being that there are certain audio-modulating-frequency/deviation combinations at which the carrier will seemingly disappear!

http://www.qsl.net/g3oou/fmdeviation.html has more details.

There's some stuff about the technique in the old ARRL/RSGB radio handbooks too. I'll spare you the underlying math, since not everyone's happy with sines, cosines, and sums-to-infinity in calculus.

Jon_G4MDC · 27th Apr 2018, 10:36 am

Easily done with one of the ~£10 DVB-TV dongles and associated SDR software.
Use it as the poor man's spectrum analyser.

Radio Wrangler · 27th Apr 2018, 10:51 am

With the dongle and FFT programme you can also see the other Bessel null relationships where different sidebands drop to zero. If the calibration of your audio generator is good, this is a very accurate method..... and how do you suppose deviation meters are calibrated?

David

26th Apr 2018, 9:38 am	#1
Chris 1155 Tetrode Join Date: Dec 2017 Location: Reigate, Surrey, UK. Posts: 65	FM deviation Hello, I want to check and possibly adjust the deviation on my FT221r. Does anyone have any advice on how to do this without a dedicated deviation meter. Thanks and 73 Chris G4BYZ

26th Apr 2018, 10:19 am	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: FM deviation There is a trick you can do simply using a calibrated audio-frequency sinewave oscillator and another (SSB/CW) receiver. This makes use of the mathematical "Bessel equation" analysis of FM, one property being that there are certain audio-modulating-frequency/deviation combinations at which the carrier will seemingly disappear! http://www.qsl.net/g3oou/fmdeviation.html has more details. There's some stuff about the technique in the old ARRL/RSGB radio handbooks too. I'll spare you the underlying math, since not everyone's happy with sines, cosines, and sums-to-infinity in calculus.

27th Apr 2018, 10:36 am	#3
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: FM deviation Easily done with one of the ~£10 DVB-TV dongles and associated SDR software. Use it as the poor man's spectrum analyser.

27th Apr 2018, 10:51 am	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: FM deviation With the dongle and FFT programme you can also see the other Bessel null relationships where different sidebands drop to zero. If the calibration of your audio generator is good, this is a very accurate method..... and how do you suppose deviation meters are calibrated? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done