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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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26th Apr 2018, 9:38 am | #1 |
Tetrode
Join Date: Dec 2017
Location: Reigate, Surrey, UK.
Posts: 65
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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 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,005
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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 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,015
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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 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,894
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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
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