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[aprssig] Deviation meters

John johnk.mchugh at att.net
Thu Apr 5 14:52:20 UTC 2007


Alan P. Biddle wrote:

>Scott,
>
>Back in the Dark Ages, about 15 years ago or so, TAPR produced a very nice
>deviation meter kit.  
>

Alan et al,

Back in the Darker Ages we did it mathmatically with Bezzel Functions.
For those that don't know about this it is very simple to set the 
deviation using an audio tone applied to the mike jack and a second 
selective radio/receiver.
Mr. Bezzel proved, mathmatically, that there is a relationship between 
modulation index and carrier on FM and PM radios. The ARRL Handbooks of 
the late 90's show the graphs of Bezzel functions (on page 12.7 in my 
1997 version)
.
The technique is to set the radio you want to set the mod on to a 
frequency. Then with a second radio that has narrow selectivity (IC 706, 
FT 100 or similar set to CW) monitor the carrier from the first radio. 
You can also use a VHF/UHF down converter and an HF radio to monitor it
Set the audio generator to 2khz, then turn up the deviation control 
slowly from zero until you hear the carrier null out. You then have the 
deviation set to 4.8KHz. I've used this method many times to set radios.

The Bezzel function for the first carrier null is 2.405. The formula is 
(Required Dev/ 2.405). So if you want to set the deviation to 3.5KHz, 
use an audio tone of  3.5/2.405 or  1.455KHz. Oh, you don't have a 
1.455KHz tone, then use the mark tone from a KAM, usually 1600Hz on HF. 
There must be software that will produce variable audio tones from a 
sound card, anyone know of such a program?

John
-- 

John Mc Hugh, K4AG
Coordinator for Amateur Radio  
National Hurricane Center, WX4NHC
Home page:- http://www.wx4nhc.org





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