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[aprssig] Deviation meters

Stephen H. Smith wa8lmf2 at aol.com
Wed Apr 4 22:58:08 UTC 2007

scott at opentrac.org wrote:
>  I'd like to be
> able to point people to an affordable option that's somewhere between the
> extremes of 'service monitor' and 'set it by ear'.
The peak voltage seen at the receiver discriminator  is proportional to 
the transmitted deviation (at least as long as you don't deviate beyond 
the RX passband).   On any radio with direct discriminator access, such 
as through the 6-pin mini-DIN data/packet jack, a deviation meter could 
basically be a calibrated AC millivoltmeter.   I envision something like 
a microcontroller with an A/D input driving a multi-segment bargraph LED 
via multiple single bit output lines.   If you want to get fancy, one 
could have a RED-YELLOW-GRN -GRN-RED sequence to corrrespond to 1 
through 5 KHz peak values.

Calibrating this device to the output of a particular radio's 
discriminator output would be fairly easy.   While receiving a constant 
unmodulated carrier, switch the radio up or down 5 KHz.  The resulting 
constant voltage change on the discriminator output is the same as the 
peak voltage change would be under 5KHz peak deviation with audio. 

One could have a cal program similar to the config program for an 
OpenTrack or TinyTrak that would prompt the user to first switch the 
radio up 5 KHz to read the delta in discr voltage. It would then prompt 
the user to press "-" or "+" until the top LED in the display just 
lights up, establishing a multiplier constant for the value read from 
the A/D converter. 

If you plug this device into a "DC-to-light" radio like an Icom 706, 
FT-100 or TS-2000 or a VHF/UHF multibander like a D700 (all have the 
direct discriminator data/packet connector) one could read deviation on 
any band. 

Of course, one could just connect a peak-reading AC voltmeter to the 
packet jack after determining what DC voltage is created by tuning 5 KHz 
off frequency. 

Stephen H. Smith    wa8lmf (at) aol.com
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