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[aprssig] Deviation meters

'Scott Miller' scott at opentrac.org
Wed Apr 4 22:15:50 UTC 2007

> Audio drive to the transmitter play an equally, and maybe more, 
> important role.

And this is really what I'm hoping to come up with a tool to set - i.e., set
your tracker/TNC to calibrate mode (alternating tones) and set the audio
drive level to get appropriate deviation.  Assuming the transmitter's not so
out of whack that at 3.2 kHz deviation it's clipping the high tone already.

> More important is having the transmitted amplitude of the two 
> tones in 

You're preaching to the choir.  =]  Of course, as long as we've got D700's
and such out there, there's no getting around the fact that some stations
will transmit with no pre-emphasis.  I made a point of adding a pre-emphasis
circuit to my T2-135 board, since the DR-135T doesn't provide pre-emphasis
on the TNC input (not the one from the internal header, anyway).  I may add
a jumper on future versions to bypass it if needed, though.

> In the absense of a service monitor or oscilloscope, and if a simple, 
> peak reading device is used, adjust the audio drive until the desired 
> level is achieved, then back it off just a tad.

This is what I've encouraged users to do.  Still, it'd be nice to have a
small board that'd give you at least a basic indication that you've got it
set right, even if it wasn't very sensitive and required the radio to be
tuned to a known frequency.  I've got an Agilent service monitor, myself,
and it's wonderful for checking my relative tone levels and deviation, but
even used and several years old it was more than $3,000.  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'.


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