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[aprssig] Tuning a TNC

Chris Howard w0ep at frii.com
Sat Sep 15 13:46:04 UTC 2007

After Bob brought up the issue of tuning a TNC I thought I would try
something.  (I really should have googled first, but sometimes
it is good for a guy to leap before looking.)

I hooked receiver speaker output to line-in on my computer
and downloaded/installed a free audio spectrum program.

Looking at the audio frequency display didn't seem to be
very helpful in telling if an incoming signal was good or not.
At the first part of transmission I got a forest of 
peaks.  Then during the data part I got one large
hump of signal between 950 and 2200 hz.

On the time/frequency display (like an O-scope) I got
a nicer picture of a sine wave.  In that case I can
see sometimes the waves are a little bit flat on the top.

I'm wondering why the audio frequency display does not
show two humps centered on 1200 and 2200 instead of one
big hump?

Googling for "tnc deviation" and other variants I found
a number of good web pages on the subject.

http://www.febo.com/packet/layer-one/transmit.html from John Ackermann
http://www.zianet.com/elm-st-station/deviate.html from K5YE
http://www.vk1od.net/afskoverload/afskoverload.htm from VK1OD

VK1OD's page has a similar display of the "one big hump" picture
in the audio spectrum.

Chris Howard

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