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[aprssig] FW: Tuning a TNC

Robert Bruninga bruninga at usna.edu
Tue Aug 24 16:45:36 UTC 2010


With today's sound cards in every PC, surely there is a free
O-Scope display.  If so, then there is no excuse for any ham to
not use this tool for cleaning up some of the trash we see on
144.39.

> I hooked receiver speaker output to line-in on my PC
> 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.
>
> I'm wondering why the audio frequency display does not
> show two humps centered on 1200 and 2200 instead of one
> big hump?

If you had continuous 1's or 0's in a row, you would see the two
clear tones, but with it going back and forth between the two
tones at a rate of 1200 times per second, it slurs the energy
between them.

> 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.

AMEN!  It is best to look at the O-scope display.  Things to
look for are:
1) Amplitude compared to say a D7 or D700 (I simply use them as
a benchmark right-or-wrong)
2) Amplitude between the 1200 and 2200 Hz tones
   (should be same (for D7/D700)***)
3) Each sinewave should be clean (not flattopped or skewed)

*** Note, but then if you are looking on a speaker output, then
the tones have been de-emphasized and so its hard to say what
you are seeing as to what is equal or not equal.  But by
watching SOME signals compared to some OTHER signals, it is easy
to see the HUGE differences between them, and why some signals
perform FAR BETTER than some other signals (under weak signal
conditions).

With today's sound cards in every PC, surely there is a free
O-Scope display.  If so, then there is no excuse for ANY ham to
not use this tool for cleaning up some of the trash we see on
144.39.

Bob, WB4APR





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