[OZAPRS] HF APRS Frequency accuracy/stability

[Richter, Mike W]

"TNC's have their own filtering."

 - that would explain the need for absolute stability !

I assumed that Receiver selectivity in the IF was the last filtering

Mike 


-----Original Message-----
From: Darryl Smith [mailto:Darryl at radio-active.net.au]
Sent: Saturday, 6 December 2003 09:36 pm
To: ozaprs at marconi.ics.mq.edu.au
Subject: RE: [OZAPRS] HF APRS Frequency accuracy/stability


Mike

The problem is that you are dealing with the radio pass bands. What
happens
in TNC's is that they have their own filtering.

In order to improve noise performance, the pass band in front of the
demodulator is quite narrow... The narrower, the better. 

With decoding, there are a number of ways to do this. Filtering with
energy
detection is common. So is mixing with another frequency. A number of the
analog techniques require the tones to be as close as possible, or the
tones
to be at least symmetrical. 

If the tones are not symmetrical from a center frequency with the energy
detection, then the detector will be biased towards one end.

Darryl
_______________________________________________
ozaprs mailing list
ozaprs at marconi.ics.mq.edu.au
http://marconi.ics.mq.edu.au/cgi-bin/mailman/listinfo/ozaprs