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[aprssig] Proper deviation levels

Ray Wells vk2tv at exemail.com.au
Tue Nov 28 04:35:40 UTC 2006

Stephen H. Smith wrote:

> scott at opentrac.org wrote:
>> I know this just came up recently, but I can't find it.  What's the 
>> accepted
>> standard deviation for APRS?  Or more specifically, what should the
>> deviation be for the high and low tones, so I can check pre-emphasis as
>> well?
>> And thanks to everyone who made suggestions for comm test sets.  I 
>> got my
>> HP/Agilent 8920A today and I'm having loads of fun with it.  And when 
>> I'm
>> not using it for testing, it makes a pretty decent (if rather 
>> expensive) FM
>> radio for listening to music in the lab.  =]
>> Scott
>> N1VG
>> _______________________________________________
>> aprssig mailing list
>> aprssig at lists.tapr.org
>> https://lists.tapr.org/cgi-bin/mailman/listinfo/aprssig
> The peak deviation which will result from the higher of the two tones 
> (2200Hz) should be about 3.5-3.75 KHz  .  This should result in the 
> low tone (1200) deviating about 2.5-2.9 KHz.   [Deviation should be 
> proportional to the modulating freq so the high tone should be 
> 2200/1200 of the deviation of the low tone.]      Theoretically the 
> high tone could be deviated a full 5 KHz but there will then be no 
> margin for transmitter or receiver frequency  error if the receiver 
> has a narrow passband.   One is much better off slightly 
> under-deviated in order to tolerate frequency error at either end.
> -- 
> Stephen H. Smith    wa8lmf (at) aol.com
> EchoLink Node:      14400    [Think bottom of the 2M band]
> Home Page:          http://wa8lmf.com  --OR--   http://wa8lmf.net
> NEW!   TNC Test CD
>  http://wa8lmf.net/TNCtest
> JavAPRS Filter Port 14580 Guide
>  http://wa8lmf.net/aprs/JAVaprsFilters.htm
> "APRS 101"  Explanation of APRS Path Selection & Digipeating
>  http://wa8lmf.net/DigiPaths
> Updated "Rev G" APRS            http://wa8lmf.net/aprs
> Symbols Set for UI-View,
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I concur with Stephens remarks but add another two comments.

The ratio Stephen mentions only holds true if the radio conforms to 
6dB/octave. Consider both tx and rx in this regard (my old FDK Multi-750 
had almost no high tone after deemphsis).

The second consideration applies if audio is fed into the microphone of 
the tx. If you have sufficient drive level to push the tx audio into 
limiting/compression, you may very well end up with equal tone levels on 
your service monitor. Such a signal will produce high tone at 
approximately half the level of the low tone after deemphasis in a rx, 
rather than both tones being approximately equal, as presented to the 
TNC. XR2211/2206 based TNC's don't take kindly to the high tone being 
lower in level than the low tone.

Ray vk2tv

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