[aprssig] Deviation meters

Ray Wells vk2tv at exemail.com.au
Wed Apr 4 16:58:41 CDT 2007

'Scott Miller' wrote:

>Since proper deviation is one of the most critical (and most neglected)
>settings for an APRS transmitter, I thought I'd look into the feasibility of
>coming up with a simple meter kit.
>So far I haven't found any existing, simple kits, but I seem to remember one
>that was based on a single LED.  Anyone remember that one, or know where I
>can find it?  If there's already something suitable out there, I don't want
>to have to reinvent it.
>aprssig mailing list
>aprssig at lists.tapr.org
Deviation certainly raises its head periodically, but deviation, alone, 
is not the only consideration.

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

A transmitter can be set to deliver no more than, say, 3.5khz deviation, 
which is probably close to ideal for data in a nominally +/-5khz system.

However, just having the correct deviation will not guarantee success.

More important is having the transmitted amplitude of the two tones in 
the correct ratio, which is approximately 2 to 1 in favour of the high 
tone. This assumes that pre-emphasis/de-emphasis is being employed on 
the network at the rate of 6dB/octave, something that is more the norm 
than the exception.

The tones arriving at the receiving end TNC/modem must be approximately 
equal in amplitude. This is the most important point to consider.

Getting back to our transmitter that is delivering 3.5khz deviation. If 
the audio drive to that transmitter is sufficiently high, its audio 
limiter/compressor can result in a transmitted signal where the 
amplitude of both TNC tones is equal. At the receiving end, and after 
de-emphasis, the TNC is presented with a signal in which the high tone 
is approximately half the level of the low tone. Some decoders simply 
baulk at this and won't decode. The XR2211 comes to mind.

Most systems are more tolerant of incorrect (within reason) deviation 
that they are of grossly uneven tone levels, and the latter is more 
easily achieved!

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.

Ray vk2tv

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