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[aprssig] Clock/timing accuracy when generating an APRS signal

Richard Corfield richard.corfield at gmail.com
Wed Aug 31 12:28:09 UTC 2011


Looking at the AX25 standard it seems designed to withstand some amount of
error in the timing accuracy of the devices sending or receiving the
message. There can be no more than 5 bits before a transition between the
two tones, so if a device can adjust based on the transitions or if it can
adjust using the special flags it should be able to synchronise.

I've found in experiments the opposite. I need an accurate clock source in
my generator for it to be received - at least by the AGWTracker software.

What typical errors can I get away with for most receivers if any? I assume
I am right in my assessment and it is not a slight change in ratios between
different timings that effected it. Changing to an accurate clock meant
changing clock speed, so recalculating delays, so different rounding errors.

Thanks
  - Richard
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