ADC Input Schematic

[Rob WX9O]

Great work on the digipeater.  I noticed that you changed the input circuit for the ADC from the BeRTOS schematics.  Why was that change made? I am working on a TNC board as well -- it's a simple KISS based TNC using BeRTOS code and code from KI4MCW.

Thanks,

Rob

wx9o

[Alejandro Santos]

Thanks! That change was made after reading a comment on StackExchange
about a DC bias circuit, here:

http://electronics.stackexchange.com/questions/14404/dc-biasing-audio-signal

Both designs available in my website should, in theory, have the same
effect: act as a low pass filter to remove all the signals from 75 Hz
upwards. Since I have not any electronics background, the suggested
modification was designed by another amateur radio.

73 LU4EXT

2013/4/20 Rob WX9O <rob....@gmail.com>:

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Alejandro Santos

[Rob Riggs]

Thanks Alejandro.  The circuit I am working with seems to have very clean power, all VCC connections have decoupling caps, and the filtering done inside the BeRTOS AFSK code seems to be good enough for any noise that might appear there.  The one area I am still struggling with is finding a compromise that will allow both HT audio and mobile/base TNC connections to work.  For the HT audio connections, the 100nF cap with 1KOhm resistors seems to work quite well.  But the audio levels on the TNC connection on my base station are completely suppressed by this configuration.  For that, a 10uF cap (or larger) and either 4.7K or 10K resistors work best.  But the TNC cannot decode HT audio with this confifuration.

I, too, do not have a formal background in electronics.  But the cap + resistor divider looks an awful lot like a high-pass filter to me.  An 100nF & 1KOhm would be close to the AFSK frequencies.  The circuit I am designing is very much intended for HT use, so it is not too much of a problem to optimize it for the HT.  But I would prefer to find a compromise that allowed both to work.

What radios are you able to test your digipeater with?

Cheers,

Rob

wx9o