This software AX.25 decoder is loosely based on the serial port powered decoder that was quite popular a few years ago.  It consists of a PIC 16F62x micro controller, MAX232 serial interface and a 741 op-amp.
The design is very easy to build and it's performance rivals decoders using the sometimes hard to find MX614 modem.
The secret to it's operation is using the MAX232 charge pump +/- 9 volt outputs to power the op-amp, allowing dual supply configuration as a zero crossing detector.

Audio containing packet data is AC coupled to the non-inverting input of the op-amp, configured as a comparator.
As the name implies, when the waveform crosses the zero volt level either rising or falling, the comparator output swings it's full voltage range, forming the edge of a square wave.
The time between two of these swings is 1/2 of the instantaneous frequency. The square wave output is level shifted by the MAX232 from +/- 9 volts to 0-5 volts which is an acceptable level for the PIC input.

The decoder can use one of two different programs:
The original PC version for displaying received packets to a computer terminal program.
The GPS WPL version that displays received stations with position information as way points on a mapping GPS screen.

* All GPS versions now support compressed mic-e data.

The hardware also has two configurations: The original described in the left paragraph and a new, simpler design that uses the PICs built in comparator as a ZCD. The result is a decoder than consists of one IC!

* Simpler model now has both terminal and GPS firmware available just like the full blown ZCD model.

And finally, there's a link to WB8WGA's PIC 16F88 based TNC that offers a lot of bang for the buck.

Mapped APRS stations using the GPS version

File packs contain program, schematic and instructions
Original ZCD decoder for computer display of packets This ZCD decoder plots standard and Mic-E APRS stations on GPS Simpler one chip ZCD decoder
Both PC and GPS versions
Link to WB8WGA's Simple ($25) ZCD PIC TNC

Questions and inquiries can be sent to: Ask Mike

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This page was updated 12-19-2009