K4DSP FSK Regenerator

This page contains the schematic diagram and the software source code for a simple FSK regenerator that I designed. This circuit takes AFSK from the sound card and produces FSK and PTT outputs suitable for driving most modern transceivers. Some RTTY programs use the RTS or DTR lines from a serial port to produce FSK, but some programs may not have that provision. And some newer computers don't have serial ports at all. You may find this circuit useful if you want to use FSK with your radio and don't have any other way to generate the FSK signal.

Some features of this design:

Operation

The AFSK audio signal from the sound card is presented to J1 and is converted into a square wave by the comparator circuit at U1. This signal is then fed to U2, the Atmel ATTiny45 microcontroller, which measures the frequency of the incoming signal. When a MARK (2295 Hz) signal is detected the FSK output is set to MARK, and when a SPACE (2125 Hz) signal is detected the FSK signal is set to SPACE. The PTT output is activated whenever a signal is present at the input, and is deactivated when the signal disappears. When a signal is present at the input the circuit draws about 4 mA from the batteries, but when there has been no input signal for 30 seconds the microcontroller powers down and the current drops to less than 20 microamps. At this level no power switch is needed, and the batteries will last for many months. When a signal is present again the microcontroller "wakes up" and resumes operation.

Schematic in PDF

Source code in ZIP format

Theory and Operation Notes in PDF

The Atmel microcontroller has built-in flash memory which must be programmed with the file regenerator.hex before it will function. If you don't have access to a device programmer and would like a pre-programmed part please contact me at the email address below. I will provide a programmed Atmel microcontroller for U2 for $5 postage paid in the U.S. and $6 outside the U.S.