Low cost programmable FSK modem

This paper presents a modulator — demodulator system (modem) for data transmission via radio link using frequency shift keying (FSK) modulation. The modem was implemented in a low cost mid-range microcontroller. Results for the use of the modem in a railway end of train sensing unit and on the Automatic Position Report System (APRS) are showed. Both applications have slight different FSK specifications, corrected easily in each case with software adjustments. In both cases the system works very well, proving that a robust, flexible, easily reconfigurable and low cost device was obtained.