From AFSK to Goertzel
Summary
This article explains how to detect Bell 202 AFSK tones on a small MCU using the Goertzel algorithm instead of a full FFT. It presents an intuitive look at DFT, describes a second-order resonator interpretation, and provides a compact C implementation for a two-tone (1200 Hz and 2200 Hz) detector used in AX.25 packet radio. The piece includes visuals and links to PacketRF for practical, open-source reference.