FSK Coherent Demodulation Using Second-Harmonic Injection Locked Oscillator

We present an alternative coherent demodulation of frequency shift keying (FSK) signals based on the use of a super harmonic injection locked oscillator (ILO). Demodulation consists of multiplying the incoming FSK signal by the output of the ILO using an up-conversion mixer. The product includes a second harmonic signal that locks the oscillator, closing a non-linear feedback loop, and synchronizes its output with the incoming FSK signal. In addition to the second harmonic, the product also includes a base band signal that reproduces the original modulation. We built and tested a hybrid prototype of the demodulation system for the frequency range from 250 to 300 MHz. The circuit consists of a second-harmonic differential output ILO with varactor diodes as tuning elements and a four quadrant analog multiplier. Our results demonstrate the reliability and robustness of the demodulator system.