Patent ID: 8811542

Claim:
A process for demodulating a low frequency half-duplex (HDX) frequency shift keying (FSK signal, comprising: receiving the HDX FSK signal using a resonant antenna, where the HDX FSK signal comprises a plurality of HDX FSK signal frequencies distributed around a center frequency nominally at mid-point between the lowest and highest HDX FSK signal frequencies; upwardly mixing the HDX FSK signal to an intermediate frequency FSK signal having a center frequency higher than the center frequency of the received HDX FSK signal by mixing the HDX FSK signal with a fixed local oscillator frequency; filtering the intermediate frequency FSK signal using at least one ceramic bandpass filter, wherein the at least one ceramic bandpass filter has a 6 dB bandwidth of at least +/−10 KHz centered at the intermediate frequency; and demodulating the filtered intermediate frequency FSK signal to produce a binary output; wherein the HDX FSK signal frequencies are 134.2 KHz and 124.2 KHz.