Digital demodulation method for multi-tone FM signal in track circuit system

A demodulation method for narrowband multi-tone FM signal based on high-pass FIR filter in new type digitally coded track circuit system is proposed. Frequency responses of some high-pass FIR filters are partially linear in their transition bands. A linear fitting algorithm is applied to the frequency response of a 10-order high-pass FIR filter with 10 kHz sample rate and a transition band from 800 Hz to 3800 Hz. The transition band of the filter is used for frequency discrimination and recovering the modulating signal. The digital train control codes can be further attained from spectrum analysis result of the modulating signal. Simulation shows that the method does not need high order FIR filter and has high linearity and low computation complexity. It can easily recovered the modulating signal with 0 dB SNR and 14 dB SIR. The train control codes are finally obtained through spectrum estimation algorithm.