A non-uniform sampling tangent type FM demodulation

Frequency modulation (FM) is widely used in applications such as FM broadcasting, Global System for Mobile (GSM, whose modulation method, Gaussian Minimum Shift Keying, is a kind of FM) etc. Using digital signal processing (DSP) to demodulate FM signals has advantages over analogue methods in respect of the easiness of design, adjustment, stability, reliability, and upgrading. The current digital methods employ an even spaced (uniform) sampling method and process each sample. When the modulation index increases, the modulated signal bandwidth expands. So the sampling rate needs to be increased as well. This leads to increased computation time especially for division and arctangent. In this paper, we propose a non-uniform low-sampling method, whose overall sampling rate is not a function of the modulation index. Its minimum rate can be as small as four times the bandwidth (B) of the non-modulated (message) signal. Furthermore, the minimum rate of division and arctangent computations are only 2B Hz. As a result, the proposed method can significantly reduce the overall computation for FM demodulation.