A 5-MHz IF digital FM demodulator

Key requirements for digital frequency-modulation (Fhf) demodulators are wide spurious-free dynamic range in the intermediate-frequency quantizer, linear-phase passband filtering, and accurate frequency discrimination. The proposed FM demodulator implemented digitally achieves high linearity by numerical differentiation performed at a 112x oversampling rate, suppresses adjacent channels by placing zeros of the SINC function on them, and rejects amplitude-modulation (AM) components by numerical division, A 5-MHz FM demodulator integrated with a fourth-order bandpass delta-sigma front end exhibits 74.7 dB signal-to-noise ratio, -80.7 dB total harmonic distortion, and 61 dB AM rejection within a 9-kHz message bandwidth. The 0.65-mu m CMOS chip occupies 3.5 x 3.5 mm(2) of active area and consumes 180 mW with 4-V supply when clocked at 20 MHz.