A mixed analog-digital secondary channel FSK modem

The authors describe a fully integrated circuit that performs the functions of a 75-b/s narrowband FSK (frequency-shift-keying) modem, along with the necessary filtering functions to provide greater than 30-dB channel separation between FSK and QAM (quadrature amplitude-modulation) signals. Switched-capacity circuits are used extensively to implement the QAM filters and the FSK transmitter, and a reduced-instruction-set digital signal processor is employed to realize the FSK receiver. An important aspect of the digital filters realized on the chip is that each multiplier coefficient is optimized to have fewer than three nonzero bits in the signed-digit representation. With layout precautions and separate bonding pads for voltage supplies, the crosstalk between QAM and FSK signals and the coupling of the signal processor switching noise into analog signals are minimized. Less than 0.5 dB performance degradation of signal-to-noise ratio with 10/sup -6/ bit-error rate is introduced for 19.2-kb/s modems. The chip is fabricated with a 3- mu m, double-polysilicon, single-metal, p-well CMOS process.<>