Switched-capacitor FSK modulator and demodulator in CMOS technology

A switched-capacitor FSK modulator/demodulator built in silicon-gate CMOS technology is described. The modulator is based on a programmable harmonic oscillator using two stray-insensitive integrators. The centerpiece of the FSK demodulator is a switched-capacitor voltage-controlled oscillator. A simple post-detection processor restores the digital data. Both circuits have been designed for the 600-baud modem channel with 1500 Hz center frequency and /spl plusmn/200 Hz frequency shifts, but the demodulator operates in the 1200-baud channel as well. Due to dynamic biasing the operational amplifiers feature high slew rate, high voltage gain, and low power for capacitive loads.