Design and implementation of a CMOS VCXO for FM stereo decoders

A circuit technique to simulate large variable capacitance of both positive and negative polarities over a given frequency range is discussed. The simulated capacitance can be varied by voltage control from -60 to +100 pF. The capacitor-simulating circuit is connected in parallel with a resonator to tune its parallel resonance. An oscillator with grounded resonator is also developed. Together with the variable capacitor, a voltage-controlled crystal oscillator (VCXO) is realized. The oscillation frequency of the oscillator can be tuned continuously from 452 to 461 kHz by voltage control. Detailed analyses to completely characterize the oscillator with a simple expression are presented. The prototype of the VCXO has been fabricated in a 4- mu m standard CMOS process.<
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