An adaptive analog continuous-time biquadratic filter is realized in a 2pm digital CMOS process for operation at 300kHz. The biquad implements the notch, bandpass, and lowpass transfer functions. The update method is based on a least means square (LMS) algorithm which adapts the notch frequency to minimize the power at the notch filter output. Applications include FM demodulators (linear and FSK), clock extractors, and frequency acquisition aids for PLL's and Costas loops. Measured results from experimental prototypes are presented.
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