Mixed-mode automatic tuning scheme for high-Q continuous-time filters

An automatic tuning scheme using envelope detectors and simple mixed-mode circuitry is discussed. The method is developed especially for filters with large Q values and can be used at very high operating frequencies. The circuit was designed in a 1.2 m CMOS process with a 3 V single supply voltage. As a demonstration filter, a second-order gm-C bandpass section is used, but the method can also be applied to lowpass and highpass sections. The measured frequency tuning error is less than 0.3% for filters with Q>80 operating in the range of 105 MHz-120 MHz.

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