A practical quality factor tuning scheme for IF and high-Q continuous-time filters

An important aspect of reducing power and area of RF/IF systems is putting high-frequency filters on-chip instead of using off-chip crystal, ceramic, or SAW filters now used. The largest challenge facing designers is the vulnerability of high-frequency and high-Q continuous-time filters to parasitics and process variations, requiring automatic tuning of both center frequency and quality factor. Automatic center frequency tuning is well documented, with accuracies of <1% error. Q-tuning is not as well defined, with the best reported results at about 20-30% error. The Q-tuning method proposed here improves the accuracy of Q-tuning, with experimental Q-tuning error about 1%. The basis for the proposed method is the magnitude-locked-loop (MLL) Q-tuning technique. The scheme removes the peak detectors and utilizes the continuous-time adaptive LMS algorithm to update the biquad quality factor. As a test verification of the proposed Q-tuning scheme, a fourth order OTA-C 10.7 MHz bandpass filter is designed and fabricated in a 1.2 /spl mu/m n-well CMOS process. The filter consists of two cascaded biquads, each with a desired quality factor of 20 and the same center frequency. Center frequency tuning is provided by the conventional phase-locked loop using a voltage controlled oscillator scheme, and Q-tuning is by the proposed scheme. The reference signal for the Q-tuning circuit is simply the frequency control oscillator's output.