Frequency and phase tuning of continuous-time integrated filters using common-mode signals

A system for frequency- and Q-factor tuning resulting in a self-tuned filter is proposed. The method uses a common-mode (CM) signal as reference. No master filter is required since CM reference signals are directly applied to the main filter built as two separate but identical single-ended paths. Since the two filters are built as cascade designs the tuning signal can be conveniently applied to the inputs of the two highest-Q biquads. By summing the outputs of these biquads the differential signals cancel and the resulting CM tuning signal is used in classical frequency- and Q-tuning schemes. CM tuning signals are rejected at the output of the filter by a differential buffer. For a monolithic realization, a 7th-order equiripple delay filter with cut-off frequency tunable from 5 MHz to 40 MHz and the maximum and of 2 has been selected. It is built with a single-ended double-input CMOS triode transconductor (GM) and a "lossy biquad" which requires no common-mode feedback (G/sub M/) circuitry. The filter operates from a single 5 V power supply and dissipates 200 mW. The simulated signal-to-noise (S/N) ratio equals 70 dB for an input range of 1 V/sub pp/. The advantages of the proposed approach include on-line tuning, simpler circuitry by eliminating the master filters and CMF circuits as well as reduced power, noise and die area.