Active Q-control for improved insertion loss micromechanical filters

The use of active feedback in closed-loop with two electrodes of a four-electrode capacitive-gap transduced wineglass disk resonator has enabled boosting of the effective resonator Q and independent control of insertion loss across the two other electrodes. To demonstrate the utility of this approach, two such Q-boosted resonators wired as a parallel-type micro-mechanical filter achieve a tiny 0.001% bandwidth passband centered around 61 MHz with only 2.7 dB of insertion loss - something not possible with the intrinsic resonator Q of 57,000, but quite possible with Q´s actively boosted to 670,000. In the feedback loop, the gain and phase of the active circuit are controlled to precisely add or cancel resonator damping while maintaining a suitable loop gain (i.e., less than unity if the feedback is positive). Unlike past kHz-frequency efforts, the split electrode design used here removes the amplifier feedback loop from the signal path allowing independent control of input-output coupling, Q, and frequency, the last of which comes about via either electrical stiffness changes or intentional changes in active circuit phase shift. The ability of this scheme to both raise and decrease resonator Q not only allows creation of narrow channel-select filters with insertion loss lower than otherwise achievable with un-boosted Q´s, but also opens the possibility of maximizing the dynamic range of a communication front-end without the need for a variable gain LNA.