Tunable filters can significantly reduce the complexity of future software-defined frequency- and bandwidth-agile wireless systems. Achieving wide frequency tuning range and a high unloaded quality factor (Qu) at the same time has long been a challenge for tunable filter design and implementations. Highly loaded evanescent-mode cavity filters offer a great balance between the two requirements.
As a graduate student, Dr. Liu pioneered the use of electrostatic micro-electromechanical actuators for the implementation of high-Q tunable evenescent-mode filters. An electronic tunable 2-pole filter was demonstrated for 3-4.7 GHz with 0.7% fractional bandwidth, insertion loss of 3.6-2.4 dB, and extracted Qu of 300-600. Further works demonstrated tunable bandpass and bandstop filters with similarly high Qu and tuning range for frequencies up to 20 GHz.
At UC Davis, we have continued the work exploring various aspects of the design and fabrication of novel tunable filters. We have demonstrated a design strategy for integrating lumped tuning elements with cavity filters. We have also introduced a frequency and bandwidth tunable filter design method based on dispersive coupling structures, showing bandwidth tunability of 0-10%. At the 0% bandwidth state, this coupling structure effectively shuts off the filter and can work as an RF switch without relying on an actual switch. More recently, we have also demonstrated a W-band (75-110 GHz) waveguide tunable filter with recording-breaking tunability and insertion loss.