Low loss K-band tunable bandpass filter using micromachined variable capacitors

In this paper, a low loss K-band tunable bandpass filter with micromachined variable capacitors has been designed and fabricated, and its electrical performance has been measured. All elements of the tunable filter were fabricated using a surface micromachining technology and subsequent releasing process. The fabricated chip size was 2.75 mm×1 mm. The measured initial center frequency was 20.7 GHz with a low insertion loss of 2.2 dB. In the analog operating region of the capacitor, the center frequency could be tuned from 20.7 to 18 GHz, which means a tuning-ratio of 13%. The maximum analog-mode capacitance ratio is 1.31 with an actuation voltage of 35 V. With a bias voltage of 60 V, the membrane collapsed to the bottom electrode and the center frequency was changed to 15.5 GHz, thus the tuning ratio is 25%. In this case, the capacitance ratio was calculated to be 1.9.