Miniature and tunable filters using MEMS capacitors

Microelectromechanical system (MEMS) bridge capacitors have been used to design miniature and tunable bandpass filters at 18-22 GHz. Using coplanar waveguide transmission lines on a quartz substrate (ε_r = 3.8, tanδ = 0.0002), a miniature three-pole filter was developed with 8.6% bandwidth based on high-Q MEMS bridge capacitors. The miniature filter is approximately 3.5 times smaller than the standard filter with a midband insertion loss of 2.9 dB at 21.1 GHz. The MEMS bridges in this design can also be used as varactors to tune the passband. Such a tunable filter was made on a glass substrate (ε_r = 4.6, tanδ = 0.006). Over a tuning range of 14% from 18.6 to 21.4 GHz, the miniature tunable filter has a fractional bandwidth of 7.5 ± 0.2% and a midband insertion loss of 3.85-4.15 dB. The IIP₃ of the miniature-tunable filter is measured at 32 dBm for the difference frequency of 50 kHz. The IIP₃ increases to >50 dBm for difference frequencies greater than 150 kHz. Simple mechanical simulation with a maximum dc and ac (ramp) tuning voltages of 50 V indicates that the filter can tune at a conservative rate of 150-300 MHz/μs.