High-Q 3D tunable RF MEMS filter with a constant fractional bandwidth

A 2-pole waveguide tunable filter for C-band applications has been designed, fabricated and measured. The frequency shift is realized by integrating Micro Electro-Mechanical Systems (MEMS) switched capacitors in the capacitive area of each resonator. The cavity design and the MEMS shape are optimized to limit losses and get a high unloaded quality factor (Qu). The inter-resonator coupling (k12) is magnetic and constant over the frequency range leading to a quasi-constant Fractional BandWidth (FBW). Measurements result in a 1.1% FBW resonance frequency shifting from 5.2 to 4.4 GHz. Insertion loss is respectively 1.8-4.2 dB corresponding to a Qu of 620 at 5.2 GHz and of 250 at 4.4 GHz.

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