Tunable MEMS hybrid coupler and L-band tunable filter

Simultaneous control of frequency and coupling coefficient of a directional coupler provides a wide band tunable hybrid coupler. This hybrid coupler is combined with identical two tunable band rejection filters (BRF), to achieve a small-size wide-band tunable band pass filter (BPF). The BPF is called a reflection-type BPF and has a feature of converting rejection frequency bands of the BRF to pass frequency bands. In this paper, we derive the minimum insertion/reflection loss of the lumped element BPF and BRF, considering only the equivalent series resistance (ESR) of tuning devices. The results showed their minimum insertion loss was -5.2 dB and reflection loss was -0.35 dB if the ESR is 1 Omega. Considering the results, we prioritize a reflection-type filter as a prospective low insertion loss tunable BPF. Even though the BRF could be miniaturized and be adapted to wide frequency region by adopting lumped elements, the hybrid coupler might restrict the wide tuning range. Then we propose new tunable hybrid coupler, which alters its coupling coefficient as well as its center frequency, to provide equal transmission and coupling coefficients in wide frequency range. Validation has been done by fabricating tunable BPF composed of 1.2/2 GHz selectable MEMS hybrid coupler and 800 MHz-2.4 GHz seven-band MEMS tunable BRF. The total loss was around -5 dB and filter size was as small as 5.2 x 12.3 mm².