Simulation and realization of solid-state electrically tuning filters

The solid-state electrically tuning filters are main elements of different multifrequency communication systems with high reliability. New approach to designing electrically tuned band-pass filters that is based on use U-shaped microstrip resonators with several varactors connected to each resonator is proposed. This approach allows to creating prototypes which exceeds world level of varactor-tuned filters.

New formula for quality-factor of microwave resonators with transmission line segment and lossy capacitor was established. The formula allows us to decrease a insertion losses variation of tunable filters with varactors.
The question of increasing the tuning range of microstrip resonators with ferroelectric capacitors is considered which allows using them in upper part of the centimeter band (X-band, Ku-band). It is determined that step-irregular resonators with ferroelectric capacitors can provide 28% tuning range in X-band and 22% tuning range in Ku-band.

On the basis of the new approach, experimental tunable filters have been developed whose characteristics surpass those of commercial varactor-based filters: the center-frequency insertion loss ILo is reduced by more than a factor of 1.5; the third-order intermodulation – distortion factor IP3 is increased by 14 dBm, from 10 dBm to 24 dBm; power handling is increased from 1 mW to 25 mW; bandwidth BW(3 dB) is reduced from 4% to 1.6%.
For the first time, in filters based on silicon varactors, a dissipation-loss that corresponds to that of filters based on p-i-n diodes has been obtained.