Compact tunable low-pass to CFBW bandpass switchable filter using concentric resonators

Abstract

A tunable low-pass to bandpass switchable filter is presented in this study. The proposed structure consists of two concentric resonators. Along with varactors, outer resonator is also loaded with a PIN diode switch. The switching conditions of the PIN diode - OFF lead to a low-pass filter (LPF) and ON lead to a bandpass filter. At the ON condition of the switch, the outer resonator behaves as two resonators which are coupled to each other through two ways - inductively coupled through via hole and capacitively coupled through the varactor loaded between the resonators. The coupling coefficients between the two grounded resonators are controlled using varactors to achieve constant fractional bandwidth (CFBW) tunability. The third CFBW tunable band with bandpass characteristic having three transmission zeros across the passband is achieved using inductively coupled inner resonators. A filter is designed, simulated and fabricated. The response shows a tunable LPF having a tuning range of 0.55-1.4 GHz. One tunable CFBW bandpass response with a tuning range of 0.850-1.25 GHz is achieved with a CFBW of 64%. The third tunable band with bandpass filtering behaviour is achieved with a tunable range of 1.04-1.6 GHz with a CFBW of 13.4%.