Tunable 3-pole bandpass filter with constant absolute bandwidth and >60dB rejection at desired location of stopband

This paper presents a novel design approach for tunable 3-pole bandpass filter (BPF) with constant bandwidth (ABW) and deep rejection at the desired stopband. First, by properly designing the coupling coefficient and Q(ext), the tunable 3-pole BPF is designed with the characteristic of constant ABW. Only two bias voltages are adopted to complete the tuning, among which one of them is used to tune the center frequency and the other one is adopted to control the Q(ext). Then, bandstop resonators are introduced to provide deep rejection at desired stopband. A deep rejection level >60dB can be easily controlled by varying the loaded capacitor C-bs among the stopband. Furthermore, the designed deep rejection point can be constant and produces little influence on the passband. For demonstration, a 3-pole tunable BPF is designed, fabricated, and measured. The filter can be tuned from 0.98 to 1.4GHz with constant ABW, a wide fractional tuning range up to 40% can be attained. High rejection level >60dB is realized at the pre-designed stopband point 0.65GHz. The measurement shows good agreement with the simulation.