A microstrip bandpass filter with tunable bandwidth and center frequency

Abstract This paper presents a bandwidth and center frequency tunable bandpass filter with wide tuning range. The proposed filter employs two cascade cross‐shaped resonators. The cross‐shaped resonator can generate two independent tunable transmission zeros by introducing two microwave varactors. The center frequency and bandwidth can be tuned by adjusting the DC bias voltages applied to the varactors. For demonstration, a filter example was designed, fabricated and measured. The tuning 3‐dB bandwidth from 20% to 30% is achieved when the center frequency is tuned from 3.5 to 5 GHz. Meanwhile the insertion loss is less than 4 dB with a high selectivity. The size of the filter is 0.32 λ g × 0.36 λ g where λ g is the guided‐wavelength at the center frequency.