Quasi-Elliptic Tunable Bandpass Filter With Controllable Transmission Zero Locations

This paper presents a design of quasi-elliptic bandpass filter (BPF) with center frequency tunability characteristics. To achieve quasi-elliptic response, transmission zeros (TZs) are generated at the lower and upper sides of the passband through cross-coupling between the first resonator and output port, while the location of these TZs are controllable. Frequency tunability can be achieved by tuning the resonance frequency of tunable resonators. To validate the design, a prototype of the microstrip line of the proposed structure is designed, manufactured, and measured. The measurement results show that passband center frequency is tuned from 1.65 GHz to 1.95 GHz, achieving tuning ratios of 1.182:1. In all tuning states, in-band minimum insertion loss is varying from 2.64 to 3.18 dB, while return loss is better than 11.67 dB.