A Tunable Trisection Bandpass Filter with Constant Fractional Bandwidth based on Magnetic Coupling

Mingye Fu 1, Qianyin Xiang* 1, and Quanyuan Feng 1

1 School of Information Science and Technology, 

Southwest Jiaotong University, Chengdu, Sichuan, 610031, China

Abstract ─ A tunable microstrip trisection bandpass filter with source-load coupling is proposed in this paper. Magnetic cross coupling structure based on varactor loaded open ring resonators is employed. No extra capacitor is used to adjust the coupling coefficients directly. Normalized coupling matrix M is used to calculate the frequency response, and formulas for computing the S parameters based on the resonator with unconventional phase performance are given. Coupling coefficients are investigated by computing the integral of the distributed voltage/current and it is proved that the coupling coefficients can meet the requirements of constant fractional bandwidth (CFBW). Due to the magnetic cross coupled trisection structure and the electrical source-load coupling, this tunable filter has three transmission zeros at finite frequency which can effectively improve the frequency selectivity. The measurement agrees well with the simulation. The center frequency can be tuned from 1097 MHz to 1936 MHz. In this tuning range, the insertion loss varies from 7.8 dB to 4 dB. A constant fractional bandwidth of about 5% is achieved.