A 5th-Order Constant Bandwidth Tunable Bandpass Filter With Two Cascaded Trisection Structures

A novel 5th-order cross-coupled tunable bandpass filter based on full-wavelength tunable resonator with four independent coupling regions, including two electric and two magnetic coupling regions is proposed. The slop of the coupling coefficients versus the resonant frequency can be pre-defined to meet the requirement of tunable bandpass filter with constant bandwidth. The 5th-order constant bandwidth cross-coupled bandpass filter is with two cascaded trisection (CT) structures, including one all electric type CT cell and one all magnetic type CT cell, and two CT cells share a common resonator to reduce the size. Therefore, two transmission zeros are generated besides the passband for improving the frequency selectivity. The measurement shows that, the central frequency can be tuned from 1.48 GHz to 1.88 GHz with a −3 dB bandwidth of 92±1.1 MHz, and the −20dB/−3dB rectangle factor is 1.66~1.76.