Tunable bandpass filters with constant absolute bandwidth cascaded trisection topology

Abstract

A novel cascade trisection (CT) coupling structure, based on half-wavelength resonators, is proposed for designing constant absolute bandwidth (CABW) tunable bandpass filters. The tunable resonators have both electric and magnetic dominant coupling regions with predefined coupling coefficients. The third-order and fifth-order tunable bandpass filters (BPFs), which can realize CABW and a high side cross-coupling transmission zero (Tz), are designed based on the proposed CT structure. Measurement results show that the central frequency tuning of the third-order filter is from 1.47 to 1.83 GHz with −3dB bandwidth of 108.75 ± 2.05 MHz, and the high side 3 to 25 dB attenuation slope is 327.32–623.57 dB/GHz. The tuning range of the fifth-order filter is from 1.45 to 1.92 GHz with −3dB bandwidth of 101.9 ± 1.3 MHz, and high side 3 to 30 dB attenuation slope is 442.75–843.59 dB/GHz.