Tunable Bandpass Filters with Constant Absolute Bandwidth Cascaded Trisection Topology

JingJing Yang¹, Qianyin Xiang*¹, Xiaoguo Huang², Dinghong Jia³, Quanyuan Feng¹

1. School of Information Science and Technology, Southwest Jiaotong University, Chengdu 611756 , China

2. Science and Technology on Communication Information Security /Control Laboratory, Jiaxing 314033, China

3. The 41st Institute of CETC, Qingdao 266555, China

Corresponding author: Qianyin Xiang, E-mail: qyxiang@swjtu.edu.cn

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 of the CT cell have both electric and magnetic dominant coupling region with predefined coupling coefficients, and the CT cell is feed with predefined magnetic dominant coupling. Theoretical analysis is derived to estimate coupling coefficients and performance of the proposed structure. 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. These prototypes were fabricated and measured. Measurement results show that the central frequency tuning range of third-order filter is from 1.47 GHz to 1.83 GHz with -3dB bandwidth of 108.75±2.05 MHz, and the high side 3dB to 25dB attenuation slope is 327.32~623.57 dB/GHz. The tuning range of fifth-order filter is from 1.45 GHz to 1.92 GHz with -3dB bandwidth of 101.9±1.3 MHz, and high side 3dB to 30dB attenuation slope is 442.75~843.59 dB/GHz.

Keywords: cascaded trisection; constant absolute bandwidth; tunable filter; cross coupling.