Tunable Cascaded Trisection Bandpass Filter with Constant Absolute Bandwidth

A novel cascade trisection (CT) coupling structure based on half-wavelength resonators is proposed for designing constant absolute bandwidth (CABW) tunable bandpass filters. Third-order tunable bandpass filter (BPF) which can realize CABW and a high side cross coupling transmission zero (Tz) is designed based on the proposed CT structure. Measurement results show that the central frequency tuning range of desinged 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.

I. Introduction

The advancement of multi-band, frequency agile, and software defined wireless communication systems have raised great demand for high performance and low-cost tunable filters [1]. The interference existing in the adjacent passband has bad effect on filters performance, there is a great need for suppressing these interference. Filters with cross coupled transmission zeros (Tzs) beside the passband can be used for improving the out-band rejection of the radio frequency front-end (RFFE) [2]- [3]. Recently, predefined magnetic dominant coupling and electric dominant coupling are reported for designing cascaded quadruplet (CQ) tunable filters with cross coupling Tzs [4]- [6], and the Tzs besides the tunable passband are generated. However, for low-cost transceiver, when the radio frequency (RF) signal is converted into intermediate frequency (IF) signal, filter with a single side Tz and good selectivity can be used to reject the image interference. Therefore, simple cross coupling topology with less resonator, i.e., cascaded trisection (CT) topology [7]- [8], is required for low cost RFFE design.