Compact wideband bandpass filter with equiripple response in both magnitude and delay

Abstract

Aiming at the reduction of the in-band delay variation, a wideband bandpass filter with equiripple response in both magnitude and group delay is presented and studied. The filter is composed of one section of end-connected coupled line and two short-circuited shunt transmission line stubs, all of them are uniform and equal in electrical length. Design formulas are given to calculate the characteristic impedances of the lines to meet the filtering specifications. By changing the coupling strength of the coupled line section, the group delay can be controlled to be equiripple in band, while the remaining equiripple response in magnitude unchanged. For verification, a filter sample is fabricated and measured. In experiment, it exhibits a delay variation of 0.11 ns and minimal insertion loss of 0.2 dB within its passband (|S11| < −14 dB) from 0.48 to 1.46 GHz, which validates the theoretical predictions.