Stepped Impedance Optimum Short-Circuited Stub 1–6 GHz and 6–10 GHz Bandpass Filters for Multi-standard Radio

Abstract

This article presents two ultra-broadband bandpass filter (BPF) designs for multi-standard applications, operating from 1–6 GHz and 6–10 GHz, with wide stopband characteristics. 1–6 GHz BPF (Filter 1) is based on the circuit model for an optimum distributed bandpass filter consisting of 3 shunt short-circuited stubs separated by non-redundant unit elements. By varying the length ratio between the unit elements and stubs and introducing stepped impedance in the unit elements, a 143% fractional bandwidth (FBW) is achieved, which is 20% higher than that obtained through the conventional optimum shunt short-circuited stub method. In addition, a stepped impedance elliptic lowpass filter was cascaded to achieve wide stopband characteristics from 7 GHz to 20 GHz. Filter 2 (6–10 GHz) is constructed by cascading a shunt short-circuited stub optimum high pass filter having a stepped impedance unit element with a stepped impedance low pass filter. The analytical model, transmission line equivalent circuit model, and full wave electromagnetic simulated model are presented along with the measured results. The measured in-band return loss of Filter 1 and Filter 2 is >12 dB, and the measured insertion loss is < 2 dB. The measured group delay of Filter 1 and Filter 2 is <0.8 ns.