Dual-Path Coupled-Line Dual-Band Bandpass Filter with Synthesis

Abstract

A dual-band bandpass filter topology based on the use of dual-path coupled lines is presented. It exhibits a 2nd order dual-band response with three transmission zeros that gives a wide separation of the passbands, and from the rejection bands. A simplified circuit diagram of the topology is proposed along with a global synthesis that can be used to control the transmission zeros, the center frequency, and the bandwidth of the passbands, by varying the length and impedance of the filter elements. The filter topology was implemented in the design of two dual-band microstrip filters on TRF45 substrate to validate the idea. Measurement results show that the center transmission zero is at 2.55 GHz, and the two passbands are centered at 1.81 GHz and 3.02 GHz. It provides isolation of up to 31 dB between the passbands. Keywords—Dual-mode, dual-band, microstrip, bandpass filters.