Design of microstrip diplexer constructed with step impedance resonance ring structures

A microstrip diplexer consisting of dual-mode ring filters with different feeding locations and sectional SIR ring structure is presented in this paper. The new feature of ring filter consisting of sectional step impedance resonators, of which the feeding capacitors are placed at appropriate locations to achieve the transmission zeros close to the desired frequency points, is proposed. The new type of the ring filter is not only compact in size, but it also can eliminate the superior responses of the conventional ring filter. By connecting one input of two ring filters with the quarter-wave transformer, a microstrip diplexer can be realized without altering the original filter characteristics. The experimental diplexer with two passbands is 3.6GHz and 5.0GHz. The measured insertion losses in the pass bands are less than 1.6 and 1.6dB, respectively. The isolation between two passbands is greater than 30dB and the input return losses of two passbands are about 20dB.