In this paper, we presented the design of a high performance diplexer for applications of global positioning system (GPS) at 1.575 GHz and wireless local area network (WLAN) at 2.4 GHz, simultaneously. Two bandpass filters (BPFs) using the modified stepped-impedance resonators (SIRs) operated at 1.575 GHz and 2.4 GHz are main blocks for the proposed diplexer. By discussing and analyzing the admittance of the even and odd modes, the transmission zero of the modified SIR can be found, and the location of the transmission zero can be precisely predicted, thus having a wide and deep stopband for the BPFs, in turn the high performance diplexer. Furthermore, due to the appearance of the transmission zeros near the passband edges, the passband selectivity of the BPFs as well as the diplexer can be improved. By using the impedance matching between two BPFs, a high isolation of 50 dB between two channels is obtained. The proposed diplexer was designed, fabricated and measured. The simulated and measured results had a good agreement with the proposed design concept.
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