Design of wideband diplexer using broadside-coupled filters and stepped-impedance resonators

In this study, a wideband diplexer is proposed and the associated design concepts are also briefly discussed. First, two broadside-coupled bandpass filters are employed in the design phase so as to broaden the passband bandwidths of proposed diplexer. Next by adopting the stepped-impedance resonators in the design of the bandpass filter (BPF A) for the low-band channel, one may shift its spurious passband far away from the passband of the bandpass filter (BPF B) for the high-band channel so that the stopband of low-band channel may also be extended. Based on these concepts, the wideband diplexer with 3dB fractional bandwidths of 34% and 34.5% around the center frequencies 1.71 GHz and 3.9 GHz, respectively, is implemented and examined.