Multilayer Microstrip Closely-Spaced-Channel Wideband Diplexer With Highly Selective Fourth-Order Filtering Responses

A type of multilayer microstrip diplexer with highly- selective wideband filtering channels is reported. Its closely-spaced fourth-order passbands are attributed to two circuit parts: (i) two third-order quasi-elliptic-type microstrip-to-microstrip wideband vertical transitions and (ii) a shunt two-stub dual-band bandpass filter (BPF) junction printed on the top layer. The two microstrip vertical transitions are developed by means of open-/short-circuit-ended microstrip lines and slotline stepped-impedance resonators (SIRs), respectively, and they correspond to the constituent lower and upper channels. The shunt dual-band BPF junction produces one additional transmission pole and one more pair of close-to-passband transmission zeros (TZs) for each channel to further increase their filtering selectivity. The RF theoretical foundations and design procedure of the engineered broadband diplexer are detailed. For practical-validation purposes, a proof-of-concept prototype of two-layer fourth-order microstrip wideband diplexer is simulated, manufactured, and tested. Its measured lower and upper channels has two closely-spaced sharp-rejection passbands with center frequencies of 1.496 GHz and 2.759 GHz and 3-dB fractional bandwidths of 62.83% and 24.83%, respectively, while featuring power-isolation levels above 27.39 dB from 0.5 to 4 GHz.