Design of a high-selectivity lowpass-wideband bandpass diplexer using microstrip structure of loaded lumped-element

In this paper, a novel diplexer with enhanced selectivity is proposed. The diplexer is composed of a lowpass filter and a bandpass filter, where the size of the low-pass channel is decreased by lumped-element and the bandwidth of the bandpass channel is expand by using cross-coupled stepped-impedance resonator (SIR). The selectivity of wideband bandpass channel is enhanced by a spurious response on lowpass channel's stopband. The diplexer's design method is proposed in detail and have been verified by an experimental results of the fabricated diplexer. The 3dB cut-off frequency of lowpass channel is 0.84 GHz and a bandpass channel's passband is 1.15-2.0 GHz. The measured results show low insertion loss and high-selectivity, which agree well with the simulated results.