Design and fabrication of a compact microstrip lowpass-bandpass diplexer with high isolation for telecommunication applications

This study aimed to design and fabricate a lowpass-bandpass (LP-BP) diplexer with high isolation for telecommunication applications including wireless communications. The results revealed that the −3 dB cutoff frequency of the lowpass filter (LPF) was equal to 0.82 GHz. The advantages of the LPF section include a very high suppression factor (SF) parameter (about 4.4), very sharp roll-off-rate (ROF or ζ) parameter (687), and a very high figure of merit (FOM) (about 233491). The bandpass filter (BPF) section was designed applying dual-mode resonators with triband. The central frequencies of these tribands were equal to 2.38, 3.93, and 5.65 GHz. In the following, an SMV-1247-079LF SMD varactor diode is used to adjust the proposed LPF. The tuning range for the proposed LPF is 0.3-0.72 GHz. In the proposed diplexer, the isolation between the output ports was higher than 45 dB. The results of measurements were in good agreement with those of the simulation.