Design and fabrication of a tunable microstrip lowpass-bandpass diplexer for telecommunication applications

In this article, a well-defined tunable lowpass-bandpass (LP-BP) diplexer with proper performance is provided using stepped impedance lines and square-looped resonators for telecommunication applications. Our proposed scheme is the first tunable LP filter BP filter diplexer that provides good results. The proposed diplexer has the ability to adjust the cut-off frequency of the lowpass filter (LPF) and to adjust the center frequency of the bandpass filter (BPF). The varactor diodes in reverse bias were used to make the proposed diplexer tunable. The tuning range of the cut-off frequency for the LPF is from 0.46 to 0.96 GHz. An important feature of the LPF is a high suppression factor of about 45 dB in the stopband and a sharp roll-off rate (ROF). The adjustability of the BPF part for the first channel ranges from 1.8 to 2.4 GHz; for the second channel, the ranges from 2.8 to 3.2 GHz, and for the third channel, the ranges from 5.2 to 5.6 GHz. The proposed diplexer is designed, simulated, fabricated, and finally tested and measured. The isolation between the output ports is about 34 dB. Simulation results are in good agreement with the test results and the measured values.