Design of Microstrip Lowpass-Bandpass Triplexer With High Isolation

This letter proposes a lowpass-bandpass triplexer consisting of one lowpass channel and two bandpass channels. Stepped-impedance stubs are used in the lowpass filter (LPF) design, which bring a sharp selectivity. Defected ground structures are utilized to increase the impedance of one transmission line in the lowpass section, giving improved performance. High Isolation is achieved by careful consideration of the input signal splitting structure. To illustrate the concepts, a lowpass-bandpass triplexer is designed, fabricated and measured. The cut-off frequency of the LPF is chosen to be 1 GHz. The central passband frequencies are selected to be 2.4 and 5.8 GHz. Simulated and measured results are found to be in good agreement with each other. Isolations among the three bands are better than 40 dB.