Design of Microstrip Lowpass-Bandpass Diplexer

The matching circuit design of a lowpass-bandpass diplexer may be a challenge in conventional designs because a wideband (from zero frequency to approximately the cutoff frequency of the lowpass filter) open condition is required to achieve in bandpass channel circuit. This paper proposes a new lowpass-bandpass diplexer that involves using a simple matching design that can easily meet the required matching condition in each channel circuit. The measured results are in good agreement with the simulated responses. For the lowpass channel, the measured in-band insertion loss and cutoff frequency are less than 0.25 dB and approximately 1.5 GHz, respectively. For the bandpass channel, the measured minimal insertion loss, center frequency, and 3 dB fractional bandwidth are approximately 2.42 dB, 2.4 GHz, and 7.6%, respectively. The measured isolation between the two operating bands is better than 35 dB.