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Design and Implementation of Microstrip Lowpass-Bandpass Diplexer on FR-4 Substrate

Mr. S. Srikanth , Dr. V. Jeyalakshmi

The purpose of this paper is to design a lowpass-bandpass diplexer printed on FR-4 substrate. 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. For the lowpass channel, the simulated in-band insertion loss and cutoff frequency are less than 0.1dB and approximately 1.5 GHz, respectively. For the bandpass channel, the simulated minimal insertion loss and center frequency are approximately 3.2dB and 2.4GHz respectively. The simulated isolation between the two operating bands is approximately 32 dB. The tested insertion loss of the lowpass filter and bandpass filter is 3.2dB and 8.92dB respectively. The tested isolation between the two operating bands is approximately 40dB.