Mathematical Model and Implementation of a Scalable Four-Port Filter

This paper presents a novel method for integrating multiple filters into a single board that can be reconfigured through design modifications. The primary objective is to introduce a scalable three-in-one filter, referred to as a triplexer, suitable for diverse applications. The proposed filter is well-suited to applications such as multi-band RF front ends, software-defined radios (SDRs), test instrumentation requiring selectable responses, and compact wireless sensor nodes. The manuscript develops a mathematical model for each filter, enabling adjustment of the cutoff frequency to different values. The model is then expanded to capture the interactions among the three filters and is validated in MATLAB. An experimental four-port filter sample is fabricated to validate the concept. It comprises a 2.85 GHz low-pass filter (LPF), a 5.10 GHz band-pass filter (BPF), and a 6.30 GHz high-pass filter (HPF). The proposed triplexer is designed using step impedance and coupled lines, providing a systematic design approach suitable for various applications due to its adaptability and straightforward structure. The methodology includes calculations in MATLAB, full-wave EM simulation, fabrication on RT/Duroid 5880, and measurements with a four-port network analyzer. The measured results show strong agreement with both calculated and simulated results.