Reconfigurable Microwave Coupled Resonator Band-pass Filter and Diplexer Lidiane da Silva Araujo Department of Electronics and Systems Federal University of Pernambuco Recife, Brazil lidiane sac@hotmail.com Antonio J. Belfort de Oliveira Department of Electronics and Systems Federal University of Pernambuco Recife, Brazil antonio.belfort@gmail.com Ignacio Llamas-Garro Centre Tecnol` ogic de Telecomunicacions de Catalunya Parc Mediterrani de la Tecnologia Castelldefels, Spain ignacio.llamas@cttc.es Ferm´ ın Mira Centre Tecnol` ogic de Telecomunicacions de Catalunya Parc Mediterrani de la Tecnologia Castelldefels, Spain fermin.mira@cttc.cat Michael J. Lancaster Department of Electronic, Electrical and Systems Engineering University of Birmingham Birmingham, UK m.j.lancaster@bham.ac.uk Abstract—This paper presents the design, simulation and ex- perimental results of a reconfigurable microstrip band-pass filter formed by two square open-loop resonators. A reconfigurable diplexer designed with four square spiral resonators is also proposed and preliminary simulation results are reported. The devices are designed to operate at S-band, and the reconfiguration is achieved using PIN diodes. Index Terms—reconfigurable filter, reconfigurable diplexer, PIN diodes, coupled resonators, coupling matrix I. I NTRODUCTION Current telecommunication systems should handle multi- ple standards. For example, in mobile communications to allow several technologies to operate simultaneously, where reconfigurable devices can be used to select a given service. Microstrip components are compact, lightweight, easy to inte- grate with other devices, and low cost. They are able to provide solutions for mobile systems to reconfigure both the frequency of operation and the bandwidth for communication channel se- lection. There have been previous efforts on reducing the size of telecommunication hardware through reconfiguration these include filters based on straight and bent microstrip lines. The use of open-loop and spiral resonators leads to miniaturized circuits [1]–[4]. This paper focuses on the realization of a compact reconfigurable filter and a diplexer using open-loop and spiral resonators for further size reduction compared to other works [5]–[7]. This work is supported by the Spanish government under MICINN grant RTI2018-099841-B-I00. Part of this work has been supported by the Gener- alitat de Catalunya under grant 2017 SGR 891. In section II of the paper, a two state, 0.043 dB ripple, Chebyshev two-pole microstrip filter is described. It operates at a frequency of 2.45 GHz when the PIN diodes are in the ON state. When the PIN diodes are in OFF state, the filter has an operation frequency of 2.65 GHz. The design, simulation and experiential results are discussed. In section III the design and simulation of a reconfigurable microstrip diplexer is given. The design is based on square spiral resonators for high device miniaturization. The two bands center frequency of the diplexer are 2.36 GHz and 2.63 GHz in the didoes in ON state, and 2.43 GHz and 2.74 GHz in the diodes in OFF state. Both the reconfigurable filter and diplexer are designed on 1.5 mm thick, CuClad 217 dielectric substrate. BAR50-02V PIN didoes are used along with choke inductors having a self resonance at 2.6 GHz and inductance of 62 nH. All the simulations are done with CST Microwave Studio [8]. II. THE RECONFIGURABLE FILTER A. Design The filter was originally designed without reconfiguration and following the standard coupling theory [9]. This original design is a 2-pole band-pass filter with a Chebyshev response operating at a central frequency of 2.455 GHz with 3% of fractional bandwidth (FBW ), and a -20 dB maximum re- flection loss in the passband. These specifications result in the coupling coefficient M 12 =0.05 and the external quality factors Q e1 = Q e2 = 22.16. From these values the resonator dimensions and the spacing between them can be found for this non reconfigurable filter, giving an estimate of the structure of the filter. To add the reconfiguration, so there is a second