Electronically tunable microstrip bandstop filters using a varactor-loaded open ring resonator (VLORR)

© 2017, Springer-Verlag GmbH Germany. A compact microstrip bandstop filter (BSF) with a wide electronically tunable frequency range and a constant absolute bandwidth is presented. It consists of an open ring resonator (ORR) loaded with a varactor diode and connected to a section of a microstrip transmission line. As a reverse bias is applied to the proposed varactor-loaded ORR (VLORR), the value of the resulting capacitor is controlled and the resonant frequency can be varied to the desired position. The measured tunable BSF achieves 92.6 % tuning range with a constant 15 dB absolute bandwidth of 0.27 GHz and a minimum rejection level of 37 dB. This reconfigurable microstrip BSF can be useful in microwave communication systems.