Electronically Tunable Dual-Band Bandstop Filter Using a Folded Microstrip Coupled-Line

Abstract

A coupled-line open-stub configuration is used to design a simple and compact bandstop filter. It is shown that the coupling between two lines can be utilized to obtain dual-band behavior. Electrical length of the open-stub at the first stopband frequency f1 is less than a quarter-wavelength. Furthermore, the stopbands are electronically tuned by introducing a varactor diode. A lossless transmission line model is used to analyze the filter. Two prototype bandstop filters are fabricated using a 32 mil thick RO4003C substrate with a dielectric constant of 3.55. The first filter is without any diode and provides fixed stopbands those suppress 2.4 GHz and 5.8 GHz wireless LAN signals. The second filter is electronically tunable. The first stopband is kept almost fixed at 1.42 GHz and the second stopband is varied over 2.22-3.13 GHz by a varying the bias voltage over 0.25-15 V. Design graphs and closed-form expressions are provided to facilitate the filter design for other applications.