A New Class of High-Selectivity Bandpass Filters With Constant Bandwidth and 5:1 Bandwidth Tuning Ratio

This paper presents a design for compact bandpass filters (BPFs) that feature high selectivity. This high selectivity is achieved through the use of open-/short-circuited coupled-line segments at the filter’s input and output and a pair of symmetrical parallel-coupled lines connected to a pair of open stepped-impedance resonators (SIRs) introducing three transmission zeros (TZs) on either side of the passband. In addition, two different designs for these BPFs, one with a fixed bandwidth and one with a tunable bandwidth are also presented. The characteristics of the proposed structure are analyzed using even-, odd-mode and ABCD analyses. To enable bandwidth tuning, two varactor diodes are added to the edges of the open SIRs, allowing the TZs to be adjusted around the upper band edge. The paper includes details of two prototypes that were designed, fabricated, and tested: Filter A with a constant bandwidth that covers the entire S-band (2-4 GHz) and a 3-dB fractional bandwidth (FBW) of 60%, and Filter B with a tunable bandwidth and a 3-dB FBW that varies from 12% to 60%. The filters have been measured to have insertion loss of less than 0.8 dB for Filter A and less than 1.1 dB for Filter B throughout the passband, and return loss of greater than 16 dB and 15 dB for Filter A and B, respectively. These filters have a compact size of less than
$0.113\lambda _{g}^{2}$
, and feature high selectivity with a wide 3-dB bandwidth tuning range ratio, as well as an upper stopband suppression level of more than 40 dB.