Dual-band bandpass filters (BPFs) provide the functionality of two separate filters, but in the size of a single filter. They have become an important part of many multiple-band communications systems, passing desired channels while rejecting unwanted interference and noise.
By using a pair of quarter-wave (λ/4) uniform-impedance resonators (UIRs) and a pair of λ/4 stepped-impedance resonators (SIRs), it was possible to construct a dual-band BPF with center frequencies of 1.57 and 2.07 GHz and fractional bandwidths (FBWs) of 4.46% and 4.35%, respectively. The filter, with tunable center frequencies, provides good selectivity and high out-of-band suppression, using folded resonators for extremely compact size.
A number of methods have been developed for the design of dual-band BPFs.1-8 They can be designed by using two dissimilar BPFs in parallel or cascade with common input/output (I/O) coupling structure. While this technique makes it possible to adjust the passband frequencies independently, it leads to a filter with relatively large size.1 In ref. 1, a dual-band BPF employing a short-circuited λ/4 resonator and an improved SIR was presented, using a split-ring scheme to achieve small circuit size. In ref. 2, a dual-band filter was designed with λ/4 resonators. In this approach two separate passbands were created from one initially wide passband by forming a stopband between the two bands.