Miniaturized dual-band ring bandpass filter with harmonic suppression and passband tunability

A novel miniaturized dual-band ring bandpass filter is presented in this paper. The miniaturization is accomplished by utilizing slow-wave synthesized microstrip lines, which are composed of quasi-lumped elements. The saw-toothed coupling structures are adopted to provide tight coupling at the input/output ports. The proposed filter shows an extraordinarily compact size with harmonic suppression up to the fourth harmonic frequency; the in-band response, on the other hand, is comparable to the previous designs. Benefitting from the nonlinear phase response of the synthesized lines around the cutoff frequency, the second passband can be adjusted in a simple and straightforward way. The essentials of the synthesized microstrip lines are discussed. The design methodology, circuit topology, and simulated and measured results of the miniaturized dual-band ring bandpass filter are investigated in detail.