Tunable Bandpass and Bandstop Filters Based on Dual-Band Combline Structures

New methods for the design of tunable combline bandpass and bandstop filters are presented. The first part of this paper presents a tunable bandstop filter designed using a dual-band circuit with wide passband and integrated narrow stopband. A prototype has been constructed and measured results shows that the bandwidth remained constant at 30 MHz as the filter was tuned from 3.22 GHz to 3.66 GHz. The second part of this paper presents a tunable bandpass filter designed using a dual-band circuit with both wide passband and stopband. The response corresponds to two narrow passbands, where the lower passband bandwidth and center frequency can be tuned. A prototype has been constructed and measured results demonstrate wide centre frequency tuning range and over 300% bandwidth tuning without the need for coupling adjustment.