Single-/Dual-Band Bandpass-to-Bandstop Filters With Center Frequency Tunability

This paper reports on the design and experimental validation of a new class of frequency-tunable filters with switchable bandpass-to-bandstop transfer function characteristics. Single- and dual-band filter topologies are developed by coupling dual-mode loop resonators (DMLRs) to the feedline between the RF input and output ports of the filter. The bandpass-to-bandstop reconfigurability is realized by adding/removing an in-parallel cascaded thru line using single pole double throw (SPDT) switches. Center frequency tunability is achieved by incorporating three varactor diodes in the DMLRs. For experimental validations purposes, two filters with single- and dual-band responses were implemented and tested and appear to be in a good agreement with the EM simulated performance. For the single-band bandpass-to-bandstop filter, the RF-measured tuning range for the bandpass mode of operation is 1.65-1.96 GHz, and for the bandstop mode or operation it is 1.68-1.93 GHz. The dual-band bandpass-to-bandstop filter exhibits the RF-measured tuning ranges of 1.38-1.53 GHz for the first passband and 1.82-2.08 GHz for the second passband in the bandpass mode operation. The minimum insertion loss varies between 3.3 and 6.8 dB for the first passband, whereas it is between 2.2 and 4.7 dB in the second passband. The RF-measured tuning ranges in its bandstop mode of operation were measured between 1.35-1.47 GHz and 1.75-2.01 GHz.