A Wideband Microstrip Elliptic Bandpass Filter with Flexibly Tunable Bandwidth

This paper presents a fourth-order wideband microstrip elliptic bandpass filter that is capable of flexibly reconfiguring its passband bandwidth (BW) with highly reduced number of tuning elements. The filter's BW tunability is varactor-enabled and is realized by tuning the even-mode resonant frequency of the utilized stub-loaded dual-mode resonators. Either the lower or the higher edge of the filter's passband can be tuned continuously and independently with the use of only two varactors. The demonstrated filter exhibits an elliptic-transfer-function response with tuning ranges of 250 and 520 MHz for the lower and higher passband edges, respectively, and a BW tuning range of 500–900 MHz at a center frequency of 2.4 GHz.