A Tunable Bandpass Filter With Modified Parallel-Coupled Line

A tunable bandpass filter with a wide tuning range is presented in this letter. The proposed filter is composed of two modified parallel-coupled lines and one varactor-loaded stub. Particularly, the modified parallel-coupled line is realized by a varactor connected to one end of the coupled line with asymmetric feeding and an open stub added to the coupled line's other end. Moreover, the varactor-loaded stub is introduced to control the transmission zero around the lower/higher passband skirt. The measured results show that a broad tuning range of 41.7%, from 0.95 to 1.45 GHz, was achieved. Within the tuning range, the insertion loss is from 2.4 to 2.9 dB and the return loss is greater than 15 dB.