A Compact Tunable Notch Filter With Wide Constant Absolute Bandwidth

A compact notch filter with a wide tunable frequency range is developed. This tunable filter is composed of two varactor-tuned resonators, shunted at input/output ports separately, and one quarter-wavelength transmission line connecting two varactor-tuned resonators. By enlarging impedance of the quarter-wavelength transmission line, the proposed tunable filter can obtain a wide and constant absolute bandwidth within the whole tunable frequency band. The measured results show that there is a wide tuning range, 124.3%. In addition, with insertion loss greater than 20 dB, a wide constant absolute bandwidth can be achieved at 450 MHz.