High-suppression three-pole tunable band-pass filter using square open-loop resonator

Tunable band-pass filters using microstrip square open-loop resonator have been developed at 698-802 MHz band in this paper, which show steep skirt frequency response and narrow relative bandwidth. The microstrip square open-loop resonator is employed in the three-pole tunable filter design. The testing results show the developed varactor tuned filter has a 1- dB relative bandwidth of 2.1% with an insertion loss of 10-12.5 dB, the typical attenuation in stop band is 10-15 dB higher than that of the two-pole filter, which lead to a better performance in side band rejection for the application to Cognitive Radio system.