Compact Tunable Microstrip High-Pass Filter With High Rejection Level of Stopband

A compact seventh-order tunable high-pass filter (HPF) with high rejection level of stopband is proposed in this letter. Two series resonators are added to the proposed tunable HPF and a transmission zero (TZ) is generated at the stopband, therefore the rejection level of the stopband is enhanced significantly. Varactor pair with common cathode is utilized to implement the capacitance in the middle of the proposed filter, and the varactor pair with common cathode will simplify the bias circuit of the varactors. Besides, the interdigital structure is adopted to replace lumped capcacitors, which may improve the insertion loss performance of the filter. The proposed tunable HPF is optimized by HFSS, and the simulated results show that the cut-off frequency of the tunable HPF can be tuned from 2.7 GHz to 4.3 GHz with stopband rejection level of at least 30 dB. During the whole tuning range, the return loss (RL) of the tunable HPF is greater than 12 dB.