Tunable microstrip lowpass filter with compact size and ultra-wide stopband

A compact varactor-tuned microstrip lowpass filter (LPF) with a large cutoff frequency tuning range and an ultra-wide stopband is presented. By employing a pair of parallel coupled-lines and a low-impedance stub loaded with a varactor diode, the introduced filter can successfully realise the tunable cutoff frequency function. Besides, both radial patch resonators and stepped-impedance resonators are introduced to achieve an ultra-wide stopband in all the states in this design. A varactor-tuned microstrip LPF is designed, fabricated and measured. The measured results show that the proposed LPF provides not only a large cutoff frequency tuning range of 91.3% from 0.85 to 2.28 GHz, but also an ultra-wide stopband which extends to 20 GHz.