Ultra-wideband bandpass filter with hybrid quasi-lumped elements and defected ground structure

A compact planar microstrip ultra-wideband (UWB) bandpass filter (BPF) is presented in this paper. The proposed UWB filter is realised by cascading a high pass filter (HPF) and a low pass filter (LPF). The designed five-pole HPF consists of inter-digital capacitors and short-circuited stubs and has a sharp falling edge at the cutoff frequency. Additional slots are adopted to improve the return loss performance in the pass band. The LPF is realised mainly based on a non-uniform defected ground structure array in the ground. Combining these two structures, a new UWB bandpass filter is fabricated and measured. The measured results show that the proposed BPF has a wide bandwidth from 3.0-10.8GHz, associated the 40dB shape factor as low as 1.2. A group delay variation of less than 0.15ns in the pass band from 3.0-10.8GHz is obtained. It also achieves a wide stopband with 20dB attenuation up to 20.0GHz.