A microstrip LPF with attenuation poles using hairpin structural and inter-digital capacitor

In this paper, a microstrip lowpass filter (LPF) with attenuation poles which consists of quasi-lumped elements and hairpin structural resonator is proposed. The resonator consists of a hairpin type inductor, coupled lines, and the inter-digital type capacitor and its performances has two attenuation poles. The LPF is briegly designed and optimized with LC prototype structure. Firstly, the performances of the 5-element LPF is investigated by changing the capacitances of the inter-digital capacitor. As results, the proposed LPF illustrates the sharp attenuation characteristics near the cut-off frequency. In order to improve the stopband performance, two-hairpin LPF is studied. By optimizing its design the attenuation is improved by 30 dB. Its measured results show the good agreement with the simulated ones giving the broad stopband performance.