Design of a Noval and Minimized Wide-Stopband Lowpass Filter

Multisection elliptic-function microstrip lowpass filters（LPFs） using SIRs-loaded hairpin resonators are presented.By connecting two resonators with a narrow microstrip line,the two-cell filter has a very wide stopband which greatly improves attenuation compared with the one-cell filter.It has a 3dB cutoff frequency at 2GHz and has a 10dB stopband form 2.5 to 25GHz with a size amounts to only 0.081λg×0.096λg（λg is the guided wavelength cutoff frequency 2GHz.）.The validity of this type of resonator when used in multisection filters is also proved.In order to understand the characteristics of the new filter,the parameters of its equivalent LC circuit are extracted.The new filter has advantages of compact size and wide stopband over conventional microstrip filters,and it could be used in many microwave systems.