Compact microstrip bandstop filters using stepped-impedance resonator (sir) and spur-line sections

Bandstop filters (BSFs) with high skirt selectivity as well as wide rejection bandwidth are designed by means of open-ended lambda _g/4 stepped-impedance resonators (SIRs) and spur-line sections. Base on the formula of an SIR, the frequency of a transmission zero (f_z) is accurately predicted. It is found that the impedance and length ratio can be simultaneously obtained to improve stopband performance of the filters. The transmission zero is purposely located at a lower frequency to achieve the high skirt selectivity. On the contrary, if it is adequately situated at a higher frequency, the stopband bandwidth can be increased. Also, the spur-line structure is investigated by transmission line theory and applied to provide an extra transmission zero in the stopband. With a proper choice of the even- and odd-mode characteristic impedances of the spur-line, the stopband bandwidth can be further extended. Realised by planar microstrip technology, the proposed BSF with its high skirt selectivity has a maximal value of 86.8 dB/GHz, while the other type of BSF can further widen the -20 dB stopband by 29% significantly, when incorporating the spur-line into the BSF with an SIR. Both have a size reduction benefit of better than 40% with respective to the conventional case