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

Abstract

Bandstop filters (BSFs) with high skirt selectivity as well as wide rejection bandwidth are designed by means of open-ended λg/4 stepped-impedance resonators (SIRs) and spur-line sections. Base on the formula of an SIR, the frequency of a transmission zero (fz) 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.