Miniaturized Paralleled-Coupled Microstrip Bandpass Filters with Spur-Line for Multi-Spurious Suppression

Two parallel-coupled bandpass filters with miniaturization and multi-spurious suppression were presented. The coupled stages of the filters were designed with back-to-back spur-line coupled sections. By adequately selecting the length of spur-line, an additional transmission zero can be created. Owing to the slow-wave effect of the spur-line and the over-coupled architecture for the coupled stages, both filters not only can realize size reduction of 53%, but also have spurious suppression up to 5.4f
_o
with 20 dB and 4.6f
_o
with 30 dB rejection levels, respectively.