Novel millimeter-wave bandpass filter using discriminating coupling for fundamental mode suppression

This letter presents an oversize 30 GHz bandpass filter with high selectivity. A new kind of discriminating coupling is proposed to suppress the fundamental odd-mode responses of the stub-loaded resonators, whereas the high-order mode responses remain unaffected. No extra circuit is required to realize the oversize design and the fabrication requirement can be released. Moreover, source-load coupling is adopted to enhance the selectivity. Based on this idea, a high-selectivity bandpass filter is implemented with wide stopband. The fabricated filter has an insertion loss of 2.3 dB and size of 2.7 mm×4.2 mm. The measured results validate the proposed method.