In this work, a periodic structure as a passband filter for differential input signals is introduced that suppresses the common-mode noise with a high common-mode rejection ratio on a wide range of frequencies. The unit-cell of the periodic structure consists of open-ended stubs under common-mode and short-ended stubs under differential-mode excitation. The proposed design procedure introduced in this paper can be exploited for different microwave frequency bands while the overall shape of the filter remains unchanged. To verify the design approach, we designed and fabricated a filter at the centre frequency of 7 GHz on a Rogers RO4003 substrate with a dielectric constant of 3.7 and a loss tangent of 0.0027. Our studies show for a 3-unit-cell structure with the overall size of 18mm × 40mm (0.75λg × 1.66λg), a differential fractional bandwidth of 8.6% and a rejection ratio over 55 dB are achieved while the differential in band insertion loss is around 2 dB.

Keywords