A Wideband Common-Mode Suppression Filter for Bend Discontinuities in Differential Signaling Using Tightly Coupled Microstrips

A new type of bend is proposed that reduces differential-to-common mode conversion occuring at the bend discontinuity in coupled microstrip lines for high-speed digital circuits. Simultaneously, great care has been taken to minimize the differential reflection coefficient and insertion loss, leading to an overall improved signal integrity. This is achieved by tapering the microstrip lines to tightly or very tightly coupled ones in the area of the bend. Full-wave simulations in the DC to 6 GHz frequency range show that over 9 dB and 14 dB suppression of conversion noise is achieved for tightly coupled and very tightly coupled bends, respectively. Also for these new structures, with a total length of 100 mm, the insertion loss remains below 0.6 dB. Measurements on prototype bends show very good agreement with full-wave simulations. Also time domain measurements demonstrate the significant reduction in conversion noise while keeping return loss low. Moreover, for design purposes, a dedicated circuit model which closely matches the full-wave characteristics of the proposed bends is presented.