Differential-To-Common-Mode Conversion Suppression Using Mushroom Structure on Bent Differential Transmission Lines

In this paper, we propose a method to suppress differential-to-common-mode conversion noise from right-angle bent differential lines by installing a mushroom structure on the narrow inner line. We assessed the mode conversion noise suppression characteristics of the proposed structure by analyzing dispersion diagrams. The proposed structure exhibits differential-to-common-mode conversion (Sd21) below-20 dB up to 6 GHz, while S_cd21 of conventional right-angle bent differential lines is only maintained below-20 dB for frequencies up to 1.04 GHz. In addition, we suggest that further enhancements of the mode conversion suppression bandwidth can be achieved by installing multiple distributed mushroom structures on right-angle bent differential lines. The use of multiple distributed mushrooms enabled us to achieve a wider frequency bandwidth by compensating more fully for the phase difference between the inner and outer lines. Therefore, the right-angle bent differential lines with multiple distributed mushrooms can suppress mode conversion noise under-20 dB up to 10 GHz. We assessed the performance of our proposed structure by conducting measurements in both the frequency and time domains. We obtained a good agreement between our experimental and numerical results. In addition, we measured eye diagrams to demonstrate that the proposed structures suppress differential-to-common-mode conversion noise without degrading the differential signaling quality.