Significant Reduction of Common-Mode Noise in Weakly Coupled Differential Serpentine Delay Microstrip Lines Using Different- Layer-Routing-Turned Traces

This paper proposes a new structure to reduce the commonmode noise (CMN) of weakly coupled differential serpentine delay lines (DSDLs) in microstrip line structure. In the proposed CMN reductions structure, different-layer-routing-turned traces are used as substitutes for same-layer-routing-coupled turned traces (i.e., as in the conventional scheme). The proposed reduction schemes, which are based on different-layer-routing-turned traces, include the original reduction scheme, in which all traces are impedance matched, and the improved reduction scheme, in which all traces have a narrow width. In the time domain, the peak-to-peak amplitude of the CMN obtained by the proposed solution is more than 90% higher than that obtained by the conventional scheme. Implementing the reduction scheme in DSDLs reduces differential-to-common mode conversion (|S_cd21|) by at least 20 dB (range 0.8-16 GHz). The causes of the improved signal integrity are also investigated. Therefore, an improved CMN reduction scheme for DSDLs is proposed. Regarding the frequency domain, the asymmetry can limit the CMN reduction at low frequencies. Symmetry is an important requirement for DSDLs in the proposed CMN reduction scheme. Finally, a favorable comparison between the simulated and measured results confirms the substantial CMN reduction achieved by the proposed schemes.