Differential signaling in covered coupled microstrip lines

In this work a full-wave method based on the Mixed Potential Integral Equation technique is used to analyze the propagation in the time domain of a differential signal for a coupled microstrip structure with a metallic top cover. This top cover may be due to a dense circuit packaging or it can be intentionally added to enhance the immunity of the guiding structure. We study the impact of low-frequency spurious effects that may appear in this structure both in the differential signal and in the common signal, which can be caused by line asymmetries or driver imperfections. We have found that the differential signal in this structure is not significantly affected by low-frequency spurious effects. In fact, the propagation of the differential signal is well accounted for by the conventional transmission line theory based upon quasi-TEM approximation. However, the common-mode noise is channelled by an even mode that can be bound or leaky. Thus, the common-mode noise carried by the leaky mode can have a radiative behavior and might couple into adjacent lines. Due to this effect and also to the multimode excitation, the common-mode current calculated in this structure differs significantly from that calculated by using transmissionline theory. This effect has been checked for the common-mode noise due to a time/length skew of the differential line.