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Reduction of Signal Line to Power Plane Coupling Using Controlled-Return-Current Transmission Lines
Keywords: transmission lines, return currents, power/ground planes
In printed circuit boards, transmission of high-speed digital signals is achieved using single-ended or differential transmission lines, typically using a microstrip or stripline configuration. This requires continuous reference planes, which is difficult to maintain in multilayered IC packages or boards. As a result, the signal return current may need to flow between different reference planes due to a cut-out in the reference plane or a signal via transition. The uncontrolled path of the return current results in signal integrity and electromagnetic interference problems. Traditional approach in high-speed packages and PCBs is to design high-speed nets using transmission lines, but typically not much care is given to the return path assuming that the return currents will be able return back to the source by means of a nearby decoupling capacitor or by means of the parasitic capacitance of the power planes. None of these approaches work for high-frequency signals due to the parasitic inductances, and the coupled high-frequency signals in the power/ground planes result in increased EMI and SI problems. In this paper, we present for the first time a controlled-return-current transmission line (CRCTL) design using guard traces. We emphasize that it is essential to design the return path together with the signal path for a high-speed transmission line. This is especially true for single-ended lines, which are more receptive to power/ground noise. We will demonstrate the improvement in terms of the insertion and return loss using CRCTL compared to a conventional via design in suppressing the coupling between the signal lines and power plane at a via transition.
A. Ege Engin, Assistant Professor
San Diego State University
San Diego, CA

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