The PCB Design Magazine

PCBD-Apr2017

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50 The PCB Design Magazine • April 2017 Simultaneous switching noise (SSN) is a major problem in high-speed systems. But, the underlying issue is really the management of transmission line return currents that flow on the nearby reference planes, causing the planes to bounce. High-speed design is not as simple as sending a signal from the driver to the receiver, over an interconnect. Rather, one should also consider the presence and interaction of the power distribution network (PDN) and how and where the return current flows. A logic sche- matic diagram masks details crucial to the op- eration of unintentional signal pathways vital to understanding signal performance, crosstalk and electromagnetic emissions. PCB designers, generally, take great care to ensure that critical signals are routed exactly to length from the driver to the receiving device pins, but take little care of the return current path of the signal. Current flow is a "round trip" and the critical issue is delay, not length. If it takes one signal longer for the return cur- rent to get back to the driver—around a gap in the plane for instance—then there will be skew between the critical timing signals. Return path discontinuities (RPDs) can create large loop ar- eas that increase series inductance, degrade sig- nal integrity and increase crosstalk and electro- magnetic radiation. Four factors must be considered in order to mitigate the RPDs: 1. Recognize the impact of RPDs. 2. Understand the importance of referencing. 3. Identify the location of the RPDs– path of least inductance. 4. Take corrective action to mitigate the RPDs. 1. Recognize the Impact of RPDs Ground impedance is at the root of virtu- ally all signal and power integrity problems–low ground impedance is mandatory for both. This by Barry Olney IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Return Path Discontinuities BEYOND DESIGN

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