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Signal Integrity and EMI

July 22nd, 2015

Signal Integrity refers to minimization in the degradation of high speed signal as it travels from one point to another. EMI on the other hand, is concerned (mainly) about the radiation from high speed signal ( and of course, from the associated return path), as it travels from one point to another.

A signal may be bad from Signal Integrity perspective but may still not be radiating to the extent of failing the specification. However, a set of causes that create a Signal Integrity issue also cause EMI Issue. In this article we will examine these set of common causes. Fixing a common cause results in improvement is Signal Quality as well as reduction in EMI issues. There is yet another set of fixes, that attempts to fix the EMI issue, but degrades the Signal Quality. We will examine these issues as well in this article.

Gap in Return Ground Path

Sometimes we create a gap in the Power or the ground plane which are used as return path for high speed signals. This creates an impedance discontinuity for high speed signal and creates Signal Integrity issue. It also creates and Antenna for the High Speed Signal and creates EMI Issue.

Ground and Power Plane Separation

We all know that the high frequency noise on Power Supply affects the Signal Quality. One way to reduce the high frequency ( close to 1 GHz) noise is to reduce the separation between the power and Ground plane ( as low as 2 mils). The reduction in separation increases the Capacitance between Power and Ground plane and not only improves the Signal Quality but also helps in EMI issues.

Split in Power plane

A split in power plane creates a temporary impedance discontinuity. This degrades the signal quality and also creates EMI issue by spreading out the signal. To fix the issue, reduce the separation between the split planes to as low as 6 mils and use stitching via capacitors.

Cross Talk

The Cross talk is a problem in Signal Integrity where the Signal Quality in the victim net is degraded by the coupling from adjacent net. In EMI this issue is created when is high speed net couples a victim net that goes on a connector and then to a cable.

Some fixes for EMI / ESD issues degrades the quality of the Signal. We use common mode chokes right at the connector to reduce radiation through the cable. However, the selection of right kind of common mode choke is essential and we must know the extent to which it will degrade the quality of the Signal.

Similarly, components used for ESD protection at the connectors, also degrade the quality of the signals. We must be aware of the extent to which it will degrade the signal qualty.

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