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Method to quantify coupled noise injected at transmission line discontinuities

IP.com Disclosure Number: IPCOM000200322D
Publication Date: 2010-Oct-05
Document File: 4 page(s) / 478K

Publishing Venue

The IP.com Prior Art Database

Abstract

This proposal describes a method to quantify the noise injected at via transitions caused by neighboring vias, by taking into consideration its proximity and switching timing. It can be used during the wiring phase of a module or card to minimize the noise injected at via transitions. This is accomplished by adjusting the signal delays so signal transitions concurrently at the via transition; in this manner, the noise injection occur at the time the signals are least likely to cause their eye to narrow.

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Page 01 of 4

Method to quantify coupled noise injected at transmission line discontinuities

It is a known fact that a via transition degrades the eye opening of signals. A via is also exposed to EM noise caused by neighbor vias, which further degrades the signal's eye.

The typical eye of a high speed signal is illustrated in figure 1. This plot is obtained when a given signal is plotted repeatedly over the length of a bit time. The Bit Transition is when the signal switches from one to zero or vice versa. The actual eye is the open area left between two transitions. If this area becomes too small, the receiver circuit might not be able to determine the actual state of the signal.

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Figure 1

When noise is injected into a signal, it might further degrade the eye opening if the noise event occurs during the portion of the bit time where the eye is defined. In the same manner, if the noise event occurs at the time of the bit transitions, its impact on the actual eye is reduced, as it coincides with the time the signal is itself changing value.

The eye degradation caused by noise injected at a via transition by a neighbor signal will depend on several factors: Signal amplitude, Rise Time, Via Distance, and so on, parameters which are known. By doing repeated electrical models, the relative switching time of a neighbor via can be varied to get a plot similar to the one in figure 2:

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Figure 2

The relative skew between two signals was varied and represented as a percentage of bit time. So when there is no skew between aggressor and victim, it corresponds to 0% on the X-axis. A 100% skew corresponds to a 1 bit time skew which will be the same as the 0% reading. The vertical axis plots the amount of eye closure in ps, from the undisturbed case. As it can be discerned from the plot, for this particular topology there is a penalty three times larger to the eye opening if a transition occurs near the center.

One more parameter that must be considered to determine the eye degradation caused by the noise injection from a neighbor via, is the proximity to such via. It can be shown that the amplitude of the noise injected by a neighbor via decays exponentially as the distance increases. Taking this into account, there is a maximum distance where the noise injected is small enough to be ignored, in this way, there is no need to calculate the noise contributions of all vias in a given area, but only the noise form those that are closer than this maximum limit.

The skew between two signals at the point the via transitions is measured from the location where the signals are launched by their driver circuits, usually from the pin of an IC chip. Although there might be multiple via transitions between a driver circuit in one IC chip to a receiver circuit in another IC chip, not all of these transitions degrade the signal equally. Blind vias...