DesignCon 2002

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A fundamental assumption of both QuickEye and VerifEye is that the drivers and channel are linear time-invariant systems. QuickEye and VerifEye are based on the principle of superposition, which makes use of this assumption [6]. The step response is the waveform at the far end of the channel to a single transition at the input, from low to high or high to low. To obtain the response of the overall channel, individual step responses for rising and falling edges are superposed. The result is the channel response shown in Figure 8. Figure 7: Intermediate QuickEye waveforms for the RC example. Rising edges are shown as solid curves; falling edges as dashed. When all these transitions are added together, we get the waveform in Figure 8. While it is relatively easy to calculate the effects of ISI with the QuickEye algorithm, random transmit jitter is much more difficult. The most accurate method is to do a Monte-Carlo method: each transition is displaced a random amount from the nominal time, depending on the probability distribution of the random jitter, but this greatly increases simulation time, as there needs to be at least several different jittered transitions for each bit in the original stream.
Figure 8: The response of a given channel. Equalization Requirements Several equalization techniques exist to help combat inter-symbol interference (ISI) caused by the channel. More often, long channels at high data rates in the Gb/s are beginning require feed-forward equalization (FFE) followed by a non-linear decision feedback equalizer (DFE). Typical equalization architectures can be seen in Figure 9. These examples were directly implemented in the simulation tools. The FFE assists in sharpening up the edges (pre-cursor) while the DFE utilizes decisions to assist in removing the interference from previous bits (post-cursor). The computation of the tap weights is indeed an important step, however, an equally important step is the determination of the minimum number of taps required to provide an open eye that yields acceptable BERs. Equalization requirements will be shown for both pre-layout and postlayout as well as for the measured channel.
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DesignCon 2002

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