DesignCon 2002

transitions due to jitter, it is possible to generate the information needed in much less
computation time.
First, consider a particular point on a given waveform passed through a channel. That
point is the sum of all the voltage excursions caused by the various transitions from high
to low or low to high at bit interfaces that have occurred prior to the time point in
question.
Rather than waiting for the voltage excursions for all possible combinations of bits and
then looking to see which combination caused an error, the process is shortened through
the calculation of a probability distribution for the sum of all the excursions.
Perhaps it is easiest to consider this concept in terms of what are known as “cursors”; the
deviations from the ideal waveform.
Figure 4: The two largest cursors from the step response of the RC example.
If the channel were perfect, then the response would be identical to the input; the voltage
at each point in time would be either +1V or -1V, depending on the bit in question.
Referring to Figure 4, imagine what the waveform would look like if there were a very
large number of 1s in a row, versus a large number of 0s, followed by a transition to a 1.
In the first case, the waveform would have time to settle to its correct final value of +1V.
In the second case, the waveform would be in transition from -1V to +1V because of