LeCroy SDA, Operator'S Manual

Page: 326 / 397

The SDA measures jitter by determining the timing error between the edge crossings of the data
signal under test and a reference clock, which can be derived either from the data or an external
clock. The measurement can also be performed directly relative to the edges of the external
clock. A flow diagram of the SDA jitter measurement is shown as follows:
Total jitter is derived from the histogram of timing error measurements. The histogram of
measurements is accumulated and the tails of the distribution are extrapolated to form an
estimate of the histogram for a population of 10
16
measurements. The extrapolated histogram is
then integrated to determine the cumulative distribution function which forms the sides of the
bathtub curve. The bathtub curve gives the eye opening (or 1UI - Tj(UI)) for all bit error rates
down to 10
-16
. The total jitter for a given bit error rate is found by measuring the width of the
bathtub curve at the y-axis value corresponding to the desired bit error rate. The periodic jitter
and data dependent jitter are measured directly on the sequence of timing error measurements,
and the random jitter is found by measuring the difference between the total jitter and the
components of deterministic jitter.
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