Question: Why does the deterministic jitter (DJ) seem to disappear when I measure longer patterns using a bathtub jitter method?

Answer: Bathtub jitter measurements (also known as BERTScan, and Jitter Peak on a BERTScope) are the most accurate way of measuring the total jitter (TJ) at deep BER levels, according to MJSQ[1]. Longer patterns such as PRBS-23 and PRBS-31 have jitter-inducing bit sequences that appear very infrequently, and so can look like random jitter (RJ) fooling the dual-Dirac math. If possible, an alternative is to measure a clock pattern (such as 1100) to get an accurate RJ/DJ separation. Then, using the TJ measured on the long pattern, subtract the RJ result from the clock pattern to arrive at the long pattern dual-Dirac DJ.

More information on Bathtub jitter measurements can be found here.

[1] MJSQ: Methodologies for Jitter and Signal Quality Specification is a document written as part of the INCITS project T11.2.

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Q: If you wear polarized sunglasses while driving, you have probably noticed the large spots, usually arranged in patterns, on the rear windows of other cars. What are those spots, and why do polarized sunglasses reveal them?

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     — Harry S Truman
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