LeCroy SDA, Operator'S Manual

Page: 382 / 397

Comparing Models
The plot in Figure 15 shows the bathtub curves for the measured values, as well as both
estimates. Viewed in this way, it is clear why both the effective and direct measurements for Rj
and Dj are used. Both estimates arrive at the same total jitter at the specified BER (10-12 in this
case) but they give different values of Tj at other BER values. The effective jitter values give a
very accurate prediction of total jitter for bit error rates below about 10-10, which is where they
are fitted. The direct measurement underestimates the total jitter at error rates below the specified
one and overestimates the jitter above this. Note that the effective parameters underestimate the
jitter for high bit error rates. The three curves in Figure 15 show the resulting bathtub curves from
the measured signal (blue line) and the two models: Hj(BER) for the direct Dj method (red line)
and Hje(BER) for the effective jitter method (green line).
Figure 15. Measured jitter bathtub curve (blue curve) based on the extrapolation of the measured TIE
histogram. The red and green curves are the estimated bathtub curves based on the direct Dj and
effective measurements respectively. Note that the direct Dj method underestimates the total jitter
below the BER at which it is computed.
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