LeCroy SDA, Руководство оператора

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Figure 13. The change in total jitter with BER is represented by the values Tjn in the total jitter curve
on the left. These values are plotted vs. bit error rate in the upper line in the chart on the right while
the lower line shows the variation vs. BER for a pure Gaussian. Rje and Dje are chosen such that the
curves line up.
The jitter computed using this method allows equation 1 to accurately model the jitter behavior of
systems as a function of bit error rate. This model is especially useful when computing jitter
margins in system applications.
Direct Measurement of Deterministic Jitter
Deterministic jitter can be completely characterized by measuring the threshold crossing times of
the data signal over a finite time period. The two classes of deterministic jitter are periodic and
data dependent.
Data dependent jitter is caused by system effects that are dependent on the data pattern. A
common source of data dependent jitter is the frequency response of the channel through which
the serial data signal is transmitted. In this case, data patterns with many transitions (such as a
101010… pattern) contain more high frequencies in their spectrum than patterns containing fewer
transitions (11001100…, for example). The patterns with higher frequency content will be
attenuated and phase shifted relative to the lower frequency patterns. In addition to data
dependent jitter, the rise and fall times of the data bits can be different. The detection threshold in
the receiver is normally set to the 50% amplitude (midway between the “1” and “0” levels) so
unequal rise and fall times will generate jitter. This type of jitter is known as Duty Cycle Distortion
(DCD).
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