Teledyne LeCroy SDA II - Manual (Page 31)

Operator's Manual
Cutoff Freq (1) - Shown when any filter type is selected.
Cutoff Freq (2) - Shown only when the Bandpass Filter Type is chosen.
Pattern Analysis
The DDj is measured by finding the average crossing time for each edge in the data pattern and com-
paring this to the nominal crossing time, based on the expected bit rate of the signal under test. This
measurement requires a repeating data pattern to operate. If the data pattern in the signal does not
repeat at least once in the acquired data record, the DDj parameter will indicate zero even though this
type of jitter is present. A graphical display of the DDj can be viewed by clicking the Show DDjPlot button
on the Pattern Analysis mini-dialog. This display shows the position error of the average crossing point
for each edge in the pattern. A digital representation of the pattern can also be displayed on the screen
by clicking the Show DigPatt button on the Pattern Analysis mini-dialog. Having the Digital Pattern and
the DDjPlot on the screen at the same time is useful to be able to identify exactly where in the pattern
DDj is found. Additionally, a histogram of all of the DDj values can be displayed by clicking on the Show
DDj Histogram button.
Another view that is available is the ISI Plot. This plot shows an eye pattern consisting of the averaged
waveform trace from each pattern in the data stream of a specified length. This length can be adjusted
from 3 to 12 bits. This averaging removes the effects of random jitter in the signal. What is left is an eye
diagram with all noise and random jitter removed. All that remains is the Intersymbol Interference (ISI)
and the Duty Cycle Distortion (DCD). Additionally, a specific pattern can be selected to be highlighted on
the ISI Plot.
A repeating data pattern is not always present. In these cases an alternate method of measuring data
dependent jitter that does not rely on a repeating pattern can be used. In this alternate method, the
average TIE for is calculated for edges with similar patterns of preceding N bits. This method is turned on
by unchecking the Repeating Pattern check box.
Pattern Analysis Dialog
Show DDjPlot - Check this box to display the DDj Plot.
Show DigPatt - Check this box to display the Dig. Pattern.
Show DDj Histogram - Check this box to display the DDj Histogram.
Show ISIPlot - Check this box to display the ISI Plot.
Find Pattern - Touch to automatically find the pattern.
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