LeCroy SDA, Руководство оператора

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SDA Operator’s Manual
When DDj Calc. Method is set to ISI Plot , the SDA uses a patent-pending method to measure
both forms of data dependent jitter. The method uses the history of a number of bits in the
waveform to determine their effect on the transition of a given bit. A user-selectable number of
bits (from 3 to 10) is used in the measurement. The acquired waveform is processed in segments
the size of the selected number of bits. For example, if 5 bits are selected, segments 5 UI long
are examined. For each segment, the value of the 5 bits is determined and each group of 5 bits is
averaged with segments of similar value. When the entire waveform is scanned, a set of up to 32
(for 5 bits) waveforms are created. The averaging process removes all random jitter, noise and
periodic jitter from the segments. The waveform segments are overlaid by lining up the first
transition of each of the segments and measuring the relative timing of the transitions to the last
(5th in this example) bit.
Figure 14. DDj measurement procedure. Averaged waveform segments for each pattern in
the data stream are overlaid by lining up the first data transition. The curve above shows
all of the transitions between the next-to-last bit and the last bit on the right side. The DDj
is measured by examining the width of this crossing point at the selected threshold level.
Periodic jitter is measured by examining the spectrum of the trend of TIE values. The time interval
error is measured for each edge in the data stream. Where no edges are present, as is the case
for consecutive “1” or “0”’ values, edges are inserted at the expected data transition times. These
inserted edges do not add any additional jitter since they are placed at the ideal edge locations
for the data rate. The trend of TIE is continuous and the spectrum can be computed. Periodic
jitter is the complex sum of the spectral components excluding those associated with the
repetition frequency of the data pattern and its harmonics.
The deterministic jitter is computed by adding up the periodic (Pj) and data dependent (DDj)
components. The random jitter is computed using Equation 1 and subtracting the measured
deterministic jitter from the total jitter at the selected bit error rate.
SDA-OM-E Rev H
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