LeCroy DDA 7 Zi series, Руководство оператора

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WavePro 7Zi
341
WP700Zi-OM-E-RevA
exact. On a noisy or distorted waveform, it may be that some peaks are miscounted, but as long as some are long
and some are short, the final total will be nearly correct. Finally, T is computed as:
(time at the last transition - time at the first transition)/(total n between them)
If there are 2000 edges, an average of 4T apart, the separation between first and last edge is 8000T. If our count
of n is off by 1, that is a 0.0125% error. We can tolerate up to 7 counts error (0.0875%) before the PLL will not
start locked. When the waveform is correctly equalized, this does not happen.
A highly asymmetric waveform will not have clean peaks in the distribution of its pulse widths, which also means
that many of the pulses will be nearly (n + 0.5)T. On such a waveform, we may not be able to determine T. The
possible reasons for failing to determine T (and therefore the VCO start frequency) are:
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Less than 50 edges in the waveform.
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Could not distinguish the first three peaks in the distribution of widths.
As mentioned above, you should sample at about 20x to 50x the clock frequency to make clock extraction work
reliably.
An attempt is made to start the VCO not only at the correct frequency but also at the correct phase. The phase is
pre-set such that the first edge in the waveform will occur on a falling edge of the VCO output. The first edge is
just as likely to be out of place as any other edge in the waveform, of course. If the VCO starts significantly at the
wrong phase it will either slow down or speed up for a short while until it gets to the right phase. A JitterTrack
shows this clearly. On a 4x DVD waveform we captured, which just happens to have a significantly out-of-place
first edge, the frequency is disturbed slightly for the first 15 μs or so; the frequency shift during this time is very
small, on the order of 0.1%, as the phase adjusts.
Introduction to 8B/10B
The new LeCroy Serial Bus Analyzer ("SBA" option key) features:
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Powerful search capability allows captured waveforms to be searched for a user-defined sequence of
symbols.
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Multi-lane analysis decoding of up to four simultaneously captured lanes.
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Waveform can be analyzed by other protocol analysis software packages (future capability)
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When combined with the high bandwidth capabilities of the SDA11000, this software option permits
symbolic decoding of data streams up to 10 Gb/s.
Description of Encoding and Decoding
In real-world transmission systems, there are several techniques to prevent long strings of zeros or ones from
occurring in the data stream. 8B/10B encoding and decoding is one of those techniques.
Here is a very simplified picture of what might happen when a long string of ones has a single embedded zero in
it. The waveform that results from this data stream may not make it to the threshold crossing in time to be
sampled.
When 8B/10B encoding is applied, every 8 bits are mapped into 10 bits so that long and short runs of zeros
and/or ones are avoided. In this case the new waveform might look something like this:
Running Disparity
The Running Disparity (RD) monitors the percentage of "1s" present in the message relative to the percentage of
"0s". In order to “compensate” for an increasing number of 1s relative to 0s, the system keeps track of the
Running Disparity, such that when the running disparity is “positive” (high percentage of 1s at the transmitter), the