LeCroy DDA 7 Zi series, Руководство оператора

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Operator's Manual
WP700Zi-OM-E-RevA
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The value displayed on the custom parameter line (with statistics off) is the sigma of any of the resulting
distributions for the last acquisition only. This timing jitter value is calculated internally without having to actually
histogram the values. The values displayed in nT Table mode are the sigma of the width distributions resulting
from each nT distribution for the last acquisition.
Note: Timing Jitter is always less than or equal to the standard deviation of superimposed Edge Shift
distributions.
Signals, Coupling, and Threshold Settings
Which optical recording signal, or combination of signals should be used in a calculation? How should the signal
be coupled, or the threshold set? The answers to these questions are sometimes uncertain. This section offers
tips on how to answer them.
Choice of Signals
Generally, the choice of signals depends on the aim of the measurement. For example, if the quality of the signal
direct from the media is being examined, generally the signal at the output of the photodetector should be used.
Alternatively, a conditioned signal could serve the purpose.
A “sliced” or logic conditioned signal should normally be chosen when precise timing measurements are desired
and propagation delay through the logic device (comparator) is not an issue. Timing measurement accuracy is
improved when a fast signal is used, as opposed to the slower signals at the photodetector, for the following
reasons:
x
A fast edge usually results in more accurate timing measurements because of interpolation algorithms, as
long as points are sampled along the edge.
x
A fast edge provides a threshold crossing time and, therefore, measurement accuracy more immune to
noise.
x
The use of the signal at the output of a logic device or comparator decreases the sensitivity of the
measured threshold-crossing time to the exact value of the threshold level selected.
x
The use of the signal at the output of a logic device or comparator typically solves other threshold problems
as well, in systems that dynamically adjust the threshold based on the optical recording data signal. Sliced
or logic signals facilitate the use of a fixed threshold.
Coupling
DC coupling is required only for measurements of absolute DC values. Measurements requiring it include ptop ,
pbase , pmin , pmax , and pmoda . Otherwise, AC coupling is best used on signals that are not outputs of logic
devices or comparators: those that might have varying thresholds.
Threshold Selection
If DC coupling must be used, there are some further considerations for threshold selection. While all of the optical
recognition measurements specify thresholds used to extract the pits/spaces (by recording threshold crossings),
there is a variance in the sensitivity of parameters to the exact threshold value selected. The sensitive parameters
are those that are time related or whose values depend on the exact time of the threshold crossing. Those
insensitive to the exact threshold value are parameters that use the threshold crossing time only to categorize the
parameter result according to width (that is, they use the crossing time only to find the width for determining the
nT index to which the pit/space belongs).
In the case of threshold-insensitive parameters, it usually suffices to use a fixed threshold somewhere in the
middle of the optical recognition waveform. Even if the signal’s middle shifts, the fixed threshold is usually
adequate.
Additionally, if the signal is AC coupled, it will tend not to shift much, and the fixed threshold will be perfectly
adequate.
The problem arises when what is required is a DC-coupled signal with a threshold that changes dynamically
throughout the waveform. There remains a possible solution, but the scope setup is slightly more complicated.
Consider the fact that AC coupling can be regarded as rejection of the DC component of a signal, or subtracting it
from the signal. In many systems, the threshold is determined in precisely this manner by applying a low-pass
filter to the signal, and then applying this value, with the signal itself, to the input of a comparator. If a threshold