Teledyne LeCroy AORM - Advanced Optical Recording Measurements - Manual (Page 96)

94
ISSUED: June 2013 923133 Rev A
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 appendix 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:
• A fast edge usually results in more accurate timing measurements because of
interpolation algorithms, as long as points are sampled along the edge.
• A fast edge provides a threshold crossing time and, therefore, measurement accuracy
more immune to noise.
• 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.
• 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).