LeCroy DDA 7 Zi series, Operator'S Manual

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WavePro 7Zi
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WP700Zi-OM-E-RevA
errors for peak detect signals. It can also be used for PRML signals, although Channel Emulation with a reference
signal is the recommended method if a reference signal is available. See
Using Analog Compare for a full
description of this method.
Channel Emulation without Reference
Since it does not require a reference, Channel Emulation without a reference signal is the easiest of the channel
analysis methods to use for PRML signals. This method performs channel emulation to determine the locations in
the head signal that gave the Viterbi detector the most trouble in its decision as to the maximum likelihood
sequence of samples. This means the state at a particular time, in the final surviving sequence, survived by the
smallest margin. (The "state" corresponds to deciding on a "1" or "0" bit at each position.) The margin between
keeping and rejecting what turned out to be the final choice for the state at that time is called the Sequenced
Amplitude Margin (SAM). Unlike the compare methods, Channel Emulation without reference is based only on
analyzing the quality of the head signal. It is generally assumed that the quality of the head signal is strongly
correlated to the bit error or errors in nonreturn to zero (NRZ) data. Head signal sections that produce incorrect
NRZ data are much more likely to be marginal when analyzed by the Viterbi detector. The most marginal sections
are the first flagged by Channel Emulation as having the worst SAM. See
Channel Emulation Without Reference
for a full description of this method.
Sections with no signal transitions are one exception to this rule. These may not be identified as problematic by
the Viterbi detector. However, if the Limit Run Length feature is enabled, sections with no transitions will be
identified as errors if they exceed the run length limit.
Because it is the easiest of all methods, Channel Emulation without reference is generally the best to use first.
Channel Emulation with Reference
Channel Emulation with a reference is very useful if the error being looked for occurs at least fairly frequently and
a reference head signal is available. If the error is not "hard," that is, it does not occur all the time, any acquisition
may be stored as the reference; Channel Emulation with reference will catch acquisitions for which the DDA’s
channel emulation detects a different bit sequence. If the error is hard, a separate reference must be available. To
have no errors detected, the reference must have the same bit pattern as the acquisitions with which it will be
compared. This means each sector of the reference must be either unsplit or split in the exact same place as the
matching sectors in the acquisitions with which it will be compared.
Channel Emulation without reference must assume that the final surviving sequence of bits is correct and only
points out poor quality in the head signal. But Channel Emulation with reference has a reference "correct" path
and will catch differences even if both the reference and the comparison acquisition are high quality PRML
signals. For Channel Emulation with a reference, the SAM values can be negative, indicating that a different
decision was made (less than 0 margin to the "correct" state). The threshold below which an "error" is flagged is
adjustable down to -1, to permit you to flag places where the DDA’s channel emulation determines a different bit
value for the head signal as compared to the reference head signal. See
Channel Emulation With Reference for
a full description of this method.
Channel Emulation without Reference
Channel Emulation without Reference finds a single trace and predicts the bits where the Sequenced Amplitude
Margin (SAM) -- the distance or margin the Viterbi detector has for making a decision -- is poorest. It uses a full
disk drive channel emulation to indicate how the signal ought to appear when a good reference signal is not
available. It measures the SAM of all the samples (PRML clock locations).
The software emulates a PRML channel and ranks errors by SAM value. A distance or SAM value of "0" indicates
no margin for a decision and the detector’s lack of certainty as to whether the digital bit should be "1" or "0." The
positions of the 100 worst margins are identified and can be displayed along with the SAM value of each.
Using complete disk drive channel emulation, Channel Emulation without reference predicts where the head
signal quality is the poorest in respect of a PRML channel’s ability to confidently select a "1" or "0" value.
Channel Emulation without reference starts by finding the beginning of the sector. The algorithm looks at the head
signal beginning at the Read Gate true transition (or analyze region start if Read Gate is not available) and tries to
synchronize to the VCO Synch pattern in order to establish sampling phase and expected sample levels. To
accomplish this, it is required that
VCO Synch Pattern be set correctly, and that the "Bit Cell Time" be
approximately correct. The data is then passed through the emulated channel where it is appropriately sampled.
The sampled output enters the Viterbi detector, which chooses the "sequence" of bits (history) that is the most
likely when the new bit due to this sample is appended. The difference between the mean squared distance (msd)