Notes On Using Head/Analog Compare - LeCroy WaveMaster 8600A Operator's Manual

signal for analysis.
Bit Cell Time: If specified, will be used to determine window size for comparison, otherwise a
default value is used.
Code Rate: If provided, this is used to determine the size of an encoded byte, which is needed in
determining the window size for Analog Compare.
Filter Head Signal: If enabled, an equalization filter is applied to the head signal before processing
and display. This feature should be used if the DDA does not have access to the drive's equalized
head signal.
After the setup is complete, a reference waveform should be acquired and saved by touching Store
Reference.

Notes on Using Head/Analog Compare

1. Setting Analog Threshold: The "Analog Threshold" sets how large the difference (mean
squared distance) between the reference and the acquisition must be to be recorded as an
"error". A threshold of zero means that even the slightest difference will be enough. The
threshold is normalized to full scale, which means that a setting of 1.000 specifies that the
signals must mismatch by full scale over the entire three byte wide analysis window to be
recorded as an error. DC mismatch is compensated and does not contribute to "mismatch".
If the waveform amplitude can be -full scale to +full scale, the formula relating threshold to
number of divisions mismatch (without vertical zoom) --- average over the analysis window
--- is: sqrt(thresh). For example, a threshold of 0.015 corresponds to 0.49 of a division
average mismatch within the window.
2. Analog Compare works best if it can analyze VCO Synch at the beginning of its analysis
region for both the reference and the head signal. If Read Gate is available it should be
able to do that. If Read Gate is not available, the "Analyze Region" cursors can be turned
on and the "start" marker positioned near the beginning of a VCO Synch field. This also will
let Analog Compare analyze the VCO Synch field and align reliably. If Read Gate is not
available, and VCO Synch is not available, then Analog Compare attempts to align the
signals up to about 12 bit cells difference. Note that if the start of the analysis region is far
from the t=0 (trigger) position then small speed variations can cause the waveforms to be
too misaligned for Analog Compare to be correct. For example, if the start of the analysis
region is 15,000 bit cell times after the trigger, a 0.1% speed variation gives 15 bit cell times
of jitter at the start of the analysis region. No matter how it achieves alignment, Analog
Compare attempts to maintain alignment for up to a 1% speed mismatch. Analog Compare
compensates for misalignment in the reference and acquisition based on the first Read
Gate edge (if a source of Read Gate has been specified), and if it can analyze VCO Synch,
on the end of the VCO Synch in that first section. If Read Gate is being used and the
reference contains multiple Read Gate true sections, then the timing between them in the
reference and the acquisitions must match within the alignment tolerance of Analog
Distance, about 12 bit cells.
3. The reference waveform must be the same as the waveforms it is compared to. That
means it must be the same length, acquired at the same horizontal and vertical scales. The
same regions of the new and reference waveform will be compared. That means it is not
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