Stop On Sam; Analog Compare - LeCroy WaveMaster 8600A Operator's Manual

when the signal quality is so poor that an incorrect data value would be selected by the DDA's
Viterbi detector.

Stop On SAM

The second selection mode of operation is Stop on SAM. When this mode is enabled, the DDA is
placed in what is essentially a test mode for PRML signal quality based on SAM. When in Normal
acquisition mode, the DDA will continuously acquire and analyze head signals until it finds a PRML
sample value with a SAM value below a user-specified level. When this occurs, acquisition will be
stopped, and the user can directly view the locations below the selected SAM threshold. This mode
of operation is particularly useful for capturing intermittent errors.

Analog Compare

Often the head signal locations requiring examination are a header or other non-PRML data section.
You may wish to compare these signal sections to a reference signal in order to obtain visual clues
to possible problems. However, several issues may have to be addressed before this can be done.
The reference and head signals might differ in time due to spindle speed variations. This could
make alignment and comparison of the two signals difficult.
Another issue is that differences may be so subtle that they are not very apparent --- especially so
if the signals being compared are lengthy.
A third issue is that the problem may be intermittent.
What is generally required for addressing all three issues is an automatic comparison method that
adjusts for spindle speed variations and identifies where the two signal differences are the greatest.
In the case of intermittent problems, comparisons should be made continuously until a difference
greater than a selected threshold is seen. This is just what the Analog Compare feature provides.
With Analog Compare, you select a head signal to act as the reference, and it gets stored in
memory. The maximum allowable difference between the two signals is then selected. Analog
Compare automatically aligns the two signals and identifies where a mismatch occurs. It also
counts each mismatch, storing its byte location for further review. Up to 100 mismatches can be
identified and stored in largest-to-smallest order.
As a general-purpose test method, Analog Compare can be applied to finding problems in
practically any signal, including other head signals.
General Steps of Analog Compare
1. Tell the DDA the signals you are providing: Identify in the Channel Setup the signals
that are being provided and their source --- the particular input channel or memory. Also
identify the section of the head signal you want analyzed.
2. Set up the channel characteristics: Each of the Channel Analysis methods has a
required set of head signal characteristics that need to be provided in order to perform the
necessary analysis. These may include bit cell time, code rate, PRML type, and other
characteristics.
3. Select a channel analysis method: Specific configuration requirements differ depending
on the method used. See "Setting up for Using Drive Channel Analysis" in the Disk Drive
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