X-Stream Operator’s Manual
WM-OM-E Rev I 415
Parameter
Settings
When you select nlts from the PRML dialog's With parameter field (DRIVE
ANALYSIS Æ Measure Æ Parameter Set Æ PRML) an nlts Pattern Length field
and pattern Delay field appear also. You can adjust the mantissa, exponent or
number of mantissa digits using the pop-up numeric keypad. The pattern length
should be set to the pattern period.
Although the DDA searches for the correct pattern length, the value provided
needs to be sufficiently close to the actual pattern length for nlts to perform the
search. A 1 µs pattern may, for example, accept a range of 1 µs +/- 40 ns. Within
this range, a value for nlts will be provided. Otherwise "---" appears on the screen,
indicating that no measurement can be made.
The pattern Delay setting is a percentage of the pattern length. The DDA will
internally scale the delay value entered by the ratio of the pattern length calculated
internally to the pattern entered by you. Several disk drive waveform attributes can
be measured by using different delay values. The following table provides delay
values to enter for the commonly used 127-bit pseudo-random sequence (x
7
+ x
3
+ 1 polynomial) when measuring various waveform attributes:
Waveform Attribute
Bit Cell
Location
Delay (%)
Adjacent Location 25.5 20.08%
2nd Adjacent Location -30.5 -24.02%
Initial Magnetization 45.5 35.83%
Interaction Interference -60.5 -47.64%
Notes
The pattern Delay tells the DDA where in the repeating pattern to measure NLTS.
NLTS is calculated from the correlation coefficient at that time. Correlation is
calculated at 3 sample times: the nearest sample time and one on each side. A
curve fit is performed to calculate a better estimate of the true peak height.
As described above, the nlts parameter only requires one waveform. Each
acquisition must contain only the pseudorandom repeating sequence (PRS), not a
servo wedge or a preamble. It does not matter where in the PRS the acquisition
begins; its echo properties are independent of starting point. There must be at
least two repetitions of the pattern in each acquisition. A reasonable number
would be about 25 repetitions (32 repetitions of a 127-bit sequence should fit in a
sector).
NLTS requires at least 5 samples per PW50 for acceptable accuracy. More is
better.
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