LeCroy DDA-3000A, Operator'S Manual

Page: 353 / 437

X-Stream Operator’s Manual
WM-OM-E Rev I
351
signal equalization, automatic gain control (AGC), phase lock loop (PLL), sampling, and Viterbi
detection.
For each PRML sample, a quality sequenced amplitude margin (SAM) benchmark is determined.
The lower the value of SAM the greater the difficulty for the drive channel to produce the right data
value for that sample, and the less margin there will be. A SAM value of less than zero indicates
that an incorrect data value will be selected at that sample.
PRML signals characteristically have a set of target-level values that the signal samples should
meet at the sampling times. For example, for a PR4 signal, the targets are
1 0 -1 . In order to
achieve these desired levels, the DDA’s channel emulation equalizes the head signal, either
automatically or using selected values for -3 dB cutoff and boost. The equalized head signal is then
displayed. This lets you examine the head signal after it is processed, when it will have the
characteristic PRML waveshape.
The distance of the waveform samples from the PRML target values is a first-order indication of the
quality of a PRML signal. The channel emulation annotates the head signal after equalization with
the target values. This makes visual interpretation of the quality of the head signal possible and
intuitive. As a result, you can visually inspect the head signal to an extent beyond the more obvious
indications of problems such as thermal asperities and dropouts.
In addition, several powerful ways are available for selecting how the section of the equalized,
annotated head signal will be viewed:
•
You can scroll through the head signal in the traditional time sequential mode using the
Auto Scroll feature, which provides "hands free" scrolling at a rate and in the direction
that you specify.
•
You can select which part of the head signal is to be displayed by byte number. This is
particularly useful if a data error is known to exist at a particular byte location.
•
You can review the head signal in order of poorest SAM value. This capability is
particularly practical because the areas of poorest quality are generally of greatest
interest.
The ways in which Channel Emulation can be operated are with or without a reference signal, and
Stop on SAM (on/off).
With or Without Reference
Using a reference head signal, where available, provides two major benefits. First, the head signal
under analysis can be viewed with a head signal that has a reference for improved interpretation of
waveform misshapes. The DDA will equalize both reference and head signals. It will also auto-align
the two signals when selecting the head signal to be viewed by SAM --- even if they were captured
at different spindle speeds (up to 1%).
An additional benefit is that the SAM calculation can then assume values less than zero and
indicate a data error likely at that point. Without a reference signal, it would be impossible to
determine whether an error occurs for a particular sample, only the confidence (SAM) of the Viterbi
detector in selecting between a data ‘1’ or data ‘0’ for a particular sample. In this case, the minimum
confidence level possible is zero SAM, indicating no confidence in making a selection. The correct
data can be specified with a reference, which allows SAM values of less than zero to be detected