Time/Div Settings - LeCroy WaveMaster 8600A Operator's Manual

X-Stream Operator's Manual
differences between the reference and acquired signals due to the presence of the scrambler and
not because real problems exist. If a scrambler is not present, it is reasonable to use other drive
sections from the same zone as a reference.
The head signal can be stored as a reference simply in order to have a stored trace automatically
overlapped with later acquisitions. This can be done even if the selected channel analysis method
does not require a reference, or if you do not intend to use the method. This might be done just to
scroll through waveforms using Auto Scroll to see how well they match.
Changing the source of the Head Signal or Read Gate invalidates the stored reference. You must
store another valid reference for the new setup.

Time/Div Settings

The DDA allows you to keep acquiring a head signal until it finds a problem, at which point it will
stop the acquisition so that errors can be analyzed. In order for Drive Channel Analysis to operate
properly the DDA trigger must be set up so that the correct waveform section or sections are
reliably acquired. The trigger setting must be selected to obtain the desired data on the display, in
approximately the same position, in each acquisition.
In addition, the time/div must be set properly. Two factors should be taken into account when
setting this. First, the time/div should be set to a sufficient duration so that the head signal sections
that will be analyzed are captured. Second, in order for the DDA to correctly analyze a head signal
it should have at least five ADC samples per bit cell.
For drives without ID, Read Gate only goes true to read the data of interest. If you can manually
initiate a read operation of the desired data, this is easy: trigger on the leading edge of Read Gate,
and every time the data is read, the DDA will trigger. Wait for the channel analysis to be done (the
instrument rearms if no problems are found) before performing another read.
A precaution should be taken if more than a single Read Gate block of data is to be analyzed. For
this situation, if reads are being performed repeatedly it is important to ensure that the trigger mode
selected is not one that triggers solely on the occurrence of any Read Gate. Otherwise, the Read
Gate block triggered on might sometimes be the second or later Read Gate true section.
If the drive provides Sector Pulses at the beginning of each sector it is recommended that the
DDA's Sector Pulse trigger be used to avoid this problem.
Otherwise, a SMART Trigger® can easily solve this problem, by triggering on the first (or nth) Read
Gate edge after Index.
If Read Gate is not available or its use is not desired, but looking at the head signal is, try triggering
on Index and using a trigger delay to move out to the region of interest. Use the Analyze Region
markers to delimit the area to analyze: for channel emulation methods the start marker should
always be near the beginning of the VCO Synch field. However, even 0.1% speed variance
becomes 0.8 s of jitter after 8 ms of delay. That is probably several hundred bit cells --- far more
than Analog Compare can compensate for --- which may cause the channel emulation methods to
miss the VCO Synch field entirely. Therefore this technique will not be able to look at sectors more
than 1 or 2 ms delayed from Index.
If Read Gate is unavailable and you want to look at something other than sectors of data on the
head signal, some repetitive signal such as Servo/Data can be used as Read Gate. Analog
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