Teledyne LeCroy JTA2 - Manual (Page 27)

JTA2-OM-E Rev A ISSUED: December 2003
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HISTOGRAM AND TREND CALCULATION
With the instrument configured for Histograms or Trends, the timing parameter values are
calculated and the chosen function performed on each following acquisition. The Histogram or
Trend values themselves are calculated immediately after each acquisition.
The result is a waveform of data points that can be used the same way as any other waveform.
Other parameters can be calculated on it, it can be zoomed, serve as the x or y trace in an XY
plot, or used in cursor measurements.
Acquisition Sequence
The sequence of events for acquiring Histogram or Trend data is:
1. Trigger
2. Waveform
Acquisition
3. Parameter Calculations
4. Histogram Update
5. Trigger Re-arm
If the timebase is set in non-segmented mode, a single acquisition occurs prior to parameter
calculations.
However, in segment mode an acquisition for each segment occurs prior to parameter
calculations. If the source of the Histogram or Trend data is a memory, storing new data to
memory effectively acts as a trigger and acquisition. Because updating the screen can take
significant processing time, it occurs only once a second, minimizing trigger dead-time. (Under
remote control, the display can be turned off to maximize measurement speed.)
Parameter Buffer
The instrument maintains a circular parameter buffer of the last 20,000 measurements, including
values that fall outside the set histogram range. If the maximum number of events to be used in a
histogram or trend is a number N less than 20,000, the histogram will be continuously updated
with the last N events as new acquisitions occur. If the maximum number is greater than 20,000,
the histogram or trend will be updated until the number of events equals N. Then, if the number of
bins or the histogram or trend range is modified, the instrument will use the parameter buffer
values to redraw the histogram with either the last N or 20,000 values acquired, whichever is the
lesser. The parameter buffer thereby allows histograms or trends to be redisplayed using an
acquired set of values and settings that produce a distribution shape with the most useful
information.
In many cases the optimal range is not readily apparent, so the instrument has a powerful range
finding function. If required, it will examine the values in the parameter buffer to calculate an
optimal range and redisplay the histogram or trend using it. The instrument will also give a
running count of the number of parameter values that fall within, below, and above the range. If
any fall below or above the range, the range finder can then recalculate using these parameter
values, while they are still within the buffer.