O
PERATOR
’
S
M
ANUAL
WRXi-OM-E Rev B
97
Some of these are pre-defined but can be changed. Once they are defined, the oscilloscope is ready to make the
histogram. The sequence for acquiring histogram data is as follows:
1. Trigger
2. Waveform
acquisition
3. Parameter
calculations
4. Histogram
update
5. Trigger
re-arm
If you set the timebase for non-segmented mode, a single acquisition occurs prior to parameter calculations.
However, in Sequence mode an acquisition for each segment occurs prior to parameter calculations. If the source
of histogram data is a memory, saving new data to memory effectively acts as a trigger and acquisition. Because
updating the screen can take much 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 oscilloscope maintains a circular parameter buffer of the last 20,000 measurements made, including values
that fall outside the set histogram range. If the maximum number of events to be used for the histogram 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 will be updated until the number
of events is equal to `N.' Then, if the number of bins or the histogram range is modified, the oscilloscope 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 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 oscilloscope 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 using it. The instrument will also give a running count of the number of parameter values that fall
within, below, or above the range. If any values fall below or above the range, the range finder can then
recalculate to include these parameter values, as long as they are still within the buffer.
Capture of Parameter Events
The number of events captured per waveform acquisition or display sweep depends on the parameter type.
Acquisitions are initiated by the occurrence of a trigger event. Sweeps are equivalent to the waveform captured
and displayed on an input channel (1, 2, or 3 or 4). For non-segmented waveforms an acquisition is identical to a
sweep. Whereas for segmented waveforms an acquisition occurs for each segment and a sweep is equivalent to
acquisitions for all segments. Only the section of a waveform between the parameter cursors is used in the
calculation of parameter values and corresponding histogram events.
The following table provides a summary of the number of histogram events captured per acquisition or sweep for
each parameter, and for a waveform section between the parameter cursors.
Parameters
Number of Events Captured
duty, freq, period, width, time@lev, f@level, f80-20%,
fall, r@level, r20-80%, rise
All events in the acquisition
ampl, area, base, cmean, cmedian, crms, csdev, cycles,
delay, maximum, mean, minimum, nbph, nbpw, over+,
over-, pkpk, npts, rms, sdev, dly
One event per acquisition
Histogram Parameters (XMAP and JTA2 Options)
Once a histogram is defined and generated, measurements can be performed on the histogram itself. Typical of
these are the histogram's average value, standard deviation most common value (parameter value of highest
count bin) leftmost bin position (representing the lowest measured waveform parameter value) rightmost bin
(representing the highest measured waveform parameter value).
