Teledyne LeCroy WaveExpert 100H Operators Manual - Manual (Page 133)

Wave Expert
WE-OM-E Rev A
131
Histogram Theory of Operation
An understanding of statistical variations in parameter values is needed for many waveform
parameter measurements. Knowledge of the average, minimum, maximum, and standard
deviation of the parameter may often be enough, but in many cases you may need a more detailed
understanding of the distribution of a parameter's values.
Histograms allow you to see how a parameter's values are distributed over many measurements.
They do this by dividing a range of parameter values into sub-ranges called bins. A count of the
number of parameter values (events) that fall within ranges of the bin itself is maintained for each
bin.
While such a value range can be infinite, for practical purposes it need only be defined as large
enough to include any realistically possible parameter value. For example, in measuring TTL
high-voltage values a range of ±50 V is unnecessarily large, whereas one of 4 V ±2.5 V is more
reasonable. It is the 5 V range that is then subdivided into bins. And if the number of bins used were
50, each would have a range of 5 V/50 bins or 0.1 V/bin. Events falling into the first bin would then
be between 1.5 V and 1.6 V. While the next bin would capture all events between 1.6 V and 1.7 V,
and so on.
After a process of several thousand events, the bar graph of the count for each bin (its histogram)
provides a good understanding of the distribution of values. Histograms generally use the 'x' axis to
show a bin's sub-range value, and the 'Y' axis for the count of parameter values within each bin.
The leftmost bin with a non-zero count shows the lowest parameter value measurements. The
vertically highest bin shows the greatest number of events falling within its sub-range.
Note: The range of the histogram is limited to the portion of the trace that is visible on screen. That is, if you zoom in on a
trace, the histogram will not contain data for that part of the original trace no longer visible.
The number of events in a bin, peak or a histogram is referred to as its population. The following
figure shows a histogram's highest population bin as the one with a sub-range of 4.3 to 4.4 V (which
is to be expected of a TTL signal).