HP 16533A, User Reference, Page: 101 / 152

Automatic Measurement Algorithms
This section explains top and base voltages, then defines the
measurement algorithms.
Top and Base Voltages
All measurements except Vp_p are calculated using the V
top
(100% voltage)
and V
base
(0% voltage) levels of the displayed waveform. The V
top
and V
base
levels are determined from an occurrence density histogram of the data
points displayed on the screen.
The digitizing oscilloscope displays 8-bit vertical voltage resolution. That is,
the vertical axis of the display is divided into 2
8
voltage levels. Each of these
256 levels is called a quantization level. Each waveform has 500 data points
displayed on the horizontal axis of the screen. Each of these 500 data points
has one quantization level assigned to it. The histogram is calculated by
adding the number of occurrences of each quantization level of the 500
displayed points on the displayed waveform.
The quantization level with the greatest number of occurrences in the top
half of the waveform corresponds to the V
top
level. The quantization level
with the greatest number of occurrences in the bottom half of the waveform
corresponds to the V
base
level.
If V
top
and V
base
do not contain at least 5% of the 500 data points displayed
on screen, V
top
defaults to the maximum voltage (V
maximum
) and V
base
defaults to the minimum voltage (V
minimum
) found on the display. An
example of this case would be measurements made on sine or triangle
waves.
From this information, the instrument can determine the 10%, 50%, and 90%
points, which are used in most automatic measurements. The V
top
or V
base
of the waveform is not necessarily the maximum or minimum voltage
present on the waveform. If a pulse has a slight amount of overshoot, it
would be wrong to select the highest peak of the waveform as the top
because the waveform proper rests below the perturbation.
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