Agilent Technologies 1670E Series, User Manual

Page: 147 / 650

147
Using the Oscilloscope
The Scope Auto Measure Menu
Automatic measurement algorithms
The following explains top and base voltages, then defines the
measurement algorithms.
Top and base voltages
All measurements except Vp_p are calculated using the Vtop (100%
voltage) and Vbase (0% voltage) levels of the displayed waveform. The
Vtop and Vbase 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. In
other words, the vertical axis of the display is divided into 28 voltage
levels. Each of these 256 levels is called a quantization level. Each
waveform has at least 500 data points displayed on the horizontal axis
of the screen. Each of these data points has one quantization level
assigned to it. The histogram is calculated by adding the number of
occurrences of each quantization level of all 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 Vtop level. The
quantization level with the greatest number of occurrences in the
bottom half of the waveform corresponds to the Vbase level.
If Vtop and Vbase do not contain at least 5% of the 500 data points
displayed on screen, Vtop defaults to the maximum voltage
(Vmaximum) and Vbase defaults to the minimum voltage (Vminimum)
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
Vtop or Vbase 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.