5 Measurements and Math Functions
172 InfiniiVision 7000B Series Oscilloscopes User’s Guide
Which samples are measured depends on the Interval setting:
• NCycles — the RMS (DC) value is measured on an integral number of
periods of the displayed signal. If less than three edges are present, the
measurement shows “No edges”.
• Full Screen — the RMS (DC) value is measured on all displayed data
points.
• Auto — the measurement attempts to compute a value using the NCycles
interval. If less than three edges are present, the measurement is
computed using the Full Screen interval.
The X cursors show what interval of the waveform is being measured.
Std Deviation
The Std Deviation measurement shows the standard deviation of the
displayed voltage values. It is an RMS measurement across the full screen
with the DC component removed. It is useful, for example, for measuring
power supply noise.
The standard deviation of a measurement is the amount that a
measurement varies from the mean value. The Mean value of a
measurement is the statistical average of the measurement.
The following figure graphically shows the mean and standard deviation.
Standard deviation is represented by the Greek letter sigma: . For a
Gaussian distribution, two sigma (± 1) from the mean, is where 68.3
percent of the measurement results reside. Six sigma (± 3) from is where
99.7 percent of the measurement results reside.
-2s
mean
-1s 1s
2s
68.3%
-3s
95.4%
99.7%
03s
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