Measurements 14
Keysight InfiniiVision HD3-Series Oscilloscopes User's Guide 195
AC RMS
AC RMS is the root-mean-square value of the waveform, with the DC component
removed. It is useful, for example, for measuring power supply noise.
The N Cycles measurement interval measures the value on an integral number of
periods of the displayed signal. If less than three edges are present, the
measurement shows "No edges".
The X markers show the interval of the waveform being measured.
The Full Screen (Std Deviation) measurement interval variation is an RMS
measurement across the full screen with the DC component removed. It shows the
standard deviation of the displayed voltage values.
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:
σ. For a Gaussian distribution,
two sigma (± 1σ) from the mean, is where 68.3 percent of the measurement results
reside. Six sigma (± 3σ) from is where 99.7 percent of the measurement results
reside.
The mean is calculated as follows:
where:
• x = the mean.
• N = the number of measurements taken.
• x
i
= the ith measurement result.
-2σ
mean
-1σ 1σ
2σ
68.3%
-3σ
95.4%
99.7%
0 3σ
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