Agilent Technologies 6000 Series User Manual page 196

5 Making Measurements
196
RMS
RMS (DC) is the root-mean-square value of the waveform over
one or more full periods. If less than one period is displayed,
RMS (DC) average is calculated on the full width of the display.
The X cursors show what interval of the waveform is being
measured.
n
x
2
i
i 1
=
--------------- -
RMS (dc) =
n
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.
where x = value at ith point being measured
i
n = number of points in measurement interval
6000 Series Oscilloscope User's Guide