A typical Gaussian distribution is generally plotted with the
number of sigma, or standard deviations, shown on the horizontal
(X) axis (see figure, left). When measuring the performance of a
noise generator, a critical factor in assessing its usefulness
is to compare its distribution with that of the ideal
mathematical distribution shown above. Using the histogram
feature on a spectrum analyzer makes it possible to assess the
"Gaussian behavior" of the source under test. Such a histogram
(see figure, right) was used to show recorded hits over time
superimposed over the time-domain profile of a noise source. The
peak-to-peak variation measured over time allows one to
establish the "peak factor"—a measure of how many standard
deviations the source reaches. Other factors, such as symmetry
about the mean and distribution within the standard deviations,
can also be analyzed to determine the Gaussian quality of a
noise source under test.