HP 54710A User's Reference Manual page 338

FFT Menu
FFT Basics
Figure 21-2 illustrates what an FFT does. An FFT transforms a time record
of N samples into a frequency record of N points from 0 Hz to Fs, where Fs is
the sample frequency. The resolution or the spacing between the points in
the frequency record is Fs/N.
The frequency Fs/2 is a unique frequency referred to as the Nyquist
frequency. At the Nyquist frequency there are exactly two samples on every
cycle of the input signal. Signals above the Nyquist frequency become
aliased which means that they appear as signals of a lower frequency. The
reason is that there are not enough sample points on each cycle of the signal
to determine the correct frequency.
It turns out that the points above Fs/2 are a mirror image of the points below
Fs/2. The points above Fs/2 are not displayed because they do not provide
any additional information. Therefore, N time samples results in N/2
displayed frequency points.
Figure 21-2
Sine wave in the frequency domain
21–11