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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 10-3. Sine wave in the frequency domain.
Figure
10-4
shows
an
example
.
The
upper
graticule
shows
a
1
GHz waveform
in the
time
domain,
and
the
lower
graticule
shows
the
same
signal
in the
frequency domain. By looking at the Magnify Span and Center Freq softkeys
,
you can determine the display setup and see there is strong signal content
at
1 GHz, as well as odd frequency multiples (3 GHz, 5 GHz, and so
on).
10-29
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