Agilent Technologies 93000 SOC Series Training Manual page 561

Appendix A
So, the spectrum of a signal (the view of amplitude over
frequency) is just another way of looking at the signal. This is
illustrated in the figure below.
Three-Dimensional View of a Signal
Moreover, periodic signals have a discrete spectrum (that is,
frequency components only occur at discrete values), whereas non-
periodic (arbitrary) signals have a continuous spectrum
If we view our signal over the time axis, we see the sum of all the
sinewaves.
If we view our signal over the frequency axis, every sinewave
contained in the signal appears as a vertical line. The height of the
line represents its amplitude, and the position represents its
frequency. This view is called frequency domain or spectrum. Each
line is called a component of the total signal.
It is important to understand that we have neither gained nor lost
NOTE
information about the signal—we are just presenting it differently by
changing the perspective.
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