To better understand the complexities of radio waves, it is useful to
examine how analog signals vary with time and frequency. First consider a pure
single-frequency sine wave. If an electrical sine wave with an audible
frequency is applied to a speaker, a tone will be heard. A spectrum-analyzer
graph of this pure tone would be a single straight line
. The interactive
activity illustrates such a graph. Click on Play in the activity to hear
an example of the tone.
Now imagine several sine waves all added
together at one time. The resulting wave is more complex than a pure sine wave.
There are several tones and the graph of these tones will show several
individual lines, each corresponding to the frequency of one tone.
As a
final example, imagine an extremely complex signal, like a voice or a musical
instrument. With a sufficiently large number of different tones, a spectrum
analyzer graph would look like a continuous spectrum of closed, spaced,
individual tones. Click on Sweep in the activity to hear an example of
the tones associated with many closely spaced frequencies. Picture a graph
being drawn, as the frequencies change with time.
Digital
Signals
The pattern of voltage changes versus time is called a square
wave. There are many ways to represent data with digital signals. Figure
illustrates a very simple example in which there are only two voltage levels,
which will be interpreted as either a one or a zero.
At first, it may be
difficult to imagine that the voltage versus time graph of a digital signal can
be built out of sine waves. However, remember the Fourier synthesis and that a
square wave can be built by using the right combination of sine waves.
In general, complex waveforms will have complex spectrum graphs.