Signals
Signals in time and frequency

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.


Interactive Media Activity

Interactive Activity: Tone Generator Modulation

This activity plays frequency sounds and modulated sounds.