**3.2 Analogue Signals**

A simple analogue signal is a sine wave. The 3 main characteristics of an analogue signal are:

- Amplitude
- Frequency
- Phase

Another important characteristic of a sine wave is its wavelength. A sine wave can be regarded as being a sequence of exactly identical waves. If one wavelength was sampled and compared to another there would be an exact fit. One complete wave is called a cycle.

The highest point of a sine wave is called a crest and the lowest point a trough.

Changes in any of the 3 main characteristics can be introduced to a signal and controlled electronically. Signal modification and its control forms the basis of modern data communications.

**Amplitude **

Amplitude is a signal’s strength and can be visualised as the height of the signal. Therefore, the higher the amplitude the “louder” the signal. Signal strength is typically measured in decibels, named after the inventor of the telephone: Alexander Graham Bell.

**Frequency **

Frequency is the rate at which a signal changes per second and is typically measured in hertz (Hz). Therefore, if a signal has a frequency of 100 Hz, it changes at a rate of 100 times per second. This rate of change is also referred to as the number of cycles per second.

Frequency can also be measured in kilohertz (kHz), megahertz (MHz), and gigahertz (GHz).

**Phase**

Phase is the rate at which a signal changes its own relationship to zero time, and is expressed in radians or degrees. One complete cycle of a wave begins at a predetermined point and continues until the same point is reached again. The phase shifts forward or backward along the time axis. The amount of shift can be from 0 to 360 degrees. Therefore, a shift of 360 degrees is one complete period.

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