Noise is any undesired signal that ultimately appears in the output of a communications system. Sources of noise may be grouped into two categories:
Natural noise may be further broken down into two subcategories:
Atmospheric noise [Click here to hear atmospheric noise]
Atmospheric noise originates in our atmosphere. Lightning associated with thunderstorms creates RF noise over a wide range of frequencies ( ~ 100 kHz to 20 MHz) that travels hundreds or miles. Auroral discharges in the polar regions also create noise at lower frequencies (below 0.1 MHz).
Cosmic noise comes from a source outside the Earth's atmosphere. The sun produces noise that reaches a maximum at 11 year intervals (the last maximum was early in 2001). The planet Jupiter produces copious amounts of RF noise in the 16 to 24 MHz range. Other stars and galaxies also contribute to cosmic noise.
Man-made or artificial noise can also be divided into two subcategories:
External man-made noise
Internal man-made noise
External man-made noise is produced to a greater or lesser degree by all electrical devices. Fluorescent lights, automotive ignition systems, and electric motors are some of the worst noise sources. As a general rule, "where there are sparks, there is noise". Any type of electrical discharge produces RF over a wide range of wavelengths.
Internal man-made noise is produced by the electronic components making up the communications system Two important types of internal man-made noise are:
Thermal (Johnson) noise [Click here to hear Johnson noise]
Thermal noise is created by the motion of electrons in a resistor. Because the resistor is at a temperature above absolute zero (-459 F), the electrons move randomly in the solid. This random, fluctuating movement of electrons produces a noise voltage at the terminals of the resistor.
Thermal noise is also known as "white noise" because the amplitude of thermal noise is not dependent on frequency. In other words, thermal noise contains noise signals of all frequencies and all these signals have the same average amplitude. The noise voltage that appears across a resistor at temperature T is:
k = Boltzmann's constant (1.38*10-23 J/K)
T = absolute temperature (degrees K)
B = bandwidth (Hz)
R = resistance (ohms)
The internal resistance of transistors, diodes, and other active electronic components creates thermal noise. For capacitors and inductors, the internal resistances are negligible and they can be considered to be noiseless.
Shot noise is a noise produced in active devices such as transistors. The flow of current in a transistor is not a smooth steady flow, such as one encounters when pouring water from a pitcher into a glass. It is more similar to the flow one sees when pouring lead shot into a shell. The current flow is made of discrete current carriers, electrons, and the number of electrons leaving the collector of a transistor is not constant, but varies slightly from moment to moment.
In bipolar transistors, the shot noise increases as the bias current is increased. In FET's, the shot noise is not affected by changes in bias.
Another types of noise that appears in active devices are flicker noise. Flicker noise is most troublesome at frequencies below 1 kHz and its amplitude is inversely proportional to frequency.
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