Electrical Noise
Any electrical signal that makes recovery of the information signal more difficult is considered noise. For example, “white snow” on a TV picture and “hum” in an audio signal are typical electrical noise manifestations. Noise mainly affects receiving syst
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Electrical Noise
Any electrical signal that makes recovery of the information signal more difficult is considered noise. For example, “white snow” on a TV picture and “hum” in an audio signal are typical electrical noise manifestations. Noise mainly affects receiving systems, where it sets the minimum signal level that it is possible to recover before it becomes swamped by the noise. We note that amplifying a signal already mixed with noise does not help the signal recovery process at all. Once it enters the amplifier, noise is also amplified, which is to say that the ratio of signal-to-noise (S/N) power does not improve and that is what matters. When the power of the noise signal becomes too large relative to the power of the information signal, information content may be irreversibly lost. In this chapter, we study the basic classification of noise sources and methods for evaluation of noise effects.
8.1
Thermal Noise
At the fundamental level, the numerical value of electrical current is just an average number of electrons coming out of the conductor per unit of time. This movement is caused by the external field generated by energy source, for example a battery. However, even without any external electric field, an electron cloud moves inside a material and interacts with the vibrating ions, each electron moving in Brownian motion (i.e. similar to a pinball). The random motion of each individual electron makes a micro current that, together with all the other micro currents in the given volume, adds up to a macro current with zero average value. Due to its random nature, this current does not contain information, therefore we consider it “noise”, Fig. 8.1 (left). This motion is responsible for the conductor’s temperature, hence it is known as “thermal” noise; in real conductors, it is what constitutes the conductor’s resistance. Given that the movement of electrons produces current, and current through a resistor creates voltage across its terminals, we also consider a resistor as a random noise generator. Both experiments and theory have found that the power spectrum of thermal noise is flat, which (loosely) means that each frequency component in the noise spectrum has the same power level, as shown in Fig. 8.1 (right). This conclusion is valid over a very wide range of frequencies (up to approximately 1013 Hz). Similarly to white light, which contains all colours (i.e. light frequencies), a noise signal that contains single tones at all possible frequencies is called, appropriately, white noise. Of course, it is only a very good approximation, because the implication is that, theoretically, if measured over all possible frequencies, the total noise energy would add up to be infinite.
© Springer Nature Switzerland AG 2021 R. Sobot, Wireless Communication Electronics, https://doi.org/10.1007/978-3-030-48630-3_8
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8 Electrical Noise
Fig. 8.1 Thermal noise: in the time domain (left); and the noise power spectrum density (right)
Sn (f) power spectral density [W/Hz]
displacement
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time Pn
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