Ya Bao, South Bank University 1 Noise Richard Read, The Essence of Communications Theory, Chapter 3

Ya Bao, South Bank University 2 Introduction to noise Electrical noise : any unwanted energy that accompanies a signal in a communication systems. Two types of noise: Manmade (artificial) noise, could be eliminated Natural noise, unavoidable, (atmospheric, cosmic)

Ya Bao, South Bank University 3 Noise in electronic components Thermal noise, its energy increase with temperature. The noise voltage varies in time with a Gaussian probability distribution function and mean value of zero. uniformly distributed spectrally from 0 to about Hz. Where, k=1.38* J/K Boltzmann’s constant, k is kelvin T = (273+C) kelvin, absolute temperature

Ya Bao, South Bank University 4 Additive White Gaussian Noise (AWGN) Thermal noise known as white noise. Power spectral density (PSD) of thermal noise Additive property, The total mean squared noise voltage across two resistors equals the sum of their individual mean squared voltages.

Ya Bao, South Bank University 5 Example The output noise from a high-gain amplifier obeys Gaussian distribution with zero mean, and dissipateds an average power of 100 mW in a 50-Ω resistive load. What is the probability of observing the output noise amplitude less than –1 V? mean : m x =0, and variance: σ x 2 = P*R=0.1*50=5

Ya Bao, South Bank University 6 Other noise Shot noise, only occurs in active (semiconductor or thermionic) devices Flicker noise, occurs in thermionic device. Concentrated in low frequencies not flat as with thermal noise, ---- pink noise. Can be ignored if the frequency more than 10 kHz.

Ya Bao, South Bank University 7 Signal to noise ratio The ratio of signal power to noise power. Relative level compared that of the signal. An amplifier has an output resistance of 600  and delivers 250 mV into a matched load. If the temperature of the load resistor is 27'C and signal bandwidth 10 MHz, determine: (a) available noise power; (b) signal power developed in the load; c) signal to noise ratio at the load in dB.

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