Transcript noise - London South Bank University

Noise
Richard Read, The Essence of
Communications Theory, Chapter 3
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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)
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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 1013 Hz.
Vrms  4kTBR
Where, k=1.38*10-23 J/K Boltzmann’s constant, k is kelvin
T = (273+C) kelvin, absolute temperature
Pa  kTB
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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.
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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 : mx=0, and variance: σx2 = P*R=0.1*50=5
P( x  1)  
1

5
1
2 5
e
 x 2 /[2 ( 5)]
dx  

1/ 5
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1  z2 / 2
e
dz  0.3274
2
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.
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Signal to noise ratio


The ratio of signal power to noise power.
Relative level compared that of the signal.
S
signal power
dB  10 log 10
N
noise power
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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