# Chapter 4 - Chapter 4 Noise Considerations 1 The Importance...

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1 Chapter 4 Noise Considerations

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2 The Importance of Noise in Active Network Noise is everywhere. Because of noise, the minimum detectable signal is limited. The noise figure of the low noise amplifier after the antenna determines the overall sensitivity of a receiver. An oscillator starts up from noise.
3 What is Noise Noise is a kind of power Noise power results from random processes that exist in nature. Noise is passed into the input port of an electronic circuit, and is generated by the components of the circuits. Noise spectral power density in a noisy resistor is constant from DC to infinite frequency. We call it white noise.

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4 The Random Processes Thermal vibrations of atoms, electrons, and molecules in a component at any temperature above 0° K. Flow of charges (electrons or holes) in a wire or device. Emission of charges (electrons or ions) from a surface such as the cathode of a diode or an electron tube. Wave propagation through atmosphere or any other gas.
5 Definition of Electrical Noise Electrical Noise: Any unwanted electrical disturbance or spurious signal. These unwanted signals are random in nature and are generated internally in the electronic components or externally through impinging electromagnetic radiation. Noise Expression: Noise is a random time varying electrical signal which can be denoted as ) ( t V n . Statistical Analysis of Noise : Because the noise figure is random, it is convenient to analysis noise though statistical method.

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6 Source of Noise There are three main sources of noise. Thermal Noise : The most basic type of noise, which is caused by thermal vibration of bound charges and thermal agitation of electrons in a conductive material. This is common to all passive or active devices. Short Noise : Caused by random passage of discrete charge carriers crossing a junction or other discontinuities. Flicker Noise : Small vibrations of a current due to the following factors: a. Random injection or recombination of charge carrier at an interface, such as at a metal and semiconductor interface (in semiconductor devices). b. Random changes in cathode emissions of electric charges such as at a cathode-air interface (in a thermionic tube). Flicker noise exists at lower frequencies, almost from DC extending down to approximately 500kHz to 1 MHz at a rage of -10dB per decade.
7 Statistical Expression of Noise Mean Value of Noise : The time mean value of noise is zero: () + = = T t t n T n dt t V T V 1 1 0 1 lim (1) Mean-Square Value of Noise: The value is a constant proportional to the noise power: () + = = T t t n T n Const dt t V T V 1 1 . ] [ 1 2 2 lim (2) Root-Mean-Square Value of Noise: It is the effective value of the noise voltage: 2 , n rms n V V = (3)

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8 Thermal Noise Analysis The Thermal Noise Power The thermal noise is the most basic type of noise. The available power from any arbitrary resistor was given by Nyquist as: kTB P N = (4) where,
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## This note was uploaded on 05/01/2011 for the course ELECTRICAL EE5602 taught by Professor Xuequan during the Spring '11 term at City University of Hong Kong.

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Chapter 4 - Chapter 4 Noise Considerations 1 The Importance...

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