Tham khảo tài liệu 'handbook of high temperature superconductor electronics part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 6 Noise in High-Temperature Superconductor Josephson Junctions . Macfarlane L. Hao and . Pegrum University of Strathclyde Glasgow Scotland INTRODUCTION Sources of Electronic Noise Noise is an important problem in science and engineering because it degrades the accuracy of any measurement and the quality of electronically processed signals. To understand and minimize these effects one must measure this noise simply and accurately. Perhaps the two most commonly encountered types of noise are thermal noise and shot noise. Thermal noise arises from the random velocity fluctuations of the charge carriers electrons and or holes in a resistive material. The mechanism is sometimes said to be Brownian motion of the charge carriers due to the thermal energy in the material. Thermal noise is present when the resistive element is in thermal equilibrium with its surroundings and it is often referred to as Johnson noise or Nyquist noise in recognition of two early investigators of this phenomenon 1 2 . Thermal noise is usually represented by the equation Sv f 4kTR V2 Hz 1 Current affiliation Centre for Basic Metrology National Physical Laboratory Teddington England Copyright 2003 by Marcel Dekker Inc. All Rights Reserved. wherekis Boltzmann s constant X 10 23J K Ris the resistance of the conductor T is the absolute temperature and Sv is the voltage noise power spectral density. Shot noise occurs when the current flows across a barrier. It was first discussed by Schottky 3 . It is often found in solid-state devices when a current passes a potential barrier such as the depletion layer of a p-n junction. The stream of charge carriers fluctuates randomly about a mean level. The fluctuations . the shot noise are due to the random discrete nature of the tunneling process. The shot noise has a constant spectral density of Si f 2elDc A2 Hz where e is the electronic charge X 10 19 C and IDC is the average current. In both of the above cases the noise spectral
