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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Roll-off ： ウィキペディア英語版 Roll-off Roll-off is the steepness of a transmission function with frequency, particularly in electrical network analysis, and most especially in connection with filter circuits in the transition between a passband and a stopband. It is most typically applied to the insertion loss of the network, but can in principle be applied to any relevant function of frequency, and any technology, not just electronics. It is usual to measure roll-off as a function of logarithmic frequency, consequently, the units of roll-off are either decibels per decade (dB/decade), where a decade is a 10-times increase in frequency, or decibels per octave (dB/8ve), where an octave is 2-times increase in frequency. The concept of roll-off stems from the fact that in many networks roll-off tends towards a constant gradient at frequencies well away from the cut-off point of the frequency curve. Roll-off enables the cut-off performance of such a filter network to be reduced to a single number. Note that roll-off can occur with decreasing frequency as well as increasing frequency, depending on the bandform of the filter being considered: for instance a low-pass filter will roll-off with increasing frequency, but a high-pass filter or the lower stopband of a band-pass filter will roll-off with decreasing frequency. For brevity, this article describes only low-pass filters. This is to be taken in the spirit of prototype filters; the same principles may be applied to high-pass filters by interchanging phrases such as "above cut-off frequency" and "below cut-off frequency". ==First order roll-off== A simple first order network such as a RC circuit will have a roll-off of 20 dB/decade. This is approximately equal (to within normal engineering required accuracy) to 6 dB/8ve and is the more usual description given for this roll-off. This can be shown to be so by considering the voltage transfer function, ''A'', of the RC network:〔J. Michael Jacob, ''Advanced AC circuits and electronics: principles & applications'', pages 150-152, Cengage Learning 2003 ISBN 0-7668-2330-X.〕 :$A=\backslash frac=\backslash frac$ Frequency scaling this to ''ω''c=1/''RC''=1 and forming the power ratio gives, :$|A|^2=\backslash frac\; =\; \backslash frac$ In decibels this becomes, :$10\backslash log\; \backslash left(\}\backslash right)$ or expressed as a loss, :$L=10\backslash log\; \backslash left(\backslash right)\; \backslash \; \backslash mathrm$ At frequencies well above ''ω''=1, this simplifies to, :$L\; \backslash approx\; 10\backslash log\; \backslash left(\backslash omega^2\backslash right)=\; 20\backslash log\; \backslash omega\; \backslash \; \backslash mathrm$ Roll-off is given by, :$\backslash Delta\; L\; =\; 20\backslash log\; \backslash left(\; \backslash right)\; \backslash \; \backslash mathrm$ and for an octave, :$\backslash Delta\; L\; =\; 20\backslash log\; 2\; \backslash approx\; 20\; \backslash times\; 0.3\; =\; 6\; \backslash \; \backslash mathrm$ 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Roll-off」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.