| 单词 | frequency (f) |
| 释义 | frequency (f) The Institute of Electrical and Electronics Engineers (IEEE) defines frequency as the number of complete cycles of sinusoidal variation per unit time, with the unit of time generally being that of one second. Plotting y = sin x, where x is expressed in radians, yields a sine wave as illustrated in Figure F-8. (From the Latin radius, a radian is a unit of plane angular measurement equivalent to the angle between two radii that enclose a section of a circle’s circumference [arc] equal in length to the length of a radius.There are 2 π radians in a circle.) A complete sine wave entails a cycle as measured from a point of zero (0) amplitude to a point of maximum positive amplitude (+A), through zero to a point of maximum negative amplitude (–A), and back to a point of zero (0) amplitude.The frequency of an alternating current, for example, is the half number of times the polarity of a continuous waveform reverses direction, or polarity, per second. The frequency of the waveform is the number of times per second it makes a complete cycle from zero voltage (0V) to its maximum positive voltage (+V) or amplitude, back to zero (0 V), to its maximum negative voltage (–V) or amplitude, and back to zero (0V). Frequency traditionally is measured in cycles per second (cps) or, in more contemporary terms, Hertz (Hz). See also amplitude, electromagnetic spectrum, Hz, and sine wave. 74570c06.qxd 9/11/07 12:21 PM Page 195 |
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