| 单词 | dB (decibel) |
| 释义 | dB (decibel) In physics, one-tenth (1⁄10) of a bel (B). Bel is a measure of comparative power, or loudness, i.e., a unit of relative power ratio. The number of decibels is the decimal logarithm of the relative power ratio, which is expressed mathematically as follows: dB = 10 log (P /P ) 10 1 2 where dB = deciBel, P is the power level to be measured, and P is the reference power level. If the result 1 2 of the calculation is positive (+), there is a gain in signal power. If the result of the calculation is negative (–), there is a loss of signal power, i.e., attenuation. If, for example, the ratio of power at the output port of an optical amplifier to the power at the input port is 2.000, i.e., twice the reference level, the gain in signal power is expressed as: 2.000 = +3.0103 dB If the ratio of power at the point of signal origin where the optical signal from an LED or laser enters an optical fiber to the power at the end of the fiber is 0.500, i.e., one-half the reference level, the attenuation, or insertion loss, is expressed as: 0.500 = –3.0103 dB Decibel is the commonly used relative power ratio, as the unit of measurement is smaller and more convenient than the bel.The bel is named for Alexander Graham Bell, inventor of the telephone, among other things. See also amplifier; attenuation; Bell, Alexander Graham; decibel; gain; insertion loss; logarithm; and power. 74570c04.qxd 9/11/07 12:19 PM Page 127 |
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