Kenny's Blog

Decibels 
In electronics one often wantsto represent the ratio of two signals on a logarithmic scale. For this we use the decibel, dB. If we have two powers P1 and P2

dB = 10 log10 (P2/P1)

or equivalently - when comparing two signals with the same kind of waveform -

dB = 20 log10 (V2/V1)

Note dB is a relative unit of measurement, i.e. “This amplifier has a gain of 10 dB.” dB can be positive or negative. For example, +10 dB corresponds to P2 greater than P1 by a factor of 10, and -3 dB corresponds to P2 less than P1 by approximately a factor of 2.

For an absolute measure on a logarithmic scale there are a variety of other units. A common one is dBm.

dBm = 10 log (P [mW])

Zero dBm corresponds to 1 mW of power. And in a 50 Ω system 0 dBm corresponds to 220 mVrms.

http://www.sengpielaudio.com/Rechner-db-volt.htm 
http://www8.plala.or.jp/ap2/suugaku/taisuu.html

對數 計算

http://support.microsoft.com/kb/214116/zh-tw

http://zh.wikipedia.org/wiki/%E5%AF%B9%E6%95%B0#.E8.BF.98.E5.8E.9F