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1、分贝毫瓦的概念DBm(2009-09-0908:39:36)onthelabel:DBmmeansdecibelsofdecibels.Decibelareferenced(deciBelstomW)referstotheuseoflogarithmicformofpowermeasurement,dBmtolmWasareferenee.PowerunitandP(Watt)conversionformula:(10*P)dBm=(10*P)dB-30dB(P:watts)Firstofall,DBisapurecountingunit:forpower,dB=10*lg(A/B).Forv
2、oltageorcurrent,dB=20*lg(A/B)・DBinfactagainsimple,isalarge(followedbyalongstringof0)orsmall(infrontofalongstringof0)thenumberofrelativelybriefrepresentation.Suchas:X=1000000000000000(howmany?)101gX=150clBX=0.000000000000001101gX=-150dBDBmdefinesmiliwatt・0dBm=101glmw;DBwdefineswatt.0dBw=lOlgl,W=lOlgl
3、OOO,Mw=30,dBm.TheDBalwaysdefinespowerunitsbydefault,withlOlgasthegauge.Ofcourse,insomecasessignalpower(Amplitude)canbeusedtodescribetheworkandpower,andthistimethe201gisused・Thisistrueinboththecontrolandsignalprocessingareas・Forexample,sometimesyoucanseetheexpressionofdBmV.IndBanddBmcomputing,payatte
4、ntiontothebasicconcepts・Forexample,theprevioussaidOdBw二lOlglW二lOlglOOOmw二30dBm;and,forexample,usingonedBmminusanotherdBm,theresultisdB.Suchas:30dBm-OdBm二30dB.Generallyspeaking,inengineering,betweendBanddBnotonlyadditionandsubtraction,multiplicationanddivision.Whichisthemostusedmethod:dBmminusdBmisac
5、tuallytwophasepower,signalpowerandnoisepoweristhedivisionofsignal-to-noiseratio(SNR)・DBmplusdBmisactuallyamultiplicationoftwopowers,whichisrare(Ionlyknowthisinpowerspectralconvolutioncalculations)・WhatisdBmbydBm,andlmWbylmW?InadditiontothestudentswhowritetomesothatIcanalmostkeeppacewithGerdBach，sexp
6、ression,I'velivedforsomanyyears,andIhaven'tseenanyengineeringareastoplaywiththis・ThedBistheunitofpowergainthatrepresentsarelativevalue.WhenthepoweroftheAiscalculatedtobeaslargeorsmallasB,howmanydBcanbecalculatedbyformula10,LG,A/B・Forexample:ApoweristwicelargerthanBpower,then10LGA/B二10LG2二3dB・Thatist
7、osay,thepowerofAislargerthanthepowerof3dB;ifthepowerofAis46dBm,thepowerofBis40dBm,itcanbesaidthatAislargerthanB;iftheAantennais12dBd,andtheBantennais14dBd,itcanbesaidthatAissmallerthanBand2dBissmaller
