《通信原理》课程教学资源（PPT课件讲稿，英文版）Chapter 2 Signals and Spectra

2.1 Properties of signal and Noise Decibel(db) this is a base 10 logarithmic measure of power ratios, averagepower out d B=10log - ou =10los n averagepowerin O=10B/0 out 2 dB=10 log rms out /R out 2 /R 11

11 2.1 Properties of signal and Noise • Decibel(dB) this is a base 10 logarithmic measure of power ratios,               powerin powerout 10log 10log average average P P dB in out        rms in in rms out out V R V R dB / / 10 log 2 _ 2 _ /10 10 dB in out P P  

2.1 Properties of signal and Noise dB=20log ms out 1 +10log R R rs-in out R or dB=20log amout +10og out R rns-in In Engineering Practice dB=20log rms-out =olog rms-out rms-in

12 2.1 Properties of signal and Noise                               in out rms in rms out out in rms in rms out R R I I dB R R V V dB or 20log 10log 20log 10log • In Engineering Practice:                   rms in rms out rms in rms out I I V V dB 20log 20log

2.1 Properties of signal and Noise e Decibel signal-to-noise ratio is (S|NB101oP108 signa 10logl 201og rmS-signal rs--noise

13 2.1 Properties of signal and Noise • Decibel signal-to-noise ratio is        noise signal dB P P (S / N) 10log        ( ) ( ) 10log 2 2 n t s t          rms noise rms signal V V 20log

2.1 Properties of signal and Noise Decibel measure may also be used to indicate absolute levels of power with respect to some reference level, for example, when Imw reference level is used dBm=10los actual power level ( watts) 10 30+10log(actual power level watts)

14 2.1 Properties of signal and Noise • Decibel measure may also be used to indicate absolute levels of power with respect to some reference level ，for example, when 1mW reference level is used 30 10log(actualpowerlevel(watts)) 10 actualpowerlevel(watts) 10log 3           dBm

2.1 Properties of signal and Noise When lw reference level is used the dB level is denoted dBw when ikw reference level is used. the dB level is denoted dbs When l millivolt rms level across a 75 Ohm load is used as a reference. dB level is called dBmy and is defined as dBmv=20loge ms) 15

15 2.1 Properties of signal and Noise • When 1W reference level is used, the dB level is denoted dBW; • when 1kW reference level is used , the dB level is denoted dBk; • When 1 millivolt rms level across a 75 Ohm load is used as a reference, dB level is called dBmV, and is defined as ) 10 20log( 3  Vrms dBmV