在通信系統(tǒng)中常用單位轉(zhuǎn)換

1、在通信系統(tǒng)中常用單位dBm, dBmV和dBpV的轉(zhuǎn)換引言有線電視（CATV）系統(tǒng)工作在75Q環(huán)境下。絕大部分射頻實(shí)驗(yàn)室配備的標(biāo)準(zhǔn)測(cè)試儀器則將50Q作為標(biāo)準(zhǔn) 的接口阻抗。dBmV 和 dBpV絕大部分CATV測(cè)量是以電壓為參考，以dBmV或dBpV為單位進(jìn)行測(cè)量。按照標(biāo)準(zhǔn)定義，dBmV是以 ImVrms為參考，輸出電壓（Vout）以mVrms為單位進(jìn)行測(cè)量（EQN1）。由于EQN1得到的是電壓比值，以 ImVpp為參考值時(shí)它還可以用mVpp為單位測(cè)量；或者用其它的電壓?jiǎn)挝粊?lái)表示，只要它們采用的單位相 同。dBmV = 20 logVoutEQN1從EQN1可以看出dBmV和阻抗無(wú)關(guān)。標(biāo)準(zhǔn)定義中

2、，dBpV是以IpVrms為參考，輸出電壓（Vout）以pVrms為單位進(jìn)行測(cè)量（EQN2）。由于EQN2 得到的是電壓比值，以IpVpp為參考時(shí)它還可以用pVpp為單位測(cè)量；或者用其它的電壓?jiǎn)挝粊?lái)表示，只 要它們采用的單位相同。從EQN2可以看出dBpV和阻抗無(wú)關(guān)。dBm和dBmV之間的轉(zhuǎn)換EQN3給出了 dBm的定義。Pout的測(cè)量單位是mW，dBm以1mW為參考。EQN3與負(fù)載阻抗有關(guān)，如EQN4所示。大多數(shù)射頻設(shè) 備具有50Q負(fù)載和輸入阻抗。而EQN3對(duì)于任何負(fù)載阻抗而言均正確，包括75Q負(fù)載。功率取決于負(fù)載阻抗。EQN4建立了功率與輸出電壓和負(fù)載阻抗之間的關(guān)系。整理各項(xiàng)可得:求解EQ

3、N3，得出Pout，dBmEQN3.1Poui = (1 10-a)10lD求解EQN1，得到Vout，將 EQN3.1 和 EQN1.1 代入 EQN4.1,用dBm表示dBmV，當(dāng)R = 50Q時(shí)，EQN5.2EQN5.2適用于阻抗為50Q的測(cè)量裝置。當(dāng)R = 75Q時(shí)，求解EQN 5.1得:dBmV = 43.7506 + 如所知EQN5.3EQN 5.3適用于阻抗為75Q的測(cè)量設(shè)備。采用同樣的方法可以得到dBpV到dBmV之間的關(guān)系如下，dBjiV = 20log+ dBmVEQN6dBnV = 60 +dBmVEQN6.1表1和表2分別顯示了在阻抗為50Q和75Q的條件下，dBmV，

4、dBpV和dBm之間的換算關(guān)系。表1. 50C阻抗下的功率換算dBmVdB?VdBm 50口mVrmsmW 50Q868-38.992.511.3E-04969-37.992.821.6E-041070-36.993.162.0E-041171-35.993.552.5E-041272-34.993.983.2E-041373-33.994.474.0E-041474-32.995.015.0E-041575-31.995.626.3E-041676-30.996.318.0E-041777-29.997.081.0E-031878-28.997.941.3E-031979-27.998.91

5、1.6E-032080-26.9910.002.0E-032181-25.9911.222.5E-032282-24.9912.593.2E-032383-23.9914.134.0E-032484-22.9915.855.0E-032585-21.9917.786.3E-032686-20.9919.958.0E-032787-19.9922.390.0102888-18.9925.120.0132989-17.9928.180.0163090-16.9931.620.0203191-15.9935.480.0253292-14.9939.810.0323393-13.9944.670.04

6、03494-12.9950.120.0503595-11.9956.230.0633696-10.9963.100.0803797-9.9970.790.1003898-8.9979.430.1263999-7.9989.130.15940100-6.99100.000.20041101-5.99112.200.25242102-4.99125.890.31743103-3.99141.250.39944104-2.99158.490.50245105-1.99177.830.63246106-0.99199.530.796471070.01223.871.002481081.01251.19

7、1.262491092.01281.841.589501103.01316.232.000511114.01354.812.518521125.01398.113.170531136.01446.683.991541147.01501.195.024551158.01562.346.325561169.01630.967.9625711710.01707.9510.0245811811.01794.3312.6195911912.01891.2515.8876012013.011000.0020.0006112114.011122.0225.1796212215.011258.9331.698

8、6312316.011412.5439.9056412417.011584.8950.2386512518.011778.2863.2466612619.011995.2679.6216712720.012238.72100.2376812821.012511.89126.191表2. 75。阻抗下的功率換算dBmVdB?VdBm 75包mVrmsmW 75Q868-40.752.518.4E-05969-39.752.821.1E-041070-38.753.161.3E-041171-37.753.551.7E-041272-36.753.982.1E-041373-35.754.472.

9、7E-041474-34.755.013.3E-041575-33.755.624.2E-041676-32.756.315.3E-041777-31.757.086.7E-041878-30.757.948.4E-041979-29.758.911.1E-032080-28.7510.001.3E-032181-27.7511.221.7E-032282-26.7512.592.1E-032383-25.7514.132.7E-032484-24.7515.853.3E-032585-23.7517.784.2E-032686-22.7519.955.3E-032787-21.7522.39

10、6.7E-032888-20.7525.128.4E-032989-19.7528.180.0113090-18.7531.620.0133191-17.7535.480.0173292-16.7539.810.0213393-15.7544.670.0273494-14.7550.120.0333595-13.7556.230.0423696-12.7563.100.0533797-11.7570.790.0673898-10.7579.430.0843999-9.7589.130.10640100-8.75100.000.13341101-7.75112.200.16842102-6.75

11、125.890.21143103-5.75141.250.26644104-4.75158.490.33545105-3.75177.830.42246106-2.75199.530.53147107-1.75223.870.66848108-0.75251.190.841491090.25281.841.059501101.25316.231.333511112.25354.811.679521123.25398.112.113531134.25446.682.660541145.25501.193.349551156.25562.344.216561167.25630.965.308571178.25707.956.683581189.25794.338.4135911910.25891.2510.5916012011.251000.0013.333611211