Kelvin的Blog記事本

http://hi.baidu.com/0208405023/blog/item/766abc395b03322fb8998fc7.html

好下面开始：

Units: 

          E or V = Volts (can use either, as per convention standards) 
          R = Ohms (impedence/resitance) 
          P = Power (Watts, dBm, mW, or W) 
          dB = decibel ratio (log10) - all log's will be to base 10 
          W = Watts = (E.E)/R 
          u = micro = 10E-6 (for all units - E, R, P, or V) 
          m = milli = 10E-3 (for all units - E, R, P, or V) 
          dBm = decibel ratio of Watts W to one milliwatt 
                   = 10log10 (W/mW) 
          dBuV = decibel ratio of Volts to one microvolt 
                   = 20log10 {V/uV} or, for example, {E/uV} 

Conversions for 50 ohm systems:

    1. To convert dBm to dBuV add 107 dB:         dBuV = dBm + 107 dB 
    2. To conver dBuV to dBm subtract 107 dB:     dBm = dBuV - 107 dB

Proof: 
                 Remember, 0 dBm = 1 mW (milliwatt) = 0.001 Watts

        1.0         E = Square Root of (W x R)      and    assume R = 50 Ohms 
                    Note: for dBm our reference will be 1 mW 
        2.0         E = Square Root (1mW* x 50 ohms) = 0.224 Volts 
        3.0         dBuV = 20 log10 (0.224 Volts/1uV*) = 107 dBuV 
        4.0          Therefore, 107 dBuV = 0 dBm in a 50 Ohms system 
        5.0         Using this as the scale factor: dBm + 107 dB = dBuV 
                                  * Must be entered in Watts (0.001 or 1x10-3 W = 1 mW) 
                                  * Must be entered in Volts (0.000001 or 1x10E-6 V = 1uV) 

Caution:

One common mistake observed in using conversions is in summing or subtracting powers. 
Powers (Watts) can be added, voltages and log10 values cannot be added or subtracted.

As an example, in case of a 50 Ohm system:

                        5 mW = 7 dBm = 114 dBuV = 501.2 mV 
                +      8 mW = 9 dBm = 116 dBuV = 631.0 mV 
                      13 mW = 11 dBm = 118 dBuV = 794.3 mV

Where, uV stands for micro Volts

          mV stands for milli Volts

Radiated Measurement Units:

                        uV/m = microvolts per meter = Field Strength 
                        dBuV/m = decibel ratio referenced to a microvolt per meter = Field Strength 
                        AF = Antenna Factor = losses associated with the receive antenna 
                        CL = Cable Loss = signal loss within the transmission line/cable

    Field strength measurements are normally measured and calculated as follows: 
          FS(dBuV/m) = Indicated Signal Level(dBuV)+AF(dB)+CL(dB)

Field strengths in microvolts per meter (mV/m) can be calculated by taking the inverse log of the field strength measured in dBuV/m. As an example: 

                         FS(uV/m) = 10 raised to the power of {(dBuV/m)/20}

    where,

FS = Field Strength in decibels above a microvolt per meter or in microvolts per meter 
Indicated Signal Level = signal level observed on the receiver (spectrum analyzer) 
                    Note: usually read in dBuV, or in dBm and converted to dBuV. 
AF = Antenna Factor (signal loss) in dB 
CL = Cable Loss (signal loss) in dB