dbm - 1: Written by Bruce Robertson 97 Reviewed Dec 2005...

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1: Decibel dBm Definition Written by Bruce Robertson, 97. Reviewed Dec 2005 © Kingfisher International >  An Introduction  Most   communication   systems   (human   speech,   sonar,   microwave,  radio, co-ax, fiber optics, twisted pair etc) are simply described in terms of: Transmitter power  Transmission path degradation  Receiver sensitivity  It is therefore quite natural that communications engineers should use a system  of units   and  measurements  that enables  these  three elements  to  be easily  defined and calculated. Note that transmitter power and receiver sensitivity are absolute power levels (eg  Watts), whereas the transmission path degradation is a relative value (eg %  reduction), which is generally independent of the actual power level involved.  Path degradation may involve a combination of factors, such as attenuation and  dispersion. For this discussion, we will just refer to signal attenuation, or loss. The universal measurement system adopted for this purpose is the   Decibel which is a logarithmic unit. The decibel unit allows system parameters to be  easily   calculated   by   addition   and   subtraction,   rather   than   multiplication   and  division.  Example How this makes calculations simple is shown in an example of a fiber optic   transmission system: Absolute power levels in this example are expressed in  dBm  and generally refer  to input and output power levels. The 'm' refers to the reference level used, in this  case mW (milli Watts). The reference level used for optical systems is usually 1 
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mW, since the absolute transmitter power is often approximately this power level.  So a value of 0 dBm decibel is 1 mW.
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