Author: Yan Zhang

- dBW and dBm

Until this point, you have been dealing with decibels as the ratio of two powers, that is,

dB = 10·log10(P1/P2)

With decibels being a ratio of two powers, what is the meaning of decibels for absolute power?

The key to looking at absolute power with decibels is to use P2 as a reference power. To make this more explicit, let's use P2 = 1 W as a reference power. We can then say that

Power (dBW) = 10·log10(P1/1 W)
Power (dBm) = 10·log10(P1/1 mW)

Note that the power P1 must be in the same units as the reference power, exactly as with other decibel problems.

Let's try some problems.
  1. Express 250 kW in dBmW.
  2. Express 250 kW in dBW.
  3. Express 0.1 W in dBmW.
  4. Express 0.1 W in dBW.
  5. Express 0.015 mW in dBmW.
  6. Express 0.015 mW in dBW.
  7. How many dBmW equals the power of 0 dBW?
  8. How many dBW equals the power of 0 dBmW?