Power are given in dBm in order to make calculus in the Rx/Tx path easier with gain in décibel - dB.
Gain in décibel is defined by G(dB)=10log(Pout/Pin) and power in dBm by P(dBm)=10log(P/1mW) with log
is 10 based logarithme.
Thus on a Zo impédance, knowing that P=Vrms²/Zo, we have
P(dBm)=10log[Vrms²/(1000*Zo)] that is
P(dBm)=10log[Vc²/(2*1000*Zo)] and
P(dBm)=10log[Vpp²/(8*1000*Zo)]
because we can recall that the
root mean square value Vrms is peak value Vc divided by the square root of 2,
and Vc is the peak-peak Vpp divided by 2.
Thus we have the following table under a 50 Ohms load:
| P(mW) | dBm | Vrms(mV) | Vpp(mV) |
| 100 | 20 | 2236,07 | 6324,56 |
| 39,81 | 16 | 1410,86 | 3990,52 |
| 15,85 | 12 | 890,19 | 2517,85 |
| 6,31 | 8 | 561,67 | 1588,66 |
| 2,51 | 4 | 354,39 | 1002,37 |
| 1 | 0 | 223,61 | 632,46 |
| 0,40 | -4 | 141,09 | 399,05 |
| 0,16 | -8 | 89,02 | 251,79 |
| 0.063 | -12 | 56,17 | 158,87 |
| 0,025 | -16 | 35,44 | 100,24 |
| 0,01 | -20 | 22,36 | 63,25 |
|
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