IP1dB vs OP1dB

[ge]

In the equation OP1dB (dBm) = IP1dB (dBm) + Small Signal Gain (dB) - 1 dB
Where does the " - 1dB" come from? If you can point me to a site, that is just as good. Every site I have looked at pivots to IP3.

Thanks

 

[G4BCH]

Google 1dB compression point

 

[albbg]

Let's consider a device having a gain Go at an input level low enough to be far from the compression. Increasing the input level, after a while, the gain will starts to decrease. When the gain will be decreased by 1dB, that is Gain=Go-1dB, the 1dB compression point is reached. The input level at this point is called IP1dB and, since at this point the gain is Go-1dB then the output power will be OP1dB = IP1dB +(Go-1dB)