dB is a ratio for example dB = 10 log10 (P1/P2)
so increasing power from 100 watts to 200 watts is 3 dB (3.01dB actually)
But so is increasing power from 10 watts to 20 watts = 3dB
Since it is a ratio -- it is unitless

dBm is referenced to one milliwatt thus is a discrete power level

In the equation above P2 is always one milliwatt

Thus increasing power from 1 milliwatt to 2 milliwatts is 3dBm
From 1 milliwatt to 10 milliwatts is 10dBm etc

Not sure of your reference to ip3 but suspect you mean input third order
intercept point as used in amplifiers
Google ip3 to get explanations of this and how it is measured.
--
Caveat Lector (Reader Beware)


"jason" wrote in message
ups.com...
Hello All

I am beginner in RF
I may ask about some silly question and please pardon me
May I know what actually the unit of dbm and db is different from one
another?
If they are different how can we minus the gain in unit of db from a
IP3 in unit of dbm?
Kindly enlighthen
Thank you all


rgds and thanks
Jason