Let's talk loosely, and talk about money.

If I've got twice as much money than Ian has, then
I've got 3dB more.

How much do I have? Don't know.

If Ian has three times as much as Richard, then he
has 4.7 dB more than Richard, and I have 3 + 4.7 =7.7dB
more than Richard.

How much do I have? Don't know.
How much does Ian have? Don't know.
How much does Richard have? Don't know.

OK, assuming that we could deal in 1/10ths of a cent (1 milli-dollar!)
let's assume that Richard has $100 = 50dBm.

Ian therefore has 50 + 4.7 = 54.7 dBm.
And I have 54.7 + 3 = 57.7 dBm.

The answer to your question is that you can start off with an
actual reading in dBm, but everything else relative to that is
in dB only (although it does give a result in dBm).

If the above doesn't answer your question, then, sorry,
but I give up. (Which doesn't mean that my interest is 0dBm
but -173 dBm, ie, indiscernible below the noise)

"Jason" wrote in message
oups.com...
But why we can add or minus gain and IP3 which are in different unit(db
and dbm)?
Anyone knows?
Thank you

rgds
Jason