Re: dBs

On 5/26/24 19:58, Cursitor Doom wrote:

On Sun, 26 May 2024 19:25:41 +0200, Jeroen Belleman wrote:
 

On 5/26/24 19:09, Cursitor Doom wrote:

I'm feeling cognitively-declined today, probably as a consequence of my
vast age and general ignorance of matters mathematical and everything
else in fact, with the sole exception of "fatuous conspiracy theories."
Can some kind soul assist?
If my RF power meter is reading -13dbm when there's a 20dB attenuator
in line, what is the true power level, please?
I've got an exhaustive App Note from Rhode & Schwartz which claims to
cover everything about decibels, but, er, doesn't.
>
CD.

>
That would be -13 + 20 = +7dBm, provided that impedances are matched
everywhere.

 I was under the impression that one couldn't simply just add dBs to dBms?

You can. That's what decibels were invented for.
Let's spell it out then. You know 0 dBm is 1 mW. So -13 dBm is
10^(-13/10) times 1 mW, or 50 uW.
A 20dB attenuator divides power by a factor of 10^(20/10), that
is, a factor of 100. So before the attenuator, you had 5 mW.
5mW is 10*log(5) is +7 dBm.
Jeroen Belleman