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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: dBs
Date: Sun, 26 May 2024 21:48:21 +0200
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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