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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: dBs
Date: Sun, 26 May 2024 20:58:31 -0000 (UTC)
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On Sun, 26 May 2024 21:48:21 +0200, Jeroen Belleman wrote:

> 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

Oh I know you're figures are correct, Jeroen. But to check them I had to 
use look-up tables off the net:

-13dBm = 0.05mW
20dB = 100X
0.05X100 = 5mW
5mW = =7dBm

Sometimes you can just straight add-up dBs and other times you can't and I 
can never remember when it's appropriate and when it's not. To be safe, I 
revert to the method I showed above. It's longer, but at least I know I 
can rely on the result. Whoever invent dBs "to make things simpler" needs 
to have their grave desecrated and their name effaced from history IMO.