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NNTP-Posting-Date: Sun, 26 May 2024 23:21:58 +0000
From: boB <boB@K7IQ.com>
Newsgroups: sci.electronics.design
Subject: Re: dBs
Date: Sun, 26 May 2024 16:21:56 -0700
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On Sun, 26 May 2024 23:42:15 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

>On 5/26/24 22:58, Cursitor Doom wrote:
>> 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.

It was Bell Labs a long time ago I believe that came up with that
great system.  Don't shoot them !  Well, they're no longer around,
unfortunately.

Yep, Jeroen is right.
boB


>> 
>
>It's not that bad! Adding dB values works fine in the context of
>chains of gain and attenuation. It's easier to add decibels than
>to multiply gain values. One gets used to it. I can usually do
>it in my head. Anyone working in RF or in control systems gets
>proficient at it very quickly.
>
>I agree that there are situations where using decibels can get
>confusing. For example, in light detectors, optical power gets
>converted into current, so a 20dB change in optical power would
>result in only 10dB change in the electrical signal power. But
>let's not go there just yet.
>
>Jeroen Belleman