Help understanding voltage db vs power db.

That depends on context, in some contexts sometimes ln(x) is natural logarythm and log(x) is base 10.

--
  Jasen.

a log table in some other base would be cumbersome to use. base 10 logs work because ot is easy to scale the input and output of a 1..10 table.

--
  Jasen.

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** So you missed the word "legitimate" ?

Maybe because you are not.

..... Phil

amdx = d*****ad:

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** Setting a wanker you straight is a Hurculean task.

You did know that the "bel" in decibel = Alexander Graham Bell ?

So its dB not db.

Bet you got it mixed up with " dbx " plus every other stupid mistake possible.

.... Phil

Whatever field you are working decibels in ( I know mostly RF) there are reference levels. Reference levels such as dBm (0mW), dBW(1W) , or dBv ( a specified voltage [1V] across some KNOWN resistance [frequenty 50 ohms] which is still a reference power)

In this context, which is certainly my world and I think the context of the post, then dB's are being used in the electrical power context.

But it does all get back to context, which is why you need ...( there are some exceptions if just comparing two arbitrary signals)... some reference (dBm , dBW, dBV) to get the problem staked down.

0dBm = 1 mW

Normally I do not correct trivial typing mistakes but this one I will correct because it may cause confusion

log(x) meaning natural log is a convention used only in calculus textbooks IME. I don't recall ever seeing it used in an engineering context.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Correcting more errors.... 0dBW = 1 Watt, 0dBV = 1V across known resistance

a few more examples......

0dBm +30dB = 30dBm which equals 1000mW which equals 1W which equals 0dBW 3dBm = 0dBm +3dB = 2 mW 10dBm = 0dBm +10dB = 10 mW 26dBW = 0dBW +26dB = 400W

specified voltage [1V] across some KNOWN resistance [frequenty 50 ohms] which is still a reference power)

snipped-for-privacy@columbus.rr.com wrote in news: snipped-for-privacy@googlegroups.com:

Nearly everything in the Ghz RF realm I worked with was all about dBm. All attenuators, amplification calculations, losses, etc. all used the dBm reference.

It's my pleasure to review decibels along with you. Rohde Schwarz wrote a helpful Application Note (AN) on the subject: "dB or not dB? Everything you ever wanted to know about decibels but were afraid to ask..." [1]

True or false: 30 dBm + 30 dBm = 60 dBm? Why does 1% work out to be -40 dB one time but then 0.1 dB or 0.05 dB the next time? These questions sometimes leave even experienced engineers scratching their heads. Decibels are found everywhere, including power levels, voltages, reflection coefficients, noise figures, field strengths and more. What is a decibel and how should we use it in our calculations? This Application Note is intended as a refresher on the subject of decibels. [1]

Allow me to repeat the fact that you know, to wit:

That is correct.

The most common attenuators have values of 3 dB, 6 dB, 10 dB and 20 dB. This corresponds to voltage ratios of 0.7, 0.5, 0.3 and 0.1 or power ratios of 0.5, 0.25, 0.1 and 0.01. [1]

Note.

[1]

Thank you,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
She set out one day In a relative way And returned on the previous night.

Oh no!

Doubling the voltage quadruples the power, and that's 6dB either way.

I never realized how much confusion there is about dBs.

Jeroen Belleman

On 2020-04-18 14:58, Don Kuenz wrote: [...]

Arggh! NO!

6dB is doubling the voltage and quadrupling the power. 3dB is sqrt(2) times the voltage and twice the power.

NEVER does 6dB equal 3dB.

Jeroen Belleman

Yes, I totally agree doubling the voltage quadruples the power and that is a 6dB increase. 3dB is always double the power, always. I think the language and choice of wording is what is fiendish and what trips many people up (and you spotted my trip this time). :-)

piglet

No, not wrong. If a ratio is expressed in logarithm form, it can be given the same name as the Bell System's old proprietary labels on their AC voltmeters 'dBm' only if the ratio is of milliwatts (because that's what the 'm' stood for; milliwatts into the standard 600 ohm load). But, if you care to put another third letter on, or otherwise give an explanation, the nomenclature is entirely repurpose-able.

Language is open that way. Dictionaries have to conform to usages.

The main utility of technical language is brevity. Using decibels to refer to anything other than power ratios erodes the usefulness of the term.

There are lots of specialized decibel units: dbA, dbC, dBm0, dBrnC0, and so on. They're all power ratios taken under different standard measuring conditions.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Neither one is power.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

You are not disagreeing with Phil. He said dB numbers relate to power. You said the Bell System's voltmeters were labeled in dBm because that was related to a mW reference power. You are supporting his point.

Is there such a thing as dBg - dB grams? No.

--

  Rick C. 

  ++ Get 1,000 miles of free Supercharging 
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Phil Allison wrote in news: snipped-for-privacy@googlegroups.com:

After careful consideration, I have come to the conclusion that you pull this shit and nobody filters you and they respond to you. I call one guy an asshole and the entire group filters me. You guys are

*special*.

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** Au contraire, over the years there have been many calls to "kill file" me and I am sure many have.

An evil practise now known as "de-platforming".

** The huge difference is my use of a real name and identity - maybe I get a modicum of respect in return.

FYI:

I have NEVER filtered anyone, cos I like to know what is happening AND especially what is being falsely claimed about me.

Be as silly as turning out the lights while surrounded by muggers.

..... Phil