# Help understanding voltage db vs power db.

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The fact that I know: a 6db voltage gain is a 3db power gain.
If that is wrong set me straight.

I'm confused after using this calculator.
http://www.sengpielaudio.com/calculatorVoltagePower.htm

The first calculator labeled,
? Find decibel voltage gain and ratio out/in by entering input and
output voltage:?
If I put 1 volt on the input line and 2 volts on the output line, the
calculator gives a  6db gain.

The second calculator labeled,
?Find decibel power gain and ratio out/in by entering before and after
power?
In order to enter power in Watts, I'll assume a 50 ohm system and
calculate power using
V^2/R=P. As above 1volt^2/50 =0.02 Watts for input power and 2^2/50=0.08
Watts for output power.
When I enter 0.02 and 0.08 into the second calculator, I get 6db power gain.

Back to the fact that I know, a 6db voltage gain is a 3db power gain.

So, what am I doing wrong that I don't get 3db for the power?

Re: Help understanding voltage db vs power db.
On Friday, April 17, 2020 at 8:02:18 AM UTC-4, amdx wrote:

d

after

=0.08

in.

dB is a way to relate one POWER  (always power) to another POWER.  Frequent
ly it will relate one measured power to another reference power.

A reference power will be something like 0dBm.  That is an absolute referen
ce power of 1 mW.  Now you can say that a signal is 10dB lower than another
power.  In the case of 10dB lower than 0dBm you get 10dBm - 10 dB = -10
dBm.

You have to remember that dB's relate one power to another and it only work
s if you know which one is the reference power.

Now regarding voltage and power.......

remember dBs are all about power...power ......power......

10 dB multiplies power by a factor of 10.  3dB doubles the power.  3 dB alw
ays doubles the power.  6 dB multiplies the power by 4.  remember that.

Back to voltage.....

In most cases we cannot measure power so we have to measure voltage instead
.  We want to relate things in terms of power but we can tend to have volta
ge measurements available to us.  Now if you have 1V across 1 ohm that is 1
Watt

Lets call that 0dBw ( that is 0 dB relative to 1Watt  which equals.....1Wat
t)

Now if you double the voltage on that resistor you get 2 volts but the POWE
R (remember dbs are about POWER) goes up by 4 ... which is 6 dB.

You now have 6dB above the reference of 0dBW...or 6dB above 1 W ===
= 4 Watts

so when taking voltage measurements you get
20log ( voltage measured/ voltage on reference).

You have to figure out when something is an absolute....dBm, dBW

and when it is relative....dB.

Re: Help understanding voltage db vs power db.

This is what you need to have burned into your brain.  Anything else is

Decibels are all about POWER, there are no voltage decibels.

--
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Re: Help understanding voltage db vs power db.
On 18/4/20 2:44 am, Adrian Tuddenham wrote:

Sorry, but decibels are not about any unit. Not power, not voltage.
They're only about ratios. They're just a log representation of a ratio,
which is a *pure unit-less number*.

RF designers assume they're about power. Audio folk assume they are
A-weighted subjective measures of loudness. Etc.... but those are all
the assumptions of a specific community, and they're just convenient lies.

Because dB are not *about* any of those units.

Clifford Heath

Re: Help understanding voltage db vs power db.
Clifford Heath is an Idiot wrote:

-----------------------------------

** 100% WRONG.

The only quantities that can be legitimately expressed in dB are actual power or directly relate to power.

A car can have 3dB more power than another but not 3dB more speed.

** The only convenient LIAR here is YOU.

Bugger off.

Re: Help understanding voltage db vs power db.

I never thought I would end up agreeing with you - but this time I do!

--
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Re: Help understanding voltage db vs power db.

kinetic energy is 0.5mv^2

3db more Speed is about 1.41 times faster.

--
Jasen.

Re: Help understanding voltage db vs power db.
Jasen Betts wrote:

--------------------

** So you missed the word "legitimate" ?

Maybe because you are not.

.....  Phil

Re: Help understanding voltage db vs power db.
On 2020-04-18 04:48, Jasen Betts wrote:

Neither one is power.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
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Re: Help understanding voltage db vs power db.
On Saturday, April 18, 2020 at 4:32:01 AM UTC-4, Phil Allison wrote:

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.

Re: Help understanding voltage db vs power db.
On Saturday, April 18, 2020 at 7:11:15 AM UTC-4, snipped-for-privacy@columbus.rr.com wrote:

0dBm = 1 mW

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

Re: Help understanding voltage db vs power db.
On Saturday, April 18, 2020 at 7:15:32 AM UTC-4, snipped-for-privacy@columbus.rr.com wrote:

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)

Re: Help understanding voltage db vs power db.
snipped-for-privacy@columbus.rr.com wrote in

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.

Re: Help understanding voltage db vs power db.
On Saturday, April 18, 2020 at 1:32:01 AM UTC-7, Phil Allison wrote:

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.

Re: Help understanding voltage db vs power db.
On 2020-04-18 18:13, whit3rd wrote:

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
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Re: Help understanding voltage db vs power db.
On Saturday, April 18, 2020 at 6:13:21 PM UTC-4, whit3rd wrote:

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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Re: Help understanding voltage db vs power db.

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*.

Re: Help understanding voltage db vs power db.

-----------------------------------

** 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

Re: Help understanding voltage db vs power db.
On 4/18/2020 7:23 PM, Phil Allison wrote:

Many off us don't kill file you because you have a high technical
knowledge of interest to those using test equipment for testing and
repairs. It has zero to do with using your real name.
That said, your online personality just sucks, why you need to
degrade people rather than just correcting them or presenting the
facts as you see them is a character flaw.
Why would you kill file anyone, you need targets.
Mikek

Re: Help understanding voltage db vs power db.
amdx wrote:

------------

** You could not be more wrong abut that last claim.

** Your question has a false assumption.

I correct people all the time and post the facts too.

Then, if they become defensive and act all aggressive -  I have to right to shit all over them.

Fools can never learn about what they wrongly think they know already.

Fools have no right to post here at all.

And you are one of them.

....  Phil