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# What is the relation between dB AND dBm

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#### ravisankar

##### Member level 1
HELLO,ALL

WHAT IS THE RELATION BETWEEN dB AND dBm,GIVE THE RELATION EQUATION.

THANKYOU,ALL.

#### balaguru thavamani

##### Full Member level 5
what is the relation between db and dbm

0dB=30dBm
Power in dB=10log(power in watt)
Power in dBm=10log(Power in milliwatt).

ygm2000

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V
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### ygm2000

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#### IanP

relation between dbm and db

balaguru thavamani said:
0dB=30dBm
Power in dB=10log(power in watt)
Power in dBm=10log(Power in milliwatt).

Dear friend,
dB has no units ..
I think you should read that stuff too ..

Regards,
IanP

BTW:
30dBm = 0dBW

V
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### Osawa_Odessa

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#### freqzt

relation between dbc & dbm

Hi...

I agree with IanP. dB is dimensionless. The different is that dB is 10 log(value/value) and dBm is 10 log (value/1miliwatt). I think it's clear enough.

CMIIW

#### balaguru thavamani

##### Full Member level 5
relationship betwen db and dbm

yes friends dB has no unit.
Sorry I've expressed in wrong manner.

But 0dBW=30dBm not 1dBW=30dBm

##### Full Member level 5
learning decibels

you're all right. dbm is used as reference,

#### IanP

what is teh relation between db and dbm

balaguru thavamani said:
But 0dBW=30dBm not 1dBW=30dBm
Of course it is ..

#### salma ali bakr

relation between power dbm

XdBm = XdBW + 30

......

x = 10log10(P)

P = 10(x / 10)

where P is the power and x is the dBm

dBm is just the power referenced to one milliwatt (mW)

#### ravisankar

##### Member level 1
subtract dbm from other dbm

thanks to all

#### joe1986

##### Full Member level 4
calculation between db and dbm

IF U SUBTRACT DBM WITH DBM the answer is in DB.

#### jallem

relaltion db and dbm

DBM does not exist.....it is dBm....also
it is not DBm nore dbm nore dbM....

it is dBm

#### Manjunatha_hv

Hello All,

dB or not dB?
Everything you ever wanted to know

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?
A. Winter
Rohde & Schwarz

This Application Note is intended as a refresher on the subject of decibels.

I hope this helps...

---manju---

#### raygor17

##### Newbie level 4
what is 10log10

dBm= 10log10(P/1mW)
dB= 10log10(p/1W)

#### Azulykit

First of all there are a series of mistakes in the various posts above so be careful with all of the comments (this one too).

dB is a relative measurement and dBm is a specific value referenced to milliwatts so therre is no equation to relate the two terms. One often sees a variety of letters after dB indication various reference values.

Again, beware of the comments in this thread. Much of it is in error and will just add to the confusion.

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### spark115

Points: 2

##### Newbie level 3

First of all there are a series of mistakes in the various posts above so be careful with all of the comments (this one too).

dB is a relative measurement and dBm is a specific value referenced to milliwatts so therre is no equation to relate the two terms. One often sees a variety of letters after dB indication various reference values.

Again, beware of the comments in this thread. Much of it is in error and will just add to the confusion.

I agree with Azulykit; this thread is full of errors.

The dB is unitless, like saying some thing is twice (or 2x) as large as something else. Absolute numbers in logarithmic format are expressed as dB[x], where [x] is an abbreviation for the reference. For instance, dBm is a power level referenced to 1 mW. Mathmatically, 10*log10(P/1mW).

Threre's a great reference on the decibel, dB, dBm, etc. on this site:
https://www.bitweenie.com/listings/the-decibel/

That link also mentions when to use 20*log10 or 10*log10, which is a source of confusion sometimes.

js

### js

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#### ehsanelahimirza

##### Full Member level 6

in simple words

dB value can be taken as signal to noise ratio (Better the signal as compared to noise, more the dB value)
and
dBm is power output. (actual power output in miliwatt or watts is known if dBm is known)

#### Azulykit

dB is a particularly useful unit and one of the best ways to use it is to confound your peers. The above thread is a perfect example. It continues for years ......

This book has a chapter devoted to the subject if any of you re serious about really understanding the subject:

Stimson does a fine job of taking all the confusion and some of the fun out of the subject.

Azulykit

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