Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

What's the relation between dB and dBm?

Not open for further replies.


Junior Member level 2
Apr 13, 2007
Reaction score
Trophy points
Activity points
Whats the relation between dB and dBm?

Re: dB and dBm

I ve another doubt here.. My spec says that signal should be within 16dBm(+-3dB).

Is this correct?

Re: dB and dBm

xdB = 10 log (Pout / Pin ) used to measure power gain

xdB = 10 log ( P / 10^-3 ) used to measure power

Re: dB and dBm

Actualy I think it depends on what do you refer to, power or voltage. For power it is as the_risk_master said, but for voltage it is:

20*log(Vout/Vin) for dB, and 20*log(V/0.77) for dBm, not sure for 0.77!
And another thing, xdB = 10 log (P) is not true for one thing, P is in watts so it has to be devided by watts so we could use log()... It should be 10*log(Pout/Pin)...

For the second question, I am not sure what is your question, but I can help you this way:
10*log(P/1mW)=16 => P=1mw*10^(1.6)≈39.8mW this may mean that your signal must be at least 39mW...

Re: dB and dBm

Your signal must be larger than 13dBm and smaller than 19dBm That is how it is !

Re: dB and dBm


Re: dB and dBm

Hello arunapai,

Go through the attached nice PDF document on "dB Everything you ever wanted to know about decibels but were afraid to ask…"....


Re: dB and dBm

16dBm +-3dB

I have probably written it that way (dBm +-dB) a thousand times and never realized that it was possibly not technically accurate :!:
Maybe 16dBm +-3dBm :?:

13dBm ~= 20mW
19dBm ~= 79mW
You can convert that into E or I based on the load

dB is an expression of the power ratio between any two signals.

dBm ( sometimes called dBmW )has one of the signals to reference built in: 1 milliwatt and is therefore a measure of signal level. So 0dBm = 1mW.
2mW = 10xlog10(2) ~ 3dBm.

If you work it backwards you can prove that there is a square relationship between power and voltage or current which is equal to 2 x log of a number. Therefore if the ratio is in voltage or current then the formula is 20 x log(ratio).

A 'Bell' unit was too large ( like a Farad ) so they used 1/10 of a Bell or a decibel. If I remember right, it was Alexander Bell of the Telephone fame that described the exponential relationship between the power in an audio signal and it's apparent loudness.

Re: dB and dBm

Simple relation between dB and dBm is:

1dB= 1dBm+30
Hope it helps you.

Not open for further replies.

Part and Inventory Search

Welcome to