Whats the difference between DB and DBm?

[saowaluck]

DB and DBm

How difference are the DB and DBm? Does anyone help me? What is the meaning?

 

[computer_terminator]

Re: DB and DBm

dB or dBW relativ to 1W
dBm for dB relativ to 1 milliwatt. 0 dBm= 1 mW = -30dBW

 

[marcomdd]

Re: DB and DBm

dBm is used to express an absolute value of a power in a logarithmic scale.

dB is used to express power ratios (relative value of a power). Typical quantities expressed in dB are tha gain of an amplifier (the ratio of the output power to the input power), the noise figure of a two port network.

Quantities expressed in dB are dimensionless.

dB is not equal to dBW.

Regards

 

[mhamed]

DB and DBm

dBm=10log(power in watt/1mw)
and i think dB can express for absolute of a power too.

 

[wlcsp]

Re: DB and DBm

dB is a ratio, e.g. output/input. For power it is: 10log(out/in), for voltage it is: 20log(out/in). So, one cannot convert the dB output into watt/voltage if the input is not known.

dBm is a ratio to 1mW. So if any quantity is known in dBm, it can be converted into mW.

 

[wlcsp]

Re: DB and DBm

dB is a ratio, e.g. output/input. For power it is: 10log(out/in), for voltage it is: 20log(out/in). So, one cannot convert the dB output into watt/voltage if the input is not known.

dBm is a ratio to 1mW. So if any quantity is known in dBm, it can be converted into mW.

 

[marcomdd]

Re: DB and DBm

.. i think dB can express for absolute of a power too.

You think wrong !

 

[khouly]

Re: DB and DBm

dbm , dbW : represent a absolute power
dbm = 10 log(power/1mW)
dbW = 10 log(power)

db must reperst a ration , like gain , attenuation
gain in db = 10 log (pout/pin)

 

[GlamZ]

Re: DB and DBm

dB must represent a ratio,
but dBm or dBW is an absolute for certain values.
It is still a ratio though.
the ratio is power/1mW (dBm) or power/1W (dBW)
that is a ratio...

... dBm = 10 log (Power/1mW)
ie. 10 dBm = 10mW
20 dBm = 100mW
30 dBm = 1000mW.

(1000-10mW) = 29.96 dBm
(100-10mW) = 19.54 dBm

The result in dBm does not show a constant subtracted value for the constant input in mW since a dBm is a ratio

but 29.26dBm or 19.54dBm is also a constant since the ratio has a constant denominator of one, ie no repeating ratios will turn up.

IE. Both of you are correct.

 

[pico]

DB and DBm

Please follow this link.
**broken link removed**

 

[sergio mariotti]

DB and DBm

It's surprising to read on many preceding posts, lots of wrong definitions .
dB, dBm, dBc, dBV etc. are often a confused topic but really it is simple and it's a basic knowledge on electronics jobs.
dB has been "invented" to became the job more easy, not to complicate it :idea: .

I don't agree computer_terminator, mhamed.

Let me say 1st that every type of measurements, are "relative", in the sense that all type of measurements are a comparition between the actual value and the deposed standard. This is true for lenght, mass, ... voltage, current, power etc.
So for example, 3.86 [W] means 3.86 times the standard unity of the power.
In the same way, using the same example, +5.86[dBW] means 5.86 dB over 1 W , or 10^(5.86/10)=3.86 [W]

The measurement in dB is useful for quick culculation of gain and attenuation and apply to Power, (dimensionally Power is [m*l^2*t^-1]).

dB=10*Log(Pout/Pin)

Of course the dimensional equation still remain valid, so if you have Voltages or Currents (that are related to force rather than power) the dB expression became:

dB=20*Log(Vout/Vin).

Please note that we cannot speach about "dB of power" or "dB of voltage"... dB ARE dB !!

If you find on an article or data sheet an 3rd subscript after dB... like dBm, dBV, dBK, dBc etc, firstly be sure wath the author means.
There is a certain degree of freedom to invent your own subscript, but dBm, dBK, dBV are internationally assigned to a particular significance.

dBm means, how many dB of power over (or less) than 1mW.
dBV means how many dB of Voltage over (or less) than 1V.
And so...

Conclusion:

-> dB is the expression of a Gain or an Attenuation and its is dimensionless.

-> dBm is a measurement of a Power level, and indicate how many dB over a reference power of 1 mW.

-> dBV is a measurement of a Voltage level, and indicate how many dB over the reference level of 1 V.

-> dBc, read as "dB Carrier", dimensionless, indicate how many dB (for example the phase noise) is below the carrier signal.

Addendum
Why dB's and not Nepers?
Basically the answer is: the nature "works" on Nepers, the umans are sensitive to dB's
Neper=ln(10)*dB Neper = 2.30...*dB
I've read somewhere that dB is considered as the minimum discernible variation of power at wich the human hearing is sensitive.

 

[djalli]

DB and DBm

Please be aware of this ever-increasing flood of high-tech terms/notations in electronics. It will increase so just deal with it.

Engineers especially those in RF field use terms like GHz.cm for example 1GHz.cm.

Just adding some fantastic thought at these fantastic notations.

 

[semiconductor]

DB and DBm

I think this link also help you gain an adequate understanding of Decibel:

https://www.cs.berkeley.edu/~randy/Courses/CS294.S96/dB.html