deciBel miliWatt...

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Externet
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deciBel miliWatt...

Post by Externet »

Hi.
Dealt with it all my life, but never learned what is the SOURCE impedance assumed for this reference.

Explain:
0dBm is a power reference level when 0.775 Vrms appear across a 600Ω load.
With no load, the reference level is 0.775 Vrms and called 0dBu

But at what source impedance?

It is standard to equate 0dBu = 0dBm(600)

But if a 0dBm(600) signal load is disconnected, the voltage must increase; and that is not taken in account to say 0dBm(600) = 0dBu !

[ Note dBu is NOT dBµ ]

Miguel :roll:
- Abolish the deciBel ! -
Robert Reed
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Re: deciBel miliWatt...

Post by Robert Reed »

Exter

0 DBm is a referrence to power level for a given impedance.The `m' being milliwatts. For example 0 DBm (600) equates to 0.775 Vrms across a 600 ohm line ( 1 mw); 0DBm (75) is 0.274 Vrms across a 75 ohm line ( 1 mw); OdBm (50) is 0.224 Vrms across a 50 ohm line ( 1 mw). These levels are just referrence points and as the voltages go up or down, so do their corresponding DBm levels, in a positive or negative direction. They make handling incredible small levels much easier to handle , such as a receiver sensitivity of -117 Dbm rather than 0.00000025 volts. Where DB expresses a ratio, DBm gives an actual value and only has meaning when reffered to a specified impedance. The source impedance matches the load impedance in all these situations and if the load were disconnected you would no longer have the same impedance, and the line level would rise plus 6 DBm. Dont know what DBu is. The other notation with the greek `u' is in refference to microwatts and not milliwatts.
pebe
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Re: deciBel miliWatt...

Post by pebe »

Strictly speaking, the dB is the unit of measurement of Power ratios, (the Neper is a similar unit for voltage or current ratios). Modern practice has allowed a suffix, ie 'm' or 'W' to indicate absolute power levels. So 0dBm is a power level of 775mV across 600 ohms. The source impedance is irrevelant, because that is the power developed in the load.

dB cannot be used to define voltage ratios (but often is!) unless the compared voltages are across identical loads.

0dBu is used by audio engineers to show an audio voltage level of 775mV. So 0dBm is not equal to 0dBu unless the load is 600ohms.

http://rkb.home.cern.ch/rkb/AN16pp/node52.html
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Re: deciBel miliWatt...

Post by Externet »

Hi Robert.
dBu is decibels unloaded. No 600Ω load.
Not to be confused with dBµV for microvolt.

Pebe:
I mostly use audio levels expressed in dBu at my workplace.

My doubt is:
If a 600Ω loaded signal is 0dBm because it shows 0.775V across it; at the moment of disconnecting the load the voltage should increase unless the source impedance is very low or near zero.

Then, is zero Ω the assumed source impedance value to equate dBm to dbu ?
Miguel

Edited: added link to explain:
http://www.sengpielaudio.com/calculator-volt.htm

<small>[ December 12, 2005, 04:07 PM: Message edited by: Externet ]</small>
- Abolish the deciBel ! -
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Re: deciBel miliWatt...

Post by Robert Reed »

Exter
Now I am confused (easy to do). I have never seen the term DBU ever used.I have no idea of what use this would be. DBm specifies a definate power levle over a definate impedance. If in your case ,if you change from that 600 ohm impedance, the DBm reading is no longer valid. It is refferenced only to a 600 ohm impedance! Disconnecting a load will always cause the source voltage to rise. In the case of a 600 ohm source and load , it would rise 6 DB or double the voltage and even though your meter may see this as a 6 DBm increase, its meaningless as ther is no power now, just an open circuit voltge waiting to produce power.If as you stated that your source impedance was very low, again errors in readings would occur because you would obviously not have a 600 ohm line anymore. Meters that read DBm of any line Zo are only valid when used on those line Zo's.
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Re: deciBel miliWatt...

Post by Externet »

Hi Robert.
dBu is my everyday bread. Every hour of the day. I have no problem at all with it. Instead of measuring audio levels in Volts, it is expressed in dBu. No mysteries, it is the industry standard.

dBm is sort of phased out in audio measurement since early eighties when loads became high impedance with fet input operational amplifiers as the norm for input circuitry.

Try to follow me:
If a source generating 0.775 V of audio has 600Ω source impedance, and is terminated or loaded with 600Ω, there will be 0.775V across the load and is called 0dBu, and ALSO 0dBm.

Change the load to a larger impedance, say 6KΩ, and the voltage will raise.

Now choose a source generating 0.775 V of audio at 4Ω source impedance connected to the same 600Ω load. It will still be 0.775 V of audio across the load, and will still be 0dBu AND 0bdm.

Change the load to a larger impedance, say 6KΩ, and the voltage will NOT raise noticeable.

The difference is the low source impedance entering into calculation.

Or, get a piece of paper and draw the schematics of the two cases above.
It can be confusing until the concept is really grabbed, do not worry, you are not alone, there is a general misconception of the decibel, and sadly, some think they know how it is. Takes a while.
Miguel

<small>[ December 12, 2005, 08:46 PM: Message edited by: Externet ]</small>
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Re: deciBel miliWatt...

Post by pebe »

Hi Miguel,

"..If a source generating 0.775 V of audio has 600ohms source impedance, and is terminated or loaded with 600ohms, there will be 0.775V across the load and is called 0dBu, and ALSO 0dBm.."

Not so. I am not familiar enough with dBu to know whether an unloaded voltage is implied, but as far as power is concerned, if the source voltage is 0.775V and the source impedance is 600ohms, then the voltage across a 600ohms load will be half the source voltage, and so the power into the load will be a quarter, ie. -6dBm.


"...It can be confusing until the concept is really grabbed, do not worry, you are not alone, there is a general misconception of the decibel, and sadly, some think they know how it is. Takes a while."

There should be no confusion. By definition, the decibel is a measurement of power ratio.
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Re: deciBel miliWatt...

Post by Will »

How about this ? _ I knew half of it and got the other half from my references -
The bel is named after Alexander Graham Bell and is defined as the common logarithm of the ratio of two powers - If the powers are p1 and P2 then NB (i.e. N sub B - Can't do subscripts in this medium = Log(P2/P1) a preceding negative sign will be appropriate when P1 is larger than P2. A smaller and more covenient unit for small powers is the Decibel i.e. db (d sub b) and Ndb = 10Log(p2/p1)
When source and load impedances are equal (600 ohm is an accepted standard) then the attenuation can be represented as the voltage ratio multiplied by 2.0 i.e. NdB = 20 Log(V2/V1) - it has no meaning when source and load impedances are not equal.
dB1.- Abbreviation of Decibel
dBa - Abbreviation of 'adjusted decibels'
(I thought the 'a' meant attentuation)
dBc - Abbreviation of dB referred to the carrier
dBm = Abbr - of dB referred to one milliWatt where the resistive load impedance is 600 ohms unless specified otherwise
dBmV - Abbr of dB referred to one miiliVolt
Also dBrap, dBrn, dBV, dBW, dBx
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