db and dbm

Discussion in 'Wireless & RF Design' started by shahnawaz, Jan 20, 2009.

  1. shahnawaz

    Thread Starter Active Member

    Dec 30, 2008
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    hi dear all
    can you plz tell me what is difference between db and dbm.
    what is the relationship between both and how difference also.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. bertus

    Administrator

    Apr 5, 2008
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  4. leftyretro

    Active Member

    Nov 25, 2008
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    DB is a dimensionless value, just a ratio between two values, where as DBM is a ratio of a value referenced to 1 milliwatt. At least that's how I recall the difference.

    Lefty
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    That is correct.

    Eric
     
  6. RAH1379

    Well-Known Member

    Dec 13, 2005
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    +3DBs doubles the power -3DBs cuts it in half. +10 DBs its a 100 fold increase, -10DBs is 1/100th the signal strength.
     
  7. RAH1379

    Well-Known Member

    Dec 13, 2005
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    sorry, i meant a 10DB change is equal to 10 times not 100
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    Roger that.
    The origin of the Decibel is interesting unto itself because it was based on how the human ear perceives changes in audio amplitude. A.G.B determined that our ears are not linear. I've often wondered how many human test subjects A.G.B used for his experiments.
     
  9. David Bridgen

    Senior Member

    Feb 10, 2005
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    dB on its own means that the figure which precedes it is so much greater than some other, unstated, figure. It is therefore a (logarithmic) multiplier, but without knowing the unstated figure there is no way of knowing just how large either of them is.

    e.g. 3dB means a power which is twice as large as another one, or, another way of saying the same thing, a voltage which is 1.41 times as large as another one. In neither case we can't deduce the actual value of the smaller or larger voltage or power.

    dBm means that the figure which precedes it is so much greater than 1mW.
    e.g. 6dBm means a power which is 4 times greater than 1mW, or 4mW.
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    LeftyRetro (Post4) gave the correct and textbook answer.
     
  11. David Bridgen

    Senior Member

    Feb 10, 2005
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    Yes, I noticed that too - except that textbooks usually get the conventions right. dB, not DB, and dBm not DBM.

    And by the way, it's decibel, not Decibel.
     
    Last edited: May 13, 2009
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    Since we're going to get anal over this I might as well point out that the decibel translates to 1/10th of a Bel but I've never seen a Bel.
    Oops, I capitalized again! I guess I just like those caps.
     
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