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Converting dBs into dBms?

Discussion in 'Electronic Basics' started by Marcos Martinez Sancho, Apr 26, 2006.

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  1. Hi there,

    how can I convert a value in dBs into dBms (and other way around)?

    As far as I remember, it was something like that:

    db = dBm + 10

    but I am not sure. It's been almost 10 years since I don't use this stuff.

    Any input is appreciated.

    Thanks.

    Marcos
     
  2. Phil Allison

    Phil Allison Guest

    "Marcos Martinez Sancho"


    ** Bit like converting chalk into cheese - it cannot be done.

    "dBs" express a power * ratio * in logarithmic format.

    But "dBm" expresses signal (power) level referenced to 1mW in log format.




    ......... Phil
     
  3. Chris

    Chris Guest

    Hi, Marcos. Phil's right. Try starting by looking at the Wikipedia
    article:

    http://en.wikipedia.org/wiki/Decibel

    Read it all, but you're going to specifically look about halfway down,
    at the Electronics subheading. It's all there.

    dB is generally a relative measure like gain, and has no units. For
    beginners, rules of thumb are:

    Power: 10dB per decade (X10) -- approx. 3dB per octave (X2)

    Voltage: 20dB per decade -- approx. 6 dB per octave

    dBm (or dBmW) , as Phil says, is a specific power measurement,
    referenced to 1mW of power. The calculation ratios expressed in the
    Wiki article apply.

    Good luck
    Chris
     
  4. OK. I get that. All this stuff has become very rusty in my mind, sorry.

    Therefore, dBs may be applied to ratios, such as SNR.

    And dBm may be a power meaasurement.

    Let's say,

    S = 20 dBm

    N = 5 dBm

    SNR = 10 dB

    Am I correct?

    Marcos
     
  5. Chris

    Chris Guest

    Nope. To find the difference, you would just subtract. The difference
    is 15dB (it's now a relative, unitless power measurement).

    This is one of the big advantages of using dB as a measure -- you keep
    the slide rule off the desk. ;-)

    By the way, occasionally some of the basic stuff gets lost in a corner
    if you haven't used it in a while. Sci.electronics.basics is
    anonymous, but you may wait a while for a correct answer, and there's
    always a chance that someone will throw a fly in the ointment. If you
    can't ask a co-worker, or if you're in a hurry, try the Wikipedia:

    http://en.wikipedia.org/wiki/Signal-to-noise_ratio

    Although the articles are done by voluntary contributors, they're
    usually well-done, and if someone finds a problem, there's an easy way
    to resolve it by recommending an edit.

    Good luck
    Chris
     
  6. Yeah, yeah, 20 - 5 = 15 db. That's what I meant.

    Thanks.

    Marcos
     
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