Re: db Question

dansawyeror wrote:
There are three terms, idb, pdb, and vdb. They are not the same. idb and pdb have the same value: 10*log10 (i/i0), while vdb is 20*log10(v/v0).


"A decibel is a decibel is a decibel". There is only one kind of decibel, and by definition it is based on a power ratio.

Decibels are always used for comparing power levels. If you're comparing current or voltage levels, you first have to convert them into a power ratio, and that is the reason why you have to use a different formula. Your starting-point was different, but the definition of the decibel remains the same.


There is a similar-looking term, sometimes written as either "dBV", "dBmV" or "dBuV", but that has a different meaning. Anything tagged on after the "dB" indicates a standard reference level. For example, "dBV" means "a power ratio in decibels, relative to the power level of a 1V rms signal measured at the same point in the circuit."


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
.

Relevant Pages

  • Re: db Question
    ... Decibels are always used for comparing power levels. ... If you're comparing current or voltage levels, you first have to convert them into a power ratio, and that is the reason why you have to use a different formula. ... but the definition of the decibel remains the same. ... Actually what I was taught was the standard unit is the BEL, named in honor of Alexander Graham Bell. ...
    (rec.radio.amateur.antenna)
  • Re: 3 dB bandwidth
    ... >>> It is quite clear that only the power ratio is directly described by ... > the definition of the decibel changed - if it actually ever did. ... There is simply no bloody law that says Bell labs original definitions ... what is a correct definition. ...
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  • Re: 3 dB bandwidth
    ... >>> It is quite clear that only the power ratio is directly described by ... > the definition of the decibel changed - if it actually ever did. ... > 1931 The Admiralty Handbook of Wireless Telegraphy ... dBu, dBV, dB-SPL, dB-A and others. ...
    (sci.electronics.design)
  • Re: 3 dB bandwidth
    ... > Adrian Tuddenham wrote: ... >> It is quite clear that only the power ratio is directly described by ... the definition of the decibel changed - if it actually ever did. ... 1931 The Admiralty Handbook of Wireless Telegraphy ...
    (sci.electronics.design)