Re: db Question
- From: "David G. Nagel" <nagel@xxxxxxxx>
- Date: Sat, 11 Feb 2006 10:30:24 -0600
Ian White GM3SEK wrote:
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."
Actually what I was taught was the standard unit is the BEL, named in honor of Alexander Graham Bell. This is a large value hence the DECIBEL or 1/10 of a BEL is generally used.
Dave WD9BDZ
.
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