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When a physical quantity, such as power or intensity, is measured relative to a reference level it is expressed in decibels (dB), which is a logarithmic unit.dB =10 log (P1/P2)
dBm is used to express an absolute value of power and dBm is always relative to1 mW signal.dBm =10 * log (P/1mW)
Decibel (dB) is the ratio of two values whiles dBm is relative to milliwat and the reference for absolute power
dBm is a power unit in the logarithmic scale
dB is a ratio that represents gain or loss
dB is used to quantify ratio between two intensity or power values while dBm is used to express an absolute value of power.
Le dBm est unité de puissance absolue (0 dBm=1mW), alors que le dB exprime un rapport entre deux puissances : on peut par exemple dire que la puissance d'un amplificateur est de 47dBm (soit 50W), ce qui est une valeur absolue, alors que si l'on parle de son gain qui est de 20 dB par exemple, cela correspond à un rapport entre la puissance d'entrée et la puissance de sortie de 100.
Decibel (dB) and dB relative to a milliwatt (dBm) represent two different but related concepts. A dB is a shorthand way to express the ratio of two values. As a unit for the strength of a signal, dB expresses the ratio between two power levels. To be exact, dB = 10log (P1/P2).
In general dB =10*log10(X) where X is unitless.
Power in (dB) is a logarithmic ratio of powers
PdB=10* log10(P1/P2)
Since dB is a ratio of powers, dBm is defined by forming the ratio of the power you want to express relative to a reference power, which is 1 mW
Formally speaking,
PTdBm=10* log10(PT/1mW)
power or intensity, is measured relative to a reference level it is expressed in decibels
dB =10 log (P1/P2)
dBm is used to express an absolute value of power relative to1 mWdBm =10 * log (P/1mW) this is usual more simple for small quantity
more info
http://www.differencebetween.net/science/difference-between-db-and-dbm/
As per your question "is dB relative to watts and dBm to miliwatts". dB is used in comparing two powers (supppse P1 and P2, dB =10.log(P1/P2)) and the answer is expressed in "watts" or is in "watts". dB is not relative to watts but dB is meausred in watts
dBm is relative to1 milliwatt :
dBm =10.log(P1/*P2)
*P2 =1mWfor the equation above P1 is always measured and substituted in milliwatts and P2 is always substituted as1mWdBm is used in comparing powers relative to1 milliwatt
is dB is relative to watts and dBm to miliwatts
in terms of power it is20logp2/p1 and in terms of signal it is10logv2/v1
