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dB vs dBm vs dBW vs dBC | Difference between dB dBm dBW and dBC

This page compares dB vs dBm vs dBW vs dBC provide difference between dB dBm dBW and dBC units.The other useful links to difference between various terms are provided here.

RF Power is the measurement which is carrier out by engineers so often. Power meter is usually used to measure integrated RF Power across the band of test. Spectrum Analyzer is used to measure instantaneous power at various frequencies of interest.

It is measured usually in watts, but there are units such as dBm, dBW which helps represent linear scale on decibel with respect to either 1 mw or 1 watt.

dB is the ratio of either amplitude of voltage or current of the two signal. dB stands for Decibel and will be taken as log of ratio of two power values(or voltage).

dB= 10 log(P2/P1) or 20 log (A2/A1) ...Equation-1

dBm stands for decibel with respect to 1 milliwatt.


dBm value mw/Watt value
0 dBm 1 mw
30 dBm 1 Watt
40 dBm 10 watt
50 dBm 100 watt

dBW stands for decibel with respect to 1 Watt

Above explaination helps one understand difference between dB and dBm. Pls. note that unit decibel (dB) can be used for ratio of two similar quantities such as voltage. Hence both numerator and denominator should be voltage. dBm is specifically refers to power unit. It is expressed as follows:
Power (dBm) = 10* log10(Power in milliwatt)
...Equation-2

From the equation-1 and equation-2 one can easily differentiate difference between dB and dBm units.

Difference between dBm and dBW

XdBW = XdBm - 30.
Hence as per above equation,
0 dBW = +30 dBm or -30 dBW = 0 dBm

dBC stands for decibel value with respect to carrier power level
Mainly used to measure level of phase noise or harmonics or Spurious signals with respect to carrier power level.
For example,
Phase noise of modulated carrier at 1 KHz from the carrier is about 70 dBc.
Harmonics of a 6 GHz RF carrier i.e. 12GHz is about 60 dBc
Spurious of a 6GHz RF carrier i.e. fractional multiple of 6 GHz is about 60dBc.

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