What is the difference between "dBm" and "dB"?

People sometimes use dB, and sometimes use dBm. Are the two units the same? What’s the difference between “dBm” and “dB”, following this post I will explain them for you 

What is dB ?

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. Decibel is considered as a dimensionless unit because it is a ratio of two quantities with the same unit thus cancellation takes place. It is used for quantifying the ratio between two values. The best example of this is the signal-to-noise ratio.

Sound pressure level is typically measured in dB but the unit is not limited to that quantity alone. There are a lot of uses of this measuring unit particularly in engineering. Since it is applicable in measuring signals, anything that can be expressed in waves may also be measured with dB. In the disciplines of acoustics electronics, dB is liberally utilized.

dB is a ratio of two different power levers, for example the loss in a fiber optic cable. When power is measured in linear units (mW, uW or nW), dB is calculated on a log scale using this formula:

Power (dB) = 10 log (power1/power2)，Here, both P1 and P2 indicate optical power, in mW. That is, dB=10xlgP1-10xlgP2=dBm1-dBm2.

What is dBm ?

dBm is an abbreviation for the power ratio in decibel (dB) of the measured power referenced to one milliwatt (mW). Both dBm (decibel-milliwatts) and mW (milliwatts) are units of optical power. They can be converted as follows:

For example, 1 mW can be converted into 0 dBm. If we are measuring absolute power levels, the measurement is generally referenced to 1 milliwatt (mW), is expressed as "dBm" and the calculation becomes:

Power (dBm) = 10 log (power/1 mW)，Thus 1 mW = 0 dBm, 1 uW = -30 dBm, 1 nW = -60 dBm and two equal powers compared are 0dB (eg. power being the same, there is no loss.)

Conclusion: 

Typically, people use dB when expressing the ratio between two power values. Use dBm when expressing an absolute value of power. dB is the expression of a Gain or an Attenuation and its is dimensionless. dBm is a measurement of a Power level, and indicate how many dB over a reference power of 1 mW. 

The unit dB uses the lower threshold power of hearing as the reference power level, whereas dBm uses 1 milliwatt as the reference power level.

Tags:difference between dBm and dB, loss in a fiber optic cable