Topic summary
DBm
dBm or dBmW (decibel-milliwatts) is a unit of power level expressed using a logarithmicdecibel (dB) scale respective to one milliwatt (mW). It is commonly used by radio, microwave and fiber-optical communication technicians & engineers to measure the power of system transmissions on a log scale, which can express both very large and very small values in a short form. dBW is a similar unit measured relative to one watt (1000 mW) rather than a milliwatt.
The decibel (dB) is a dimensionless unit, used for quantifying the ratio between two values, such as signal-to-noise ratio. The dBm is also dimensionless, but since it compares to a fixed reference value, the dBm quantity is an absolute one.
The dBm is not a part of the International System of Units (SI) and therefore is discouraged from use in documents or systems that adhere to SI units. (The corresponding SI unit is the watt.) The level of a power P of ten decibels relative to one milliwatt may be written LP/(1 mW) = 10 dB to comply with the SI.
In audio and telephony, dBm is typically referenced relative to the 600-ohm impedance commonly used in telephone voice networks, while in radio-frequency work dBm is typically referenced relative to a 50-ohm impedance.