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DBm

For other uses, see DBM (disambiguation).
A schematic showing the relationship between dBu (the voltage source) and dBm (the power dissipated as heat by the 600 Ω resistor)

dBm or dBmW (decibel-milliwatts) is a unit of power level expressed using a logarithmic decibel (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 (1,000 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,[1][2] but since it compares to a fixed reference value, the dBm rating 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.) However, the unit decibel (dB), without the 'm' suffix, is permitted for relative quantities, but not accepted for use directly alongside SI units. Ten decibel-milliwatts may be written 10 dB (1 mW) in SI.[3]: 7.4 

In audio and telephony, dBm is typically referenced relative to the 600-ohm impedance[4] commonly used in telephone voice networks, while in radio-frequency work dBm is typically referenced relative to a 50-ohm impedance.[5]

  1. ^ Green, Lynne D. (2019). Fiber Optic Communications. CRC Press. p. 181. ISBN 9781000694512.
  2. ^ Kosatsky, Tom (2013). Radiofrequency Toolkit for Environmental Health Practitioners (PDF). British Columbia Centre for Disease Control. p. 8. Archived (PDF) from the original on 2022-10-09.
  3. ^ Thompson and Taylor 2008, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811 Archived 2016-06-03 at the Wayback Machine.
  4. ^ Bigelow, Stephen (2001). Understanding Telephone Electronics. Newnes. pp. 16. ISBN 978-0750671750.
  5. ^ Carr, Joseph (2002). RF Components and Circuits. Newnes. pp. 45–46. ISBN 978-0750648448.
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