dBm

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A schematic showing the relationship between dBu (the voltage source) and dBm (the power dissipated as heat by the 600 Ω resistor)

dBm (sometimes dBmW or Decibel-milliwatts) is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form. Compare dBW, which is referenced to one watt (1000 mW).

Since it is referenced to the watt, it is an absolute unit, used when measuring absolute power. By comparison, the decibel (dB) is a dimensionless unit, used for quantifying the ratio between two values, such as signal-to-noise ratio.

In audio and telephony, dBm is typically referenced relative to a 600 ohm impedance,[1] while in radio frequency work dBm is typically referenced relative to a 50 ohm impedance.[2]

Unit conversions[edit]

0 dBm equals 1 milliwatt. A 3 dB increase represents roughly doubling the power, which means that 3 dBm equals roughly 2 mW. For a 3 dB decrease, the power is reduced by about one half, making −3 dBm equal to about 0.5 milliwatt or 500 microwatts.

To express an arbitrary power P in watts as x in dBm, or vice versa, the following equivalent expressions may be used:

\begin{align}
  x &= 10 \log_{10} \frac{P}{ 1 \mathrm{mW}} \\
  x &= 30+10 \log_{10} \frac{P}{ 1 \mathrm{W}}
\end{align}

and

\begin{align}
  P &= 1 \text{mW} \cdot 10^{\frac{x}{10}}\\
  P &= 1 \text{W} \cdot 10^{ \frac{ x-30 }{10} }
\end{align}

where P is the power in W and x is the power ratio in dBm. Below is a table summarizing useful cases:

dBm level Power Notes
80 dBm 100 kW Typical transmission power of FM radio station with 50-kilometre (31 mi) range
62 dBm 1.588 kW = 1,588 W 1500 W is the maximum legal power output of a U.S. ham radio station.[3]
60 dBm 1 kW = 1,000 W Typical combined radiated RF power of microwave oven elements
50 dBm 100 W Typical thermal radiation emitted by a human body

Typical maximum output RF power from a ham radio HF transceiver

40 dBm 10 W Typical PLC (Power Line Carrier) transmit power
37 dBm 5 W Typical maximum output RF power from a handheld ham radio VHF/UHF transceiver
36 dBm 4 W Typical maximum output power for a Citizens' band radio station (27 MHz) in many countries
33 dBm 2 W Maximum output from a UMTS/3G mobile phone (Power class 1 mobiles)

Maximum output from a GSM850/900 mobile phone

30 dBm 1 W = 1,000 mW Typical RF leakage from a microwave oven[citation needed]

DCS or GSM 1,800/1,900 MHz mobile phone. EIRP IEEE 802.11a (20 MHz-wide channels) in either 5 GHz Subband 2 (5,470–5,725 MHz) provided that transmitters are also IEEE 802.11h-compliant, or U-NII-3 (5,725–5,825 MHz). The former is EU only, the latter is US only.

29 dBm 794 mW
28 dBm 631 mW
27 dBm 500 mW Typical cellular phone transmission power

Maximum output from a UMTS/3G mobile phone (Power class 2 mobiles)

26 dBm 400 mW
25 dBm 316 mW
24 dBm 251 mW Maximum output from a UMTS/3G mobile phone (Power class 3 mobiles)

1,880–1,900 MHz DECT (250 mW per 1,728 kHz channel). EIRP for Wireless LAN IEEE 802.11a (20 MHz-wide channels) in either the 5 GHz Subband 1 (5,180–5,320 MHz) or U-NII-2 & -W ranges (5,250–5,350 MHz & 5,470–5,725 MHz respectively). The former is EU only, the latter is US only.

23 dBm 200 mW EIRP for IEEE 802.11n Wireless LAN 40 MHz-wide (5 mW/MHz) channels in 5 GHz subband 4 (5,735–5,835 MHz, US only) or 5 GHz subband 2 (5,470–5,725 MHz, EU only). Also applies to 20 MHz-wide (10 mW/MHz) IEEE 802.11a Wireless LAN in 5 GHz Subband 1 (5,180–5,320 MHz) if also IEEE 802.11h compliant (otherwise only 3 mW/MHz → 60 mW when unable to dynamically adjust transmission power, and only 1.5 mW/MHz → 30 mW when a transmitter also cannot dynamically select frequency).
22 dBm 158 mW
21 dBm 125 mW Maximum output from a UMTS/3G mobile phone (Power class 4 mobiles)
20 dBm 100 mW EIRP for IEEE 802.11b/g Wireless LAN 20 MHz-wide channels in the 2.4 GHz ISM band (5 mW/MHz).

Bluetooth Class 1 radio. Maximum output power from unlicensed AM transmitter per U.S. Federal Communications Commission (FCC) rules 15.219.[4]

19 dBm 79 mW
18 dBm 63 mW
17 dBm 50 mW
15 dBm 32 mW Typical Wireless LAN transmission power in laptops.
10 dBm 10 mW
7 dBm 5.0 mW Common power level required to test the Automatic Gain Control circuitry in an AM receiver.
6 dBm 4.0 mW
5 dBm 3.2 mW
4 dBm 2.5 mW Bluetooth Class 2 radio, 10 m range
3 dBm 2.0 mW More precisely (to 8 decimal places) 1.9952623 mW
2 dBm 1.6 mW
1 dBm 1.3 mW
0 dBm 1.0 mW = 1,000 µW Bluetooth standard (Class 3) radio, 1 m range
−1 dBm 794 µW
−3 dBm 501 µW
−5 dBm 316 µW
−10 dBm 100 µW Typical maximum received signal power (−10 to −30 dBm) of wireless network
−20 dBm 10 µW
−30 dBm 1.0 µW = 1,000 nW
−40 dBm 100 nW
−50 dBm 10 nW
−60 dBm 1.0 nW = 1,000 pW The Earth receives one nanowatt per square metre from a magnitude +3.5 star[5]
−70 dBm 100 pW
−73 dBm 50.12 pW "S9" signal strength, a strong signal, on the S-meter of a typical ham or shortwave radio receiver
−80 dBm 10 pW Typical range (−70 to −90 dBm) of wireless received signal power over a network (802.11 variants)
−100 dBm 0.1 pW
−111 dBm 0.008 pW = 8 fW Thermal noise floor for commercial GPS single channel signal bandwidth (2 MHz)
−127.5 dBm 0.178 fW = 178 aW Typical received signal power from a GPS satellite
−174 dBm 0.004 aW = 4 zW Thermal noise floor for 1 Hz bandwidth at room temperature (20 °C)
−192.5 dBm 0.056 zW = 56 yW Thermal noise floor for 1 Hz bandwidth in outer space (4 kelvins)
−∞ dBm 0 W Zero power is not well-expressed in dBm (value is negative infinity)

The signal intensity (power per unit area) can be converted to received signal power by multiplying by the square of the wavelength and dividing by 4π (see Free-space path loss).

In United States Department of Defense practice, unweighted measurement is normally understood, applicable to a certain bandwidth, which must be stated or implied.

In European practice, psophometric weighting may be, as indicated by context, equivalent to dBm0p, which is preferred.

In audio, 0 dBm often corresponds to approximately 0.775 Volts, since 0.775 Volts dissipates 1 mW in a 600 Ω load.[6] dBu measures against this reference voltage without the 600 Ω restriction.

The dBm is not a part of the International System of Units and therefore is discouraged from use in documents or systems that adhere to SI units (the corresponding SI unit is the watt). However the straight decibel (dB), being a unitless ratio of two numbers, is perfectly acceptable.[7]

Expression in dBm is typically used for optical and electrical power measurements, not for other types of power (such as thermal). A listing by power levels in watts is available that includes a variety of examples not necessarily related to electrical or optical power.

The dBm was first proposed as an industry standard[6] in the paper "A New Standard Volume Indicator and Reference Level".[8]

See also[edit]

References[edit]

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

  1. ^ Bigelow, Stephen. Understanding Telephone Electronics. Newnes. p. 16. ISBN 978-0750671750. 
  2. ^ Carr, Joseph (2002). RF Components and Circuits. Newnes. pp. 45–46. ISBN 978-0750648448. 
  3. ^ "Part 97 - Amateur Radio". ARRL. Retrieved 2012-09-21. 
  4. ^ FCC Web Documents citing 15.219
  5. ^ Radiant Flux of a Magnitude +3.5 Star
  6. ^ a b Davis, Gary (1988). The Sound Reinforcement Handbook. Yamaha. p. 22. ISBN 0881889008. 
  7. ^ Thompson and Taylor 2008, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811
  8. ^ Chinn, H.A.; D.K. Gannett, R.M.Moris (January 1940). "A New Standard Volume Indicator and Reference Level". Proceedings of the Institute of Radio Engineers 28 (1): 1–17. doi:10.1109/JRPROC.1940.228815. 

External links[edit]