dBm
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 unit of level used to indicate that a power ratio is expressed in decibels (dB) with reference to one milliwatt (mW). It is used in radio, microwave and fiber-optical communication 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 compared to dBW, which is referenced to one watt (1,000 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.
The dBm is also dimensionless but since it compares to a fixed reference value the dBm rating is an absolute one.
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]
Contents
1 Unit conversions
2 See also
3 References
4 External links
Unit conversions
A power level of 0 dBm corresponds to a power of 1 milliwatt. A 10 dB increase in level is equivalent to a 10-fold increase in power. A 3 dB increase in level is approximately equivalent to doubling the power, which means that a level of 3 dBm corresponds roughly to a power of 2 mW. Similarly, for each 3 dB decrease in level, the power is reduced by about one half, making −3 dBm correspond to a power of about 0.5 mW.
To express an arbitrary power P in mW as x in dBm, or vice versa, the following equivalent expressions may be used:
- x=10log10P1 mW,P=1 mW⋅10x10,{displaystyle {begin{aligned}x&=10log _{10}{frac {P}{1~{text{mW}}}},\P&=1~{text{mW}}cdot 10^{frac {x}{10}},end{aligned}}}
Alternatively, using 1 W as the reference value instead of 1 mW
- x=30+10log10P1 W,P=1 W⋅10x−3010,{displaystyle {begin{aligned}x&=30+10log _{10}{frac {P}{1~{text{W}}}},\P&=1~{text{W}}cdot 10^{frac {x-30}{10}},end{aligned}}}
Below is a table summarizing useful cases:
| Power 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 | 1,500 W is the maximal 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 |
| 55 dBm | ~300 W | Typical single-channel RF output power of a Ku-band geostationary satellite |
| 50 dBm | 100 W | Typical total thermal radiation emitted by a human body, peak at 31.5 THz (9.5 μm) Typical maximal output RF power from a ham radio HF transceiver |
| 40 dBm | 10 W | Typical PLC transmission power |
| 37 dBm | 5 W | Typical maximal output RF power from a handheld ham radio VHF/UHF transceiver |
| 36 dBm | 4 W | Typical maximal output power for a citizens band radio station (27 MHz) in many countries |
| 33 dBm | 2 W | Maximal output from a UMTS/3G mobile phone (Power class 1 mobiles) Maximal output from a GSM850/900 mobile phone |
| 30 dBm | 1 W = 1,000 mW | DCS or GSM 1,800/1,900 MHz mobile phone. |
| 29 dBm | 794 mW | |
| 28 dBm | 631 mW | |
| 27 dBm | 500 mW | Typical cellular phone transmission power Maximal output from a UMTS/3G mobile phone (Power class 2 mobiles) |
| 26 dBm | 400 mW | |
| 25 dBm | 316 mW | |
| 24 dBm | 251 mW | Maximal output from a UMTS/3G mobile phone (Power class 3 mobiles) 1,880–1,900 MHz DECT (250 mW per 1,728 kHz channel). |
| 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 | Maximal 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 Wi-Fi/ISM band (5 mW/MHz). Bluetooth Class 1 radio. |
| 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 | |
| 2 dBm | 1.6 mW | |
| 1 dBm | 1.3 mW | |
| 0 dBm | 1.0 mW = 1000 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 | Maximal received signal power of wireless network (802.11 variants) |
| −13 dBm | 50.12 µW | Dial Tone for the Precise Tone Plan found on public switched telephone networks in North America |
| −20 dBm | 10 µW | |
| −30 dBm | 1.0 µW = 1000 nW | |
| −40 dBm | 100 nW | |
| −50 dBm | 10 nW | |
| −60 dBm | 1.0 nW = 1000 pW | The Earth receives one nanowatt per square metre from a magnitude +3.5 star[6] |
| −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 | |
| −100 dBm | 0.1 pW | Minimal received signal power of wireless network (802.11 variants) |
| −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 V dissipates 1 mW in a 600 Ω load.[7] The corresponding voltage level is 0 dBu, without the 600 Ω restriction. Conversely, for RF situations with a 50 Ω load, 0 dBm corresponds to approximately 0.224 volts, since 0.224 V dissipates 1 mW in a 50 Ω load.
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 unit decibel (dB), without the 'm' suffix, is accepted for use alongside SI units.[8]
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[7] in the paper "A New Standard Volume Indicator and Reference Level".[9]
See also
- dBW
- Decibel
References
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).
^ Bigelow, Stephen. Understanding Telephone Electronics. Newnes. p. 16. ISBN 978-0750671750..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
^ Carr, Joseph (2002). RF Components and Circuits. Newnes. pp. 45–46. ISBN 978-0750648448.
^ "Part 97 - Amateur Radio". ARRL. Retrieved 2012-09-21.
^ [1] FCC Part 97 Amateur Radio Service - Rule 97.215, Telecommand of model craft, section (c).
^ FCC Web Documents citing 15.219.
^ Radiant Flux of a Magnitude +3.5 Star.
^ ab Davis, Gary (1988). The Sound Reinforcement Handbook. Yamaha. p. 22. ISBN 0881889008.
^ Thompson and Taylor 2008, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811.
^ Chinn, H. A.; D. K. Gannett; R. M. Moris (January 1940). "A New Standard Volume Indicator and Reference Level" (PDF). Proceedings of the Institute of Radio Engineers. 28 (1): 1–17. doi:10.1109/JRPROC.1940.228815.
External links
- The dBm calculator for impedance matching
- Convert dBm to watts
