DBm

From Wikipedia, the free encyclopedia

This article is about a unit for power measurement. For the Dbm Database Engine, see Dbm.

The correct title of this article is dBm. The initial letter is shown capitalized due to technical restrictions.

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

dBm is an abbreviation for the power ratio in decibel (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. dBm (or dBmW) and dBW are independent of impedance (as opposed to dBV which is dependent, for example).

Since it is referenced to the watt, it is an absolute unit, used when measuring absolute power. It should not be confused with dB, a dimensionless unit, which is used when measuring the ratio between two values, such as signal-to-noise ratio.

1 Unit conversions
2 See also
3 References
4 External links

[edit] Unit conversions

Zero dBm equals one 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. To express an arbitrary power P as x dBm, or go in the other direction, the equations

$x = 10 \log_{10}(P / (P{_{1 \ \mathrm{mW}})})$

and

$P =(P_{1 \ \mathrm{mW}}) 10^{(x/10)}$ ,

respectively, should be used. Below is a table summarizing useful cases:

dBm level	Power	Notes
80 dBm	100 kW	Typical transmission power of a FM radio station
60 dBm	1 kW = 1000 W	Typical RF power inside a microwave oven
40 dBm	10 W
36 dBm	4 W	Typical maximum output power for a Citizens' band radio station (27 MHz) in many countries
30 dBm	1 W = 1000 mW	Typical RF leakage from a microwave oven
27 dBm	500 mW	Typical cellular phone transmission power
26 dBm	400 mW	Maximum output power for DCS 1800 MHz mobile phone
25 dBm	316 mW
24 dBm	250 mW
23 dBm	200 mW
22 dBm	160 mW
21 dBm	125 mW	Maximum output from a UMTS/3G mobile phone
20 dBm	100 mW	Bluetooth Class 1 radio, 100 m range (maximum output power from unlicensed FM transmitter)
15 dBm	32 mW
10 dBm	10 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
−5 dBm	316 µW
−10 dBm	100 µW
−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
−70 dBm	100 pW	Typical range (−60 to −80 dBm) of Wireless signal over a network
−80 dBm	10 pW
−111 dBm	0.008 pW	Thermal noise floor for commercial GPS signal bandwidth (2 MHz)
−127.5 dBm	0.000178 pW	Typical received signal power from a GPS satellite
−174 dBm	0.000004 pW	Thermal noise floor for 1 Hz bandwidth
−∞ dBm	0.0 W

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 implied, as indicated by context; equivalent to dBm0p, which is preferred.