Decibel milliwatts dBm, decibel-milliwatts, full written as "decibel relative to one milliwatt", is an absolute value referring to power, which is different from dB, which is only a relative value.
dBm or dBmW (decibel-milliwatt) is used to express a level or level of power, expressed in decibels (dB), and its nominal unit is 1 milliwatt (mW). It is used in radio, microwave, and fiber optic communication networks as a convenient measure of absolute power because of its ability to express very large and very small power values in a small numerical range. dBW is a similar unit that uses values in units of 1 watt (1000 milliwatts).
The decibel (dB) is a dimensionless unit used to quantify the ratio between two values, such as the signal-to-noise ratio. dBm is also dimensionless, but since it is compared to a fixed reference value, the dBm ratio expresses an absolute value.
dBm does not belong to the International System of Units (abbreviated SI), so its use in documents or systems that comply with the International System of Units is discouraged (the corresponding SI unit is the watt). However, the unit decibel (dB), without the 'm' suffix, is allowed for relative quantities but not accepted for use directly with SI units. 10 decibels - milliwatts (decibel-milliwatts) can be written as 10 dB (1mW) in the International System of Units.
In audio and telephony, dBm is usually relative to an impedance of 600 ohms, while in radio-frequency (radio-frequency) work, dBm is usually relative to an impedance of 50 ohms. These impedances are the standard settings in the relevant functional circuits.
A power level of 0 dBm corresponds to a power of 1 milliwatt. An increase of 10dB is equivalent to a 10 times increase in power. Therefore, an increase of 20 decibels is equivalent to a 100 times increase in power. A 3 decibel increase is roughly equivalent to doubling the power, which means that 3dBm is roughly equivalent to 2 milliwatts of power. Likewise, for every 3 decibel drop, the power is cut in half or so, so -3 decibels corresponds to about 0.5 milliwatts of power.
The formula for converting any power P(mW) to x(dBm) is as follows:
First convert the power to milliwatts (mW), then take the logarithm and multiply by 10.
For example, 1 milliwatt (1 mW) is converted to decibel milliwatts as 0 dBm.
1mW=10*log(1)=0dBm
1 watt (1 Watt) is converted into decibel milliwatts and is 30dBm.
1W=1000mW=10*log(1000)=10*3=30dBm
Conversely, convert x dBm to P mW:
For example, 20dBm = 1mW*10^2 = 100mW
When the power difference is half,
10^(y/10) =2*10^(x/10)
Take the logarithm of both sides and operate to get:
y - x = 10log2 ≈ 3
So if there is a difference of 3dBm, the power will be about half the difference. The larger the dBm value, the greater the power.
Example of use
For example, in the power description and setting of Bluetooth, the power unit used is dBm.
Bluetooth devices are divided into different power levels, and the corresponding maximum radio frequency signal transmission power is different.
There is an article that introduces Bluetooth power.