Dbm - Decibels Power Ratio

By Author: Jamie Hanson
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dBm is the abbreviation of the power ratio which is expressed in decibels (dB) which is referred as the measured power equivalent to one milliwatt (mW). dBm is sometimes known as dBmW as it measures the absolute power that is used in microwave, radio and in the fiber optic networks. It is used as a measurement source because it has the ability to express the largest and the smallest values in short form. As it has a reference to watt it is an absolute unit which is used for measuring absolute powers.

In comparison the dB is a unit which is dimensionless. So dB is used to show the quantity ratio between two values like the ratio between the signal and the noise. The signal and the noise ratio is a signal ratio between desired and undesired signal of noise in the calculation of the average power level of transmission.

So if the level of noise is high on the circuit the ratio of signal to noise is low but if the quality of the circuit is good then the ratio of the signal to noise will be high. This signal to noise ratio is SNR.

Formula for dBm

The formula that is used to calculate dBm is db = log10 ...
... Signal Power/ .001 watt. If there is a negative sign before the dBm it indicates a loss of signal and if there is no sign or a plus sign before the dBm then it is an indication of gain in the signal in comparison to the noise.

Dbm is used as a measurement system of absolute power in the communication work. The value of zero dBm is one milliwatt so an increase by three dBm means nearly double the power that is three dBm will be equal to two milliwatt.

But if there is decrease of three in the dBm then the power is reduced by half. If the dBm is minus three then the milliwatt will be equal to 0.5. this can also be expressed in an equation X = 10 log10 (P/ (1 mW)) and the other formula is P = (1 mW) 10 (x/10) and if these two equations are used a table can be obtained which summarizes the cases which are useful. The table can be read as it is given below.

dBm level Power

30 dBm 1 watt
26 dBm 400 milliwatt
20 dBm 100 milliwatt
04 dBm 2.5 milliwatt
- 20 dBm 0.01 milliwatt

This way a whole table of useful cases can be prepared by using the formula which has been given above and the ratio between the signal and the noise can be calculated.
Learn more about dbm and signal to noise ratio at tech-faq.com.