What's All This dBm Stuff Anyhow ? - With suitable homage to Bob Pease :-)

dBm is a logarithmic measurement of power that is used in telecommunications as a more convenient form of power measurement than the linear Watt (W) that most people are familiar with.

dB is a measurement value that represents the ratio of two powers :

Where P1 and P2 are measures of power.

In many applications the voltage (V) or current (I) is measured. Power is proportional to V² or I², giving the equivalent dB equations of :

and:

dBm is the power ratio in decibels (dB) between the measured power and the reference power of one milliwatt (mW).

For many applications, P2 and V2 assume standard values, for example, in a telecoms systems the transmit power is driving the channel impedence, which in audio telephony is typically 600 ohm, while in wireless applications 50 ohms is the most common.
Note that while dB is a relative measurement between the two powers (e.g. input and output), dBm is absolute because it is measured relative (if that makes sense) to a fixed reference power of 1 mW.

Being a logarithmic scale, dBm is really handy for calculating powers through a network so for example in audio telephony, a -13 dBm Tx power would be seen as -55 dBm at the receiver if the channel attenuation were 42 dB.

The following table shows some typical dBm / mW values used in mobile comms.

dBm	mW	Notes
36	4000	(i.e. 4W) - Typical macrocell basestation Tx power
30	1000
26	400	Typical mobile phone or indoor office basestation Tx power
20	100
16	40
10	10
6	4
3	2
1	1
-3	0.5
-10	0.1	Typical maximum received signal power
-60	10^-6	Typical minimum receive power to be able to detect a signal (in mobile comms, without a front end AGC)

The following table shows some of the basic equivalents between linear power ratio and dB.

P1/P2 (dB)	P1/P2 (linear)	V1/V2 and I1/I2
30	103 = 1000	10p
20	102 = 100	10
10	101 = 10	p
9	8	2Ö2
8	2p	2.5
7	5	2.24
6	4	2
5	p	1.77
4	2.5	p/2
3	2	Ö2
2	p/2	1.5
1	1.25	1.12
0	100 = 1	1
-10	10-1 = 0.1	p/10
-20	10-2 = 0.01	0.1
-30	10-3 = 0.001	p/100

Notes : The use of p is an approximation of Ö10 that is useful for quick calculations and accurate to 2 significant figures. The table shows the linear power doubles for a 3 dB increase and halves for a 3dB drop. The voltage and current however double for a 6 dB increase. This table can be used to give all other dB / linear equivalents : 16 dB = 10 dB + 6 dB = 10 x 4 = > P1/P2 (linear) = 40

 

-74 dB = -(70 + 4) = 1/(10⁷) => P1/P2 (linear) = 4x10^-8 The key things to learn from the tables are the following :

The absolute power doubles for a 3 dB increase and halves for a 3 dB drop.
The absolute power is multiplied by 10 for a 10 dB increase. So a 20 dB increase is equivalent to absolute power being multiplied by 100.
Given that Power = (V^2 / R) then the voltage increases by sqrt(2) for a 3 dB increase. Hence a 6 dB increase is required in order for the voltage (or current for which power = I^2.R) to double.

In terms of power and voltage, a really useful reference is the following:

    The -3dB point is the point of half power or 0.7071 voltage point

    The -6dB point is the point of half voltage or quarter power point

To revert back to a linear scale the following equation is used for power :

with the equivalent voltage equation being :

If you would like to read more on the subject Rohde And Schwartz have written a very good applications note called dB or not dB? Everything you ever wanted to know about decibels but were afraid to ask.

Also, if you would like to quickly cross-convert from dB to linear and V.V., this is a great resource: https://www.daycounter.com/Calculators/Decibels-Calculator.phtml.

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