dBm (which is at times also called dBmW), is a unit used to express absolute values of power relative to a milliwatt. The term also comes up when there is a discussion of RF measurements in any RF device or SOC device such as the Radio, Microwave and fiber Optic cabling as a way of expressing very large or very small values. dBm is also the standard used to measure telephone system power levels. dBm is similar to dB (what we refer to as decibel), except that the dB is relative to the power of the input signal, and also, dB is a dimensionless unit (its thus represents a pure value) and would be useful in quantifying ratio between two values. Decibel is allowed for use alongside other SI units but dBm is not. dBm on the other hand, only relates to a 1 milliwatt signal. This figure can also be equated with 0.775 volts RMS in a circuit with 600 Ohms impendence. A milliwatt is 1/1000 of a watt. Thus dB is a relative measurement while dBm is an absolute measurement. The amount of power in a given value of dBm is the power that which would result if one was attenuated (this is the reduction of the amplitude of the electric signal) with that number of dB. If there is a minus sign before dBm, that would be indicative of a loss. A plus sign or no sign at all would then be indicative of a gain. The concept can be expressed thus;
absolute speech level= 10 Log (P2/P1)
P2 represents the specific power level while P1 represents the reference value, which is 1 Mw. Another sound standard needs mention. Volume Unit (VU) has the same reference level as dBm, which is 1Mw. But it is to be noted that dBm is reserved only for single frequencies or tomes while VU on its part is normally used for complex audio signals such as music or speech. dBm as a power level terms is usually used only for 600 ohms circuits. The computation for bel and decibel provide avenue for comparing two power values, even though the two are not indicative of power. When a signal travels through a medium, the power at the receiving end can be easily measured. But the problem is that it is far from being easy to pinpoint what the received value indicated nor to take the received power and use the same for comparison purposes unless a standard measure is employed. For instance, in telephone operations, a 1 MW signal is applied for a frequency of about 800Hz just to test a circuit. The term dBm is then used to make sure that the resulting power measurements occurred in respect to 1 MW input signal is not lost on anyone. In many books where the term decibel-milliwatt is used, the intention is to denote a value of more than 1 Mw.
Though for many years dBm has been the industry’s standard, it is increasingly being replaced by dBu (decibel unloaded) which is a more appropriate standard for measuring signal strength with the higher impedances that are in use these days.
For more information on dBm read What is dBm?
