
The “m” in dBm references the value to an absolute value of 1 mW of power which is a standard reference throughout the fiber optic and wireless industries.

Power in absolute units can be expressed by using 1 milliwatt as the reference power in the denominator of the equation for dB. The following tabulation lists some approximations, which can be helpful if memorized.ġ/100 the power : -20 dB dB AS ABSOLUTE UNITS = dBm If the power in question is not a multiple of ten, then some estimation is required. Link budgets are therefore much easier to calculate. It is much easier to work with a loss of 137 dB rather than the equivalent factor of 2 x 10-14 mW – a number that cannot be easily manipulated.īetter still, instead of multiplying gain or loss factors as percentages, by using dB measurement we can add them as positive or negative dB values. The dB is very useful when describing signal loss due to radiation through the atmosphere. For instance, if 10 watts of power is fed into a cable but only 8.5 watts are measured at the output, the signal has been decreased by a factor ofĪn attenuator, cable or optical path that reduces its input power by factor of 0.001 has an attenuation of 30 dB or a gain of -30 dB. (notice the 3 in 30 dB corresponds to the number of zeros in the power ratio)Ĭonversely, a ratio of less than 1.0 is a loss, a negative gain, and will be expressed as a negative dB value. Consider an amplifier with an output of 100 watts when the input is 0.1 watts (100 milliwatts) The logarithmic characteristic of the dB makes it very convenient for expressing power ratios. Logarithmic Power Measurements for Wireless Links Decibel Attenuation Millimeter Wave MMW Spectrum One milliwatt output power is then zero dBm. The abbreviation dBm indicates dB referenced to 1.0 milliwatt. The most common references in the world of electronics are the milliwatt (mW) and the watt. It is often desirable to express power levels in decibels by using a fixed power as a reference. The term decibel does not in itself indicate power, but rather is a ratio or comparison between two power values. For example, if P2 is one-tenth of P1, we have: If P2 is less than P1, the ratio is less then 1.0 and the resultant (exponent) value is negative. In this equation, P1 is usually the reference power. With the constant almost always included in modern terminology and power referred to as a “ten log” function. Usually a smaller unit, the Decibel or dB, is used. Since the bel is a rather large unit, it is not useful for precise measurements. The exact relationship is given by the formula Bels = log(P2/P1), where P2/P1 represents the power ratio. As originally used, the bel represented the power ratio of 10 to 1 between the strength or intensity of two sounds, and was named after Alexander Graham Bell.īels represents a logarithmic relationship since the (common log) logarithm of 100 (to the base 10) is 2 (corresponding to 2 bels), the logarithm of 1000 to the base 10 is 3 (corresponding to 3 bels), etc. The Decibel is a sub-unit of a larger unit called the bel. Wireless link designers – just as with Fibre Optics – use Decibels to calculate power transmission losses and link budgets. Summary Decibel Power Measurements dB for Wireless Links
#Decibel 100 free
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