626
Appendix B: Decibels
Table B.2 Examples of dBm units (decibel measure of power relative to 1 mW)
Power (mW)
100
10
2
1
0.5
0.1
0.01
0.001
Value (dBm)
+20
+10
+3
0
−3
−10
−20
−30
B.2 The dBm
The decibel is used to refer to ratios or relative units. For example, one can say that
a certain optical fiber has a 6-dB loss (the power level gets reduced by 75% in going
through the fiber) or that a particular connector has a 1-dB loss (the power level gets
reduced by 20% at the connector). However, the decibel gives no indication of the
absolute power level. One of the most common derived units for doing this in optical
fiber communications is the dBm. This is the decibel power level referred to 1 mW.
In this case, the power in dBm is an absolute value defined by
Power level = 10 log
P
1 mW
(B.2)
A useful relationship to remember is that 0 dBm = 1 mW. Negative dBm numbers
designate power levels less than 1 mW, whereas positive dBm values indicate power
levels greater than 1 mW. Some examples are shown in Table B.2.
B.3 The Neper
The neper (N) is an alternative unit that is sometimes used instead of the decibel. If
P 1 and P 2 are two power levels, with P 2 > P 1 , then the power ratio in nepers is given
as the natural (or naperian) logarithm of the power ratio:
Power ratio in nepers =
1
2
ln
P 2
P 1
(B.3)
where ln e = ln 2.71828 = 8.686.
To convert nepers to decibels, multiply the number of nepers by
20 log e = 8.686.
Appendix B: Decibels
Table B.2 Examples of dBm units (decibel measure of power relative to 1 mW)
Power (mW)
100
10
2
1
0.5
0.1
0.01
0.001
Value (dBm)
+20
+10
+3
0
−3
−10
−20
−30
B.2 The dBm
The decibel is used to refer to ratios or relative units. For example, one can say that
a certain optical fiber has a 6-dB loss (the power level gets reduced by 75% in going
through the fiber) or that a particular connector has a 1-dB loss (the power level gets
reduced by 20% at the connector). However, the decibel gives no indication of the
absolute power level. One of the most common derived units for doing this in optical
fiber communications is the dBm. This is the decibel power level referred to 1 mW.
In this case, the power in dBm is an absolute value defined by
Power level = 10 log
P
1 mW
(B.2)
A useful relationship to remember is that 0 dBm = 1 mW. Negative dBm numbers
designate power levels less than 1 mW, whereas positive dBm values indicate power
levels greater than 1 mW. Some examples are shown in Table B.2.
B.3 The Neper
The neper (N) is an alternative unit that is sometimes used instead of the decibel. If
P 1 and P 2 are two power levels, with P 2 > P 1 , then the power ratio in nepers is given
as the natural (or naperian) logarithm of the power ratio:
Power ratio in nepers =
1
2
ln
P 2
P 1
(B.3)
where ln e = ln 2.71828 = 8.686.
To convert nepers to decibels, multiply the number of nepers by
20 log e = 8.686.