Professional Manufacturer of Fiber Optic Products

dB or dBm?

The actual equation used to calculate dB when the power is measured in watts is:

dB = 10 log (measured power / reference power).

It can be expressed more simply:

dB = measured power(dB) – reference power (dB)

 

the table below shows the ratio of power for dB differences in power:

dB (gain) Power ratio dB (loss) Power ratio
0 1.000 0 1.000
0.1 1.023 -0.1 0.977
0.2 1.047 -0.2 0.955
0.3 1.072 -0.3 0.933
0.4 1.096 -0.4 0.912
0.5 1.122 -0.5 0.891
0.6 1.148 -0.6 0.871
0.7 1.175 -0.7 0.851
0.8 1.202 -0.8 0.832
0.9 1.230 -0.9 0.813
1 1.259 -1 0.794
2 1.585 -2 0.631
3 1.995 -3 0.501
4 2.512 -4 0.398
5 3.162 -5 0.316
6 3.981 -6 0.251
7 5.012 -7 0.200
8 6.310 -8 0.158
9 7.943 -9 0.126
10 10 -10 0.1
20 100 -20 0.01
30 1000 -30 0.001
40 10000 -40 0.0001
50 100000 -50 0.00001
60 1000000 -60 0.000001

 

The “m” in dBm refers to the reference power which is 1 milliwatt.

dBm = 10 log (measured power / 1mw)

dB is for measuring loss, dBm is for measuring power.

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