When to use dB and when to use dBm

Occasionally one can see texts and reports about RF circuits where people mix up the terms dB and dBm.
The typical erroneous statement is something like “The power level is 7 dBm lower”. In most, probably all, cases it should instead say “The power level is 7 dB lower.”

Let’s explain by example:

Say we have a power level of 20 dBm. This means 20 dB above 1 mW, i.e. 100 mW.
And 7 dB is a factor of 5, so 7 dBm is 5 mW.
Now 20 dBm – 7 dB = 13 dBm = 20 mW. Subtracting a value in dB means a division. I.e. 100 mW / 5 = 20 mW.
But subtracting a value in dBm means taking the difference between two absolute levels: 20 dBm – 7 dBm = 100 mW – 5 mW = 195 mW = 19.8 dBm.
Thus it very rarely makes sense to say “xx dBm lower / higher” or “the power was reduced / increased by xx dBm”. Use dB in these cases instead!

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