Question 1

What is the difference between 20·log and 10·log?

Accepted Answer

Use 20·log10 for amplitude or field quantities (voltage, current, sound pressure) and 10·log10 for power. A 10× voltage ratio is 20 dB, while a 10× power ratio is only 10 dB. Because power is proportional to voltage squared, the two agree when expressed in the same domain.

Question 2

How much is 3 dB and 6 dB?

Accepted Answer

+3 dB is almost exactly double the power (×1.995), and −3 dB is half power — the classic filter cutoff point. +6 dB is roughly double the voltage (×1.995 in amplitude), which corresponds to about ×3.98 in power.

Question 3

What does dBm mean?

Accepted Answer

dBm is an absolute power level referenced to 1 milliwatt: P[dBm] = 10·log10(P / 1 mW). So 0 dBm = 1 mW, 30 dBm = 1 W and −30 dBm = 1 µW. Unlike a plain dB ratio, it expresses an actual power rather than a relationship between two values.

Question 4

How do I convert dBm to watts?

Accepted Answer

Compute the power in milliwatts as 10^(dBm/10), then divide by 1000 to get watts. For example 30 dBm gives 10^3 = 1000 mW = 1 W. To go the other way, dBm = 10·log10(power in mW).

Question 5

What is dBW and how does it relate to dBm?

Accepted Answer

dBW is referenced to 1 watt instead of 1 milliwatt, so dBW = 10·log10(P / 1 W) and dBW = dBm − 30. For example 30 dBm equals 0 dBW, because both represent 1 W.

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