dB vs dBm vs dBW vs dBc: Understanding the Differences

This page clarifies the distinctions between dB, dBm, dBW, and dBC. RF power measurement is a frequent task for engineers. Understanding these units is crucial for interpreting measurements from power meters and spectrum analyzers.

RF Power Measurement: Watts and Decibels

Power is typically measured in watts. However, dBm and dBW offer a logarithmic scale relative to 1 milliwatt or 1 watt, respectively, making it easier to represent a wide range of power levels. dB, or Decibel, represents the ratio between two power (or voltage/current) values.

What is dB?

dB stands for Decibel. It’s the logarithmic ratio of two power values (or amplitudes of voltage or current).

Equation:

dB = 10 log(P2/P1) or 20 log (A2/A1)

…

Equation-1

What is dBm?

dBm stands for decibels relative to 1 milliwatt.

The above explanation highlights the core difference between dB and dBm: dB is a ratio, while dBm is an absolute power level referenced to 1 milliwatt.

dB (decibel) can be used for the ratio of two similar quantities, like voltage, as long as the numerator and denominator have the same units (both voltage in this case). dBm specifically refers to a power unit.

Equation:

Power (dBm) = 10 * log10 (Power in milliwatt)

…Equation-2

Equations 1 and 2 clearly illustrate the distinction between dB and dBm.

What is dBW?

dBW stands for decibels relative to 1 Watt.

Relationship between dBm and dBW:

XdBW = XdBm - 30

Therefore, 0 dBW = +30 dBm or -30 dBW = 0 dBm

What is dBC?

dBC stands for decibel value relative to the carrier power level.

It is primarily used to measure the level of phase noise, harmonics, or spurious signals relative to the carrier power.

• Example: Phase noise of a modulated carrier at 1 KHz from the carrier is about 70 dBc.
• Example: Harmonics of a 6 GHz RF carrier (i.e., 12 GHz) is about -60 dBc.
• Example: Spurious signals of a 6 GHz RF carrier (i.e., a fractional multiple of 6 GHz) are about -60dBc.