In RF and microwave measurements, it is common to use units such as dB, dBm, dBc and dBW. These terms can appear confusing but are actually pretty straightforward to understand. In this short article we’ll cover the difference between these terms and settle this confusion for good!
1. dB or Decibel
In short, dB is a ratio of two quantities. Since it is a ratio, it doesn’t have any units. We typically talk about the ratio of two power levels, though in a few cases we also use the ratio of voltage levels. That’s what dB is all about.
So here’s an equation to keep in mind:
dB = 10 log(Power2/Power1)
It is a relative value between two power levels.
2. dBm or Decibel relative to 1mW power level
dBm is simply power measured relative to 1 milliwatt. So if we substitute Power1 with 1 mW in the previous equation, the result is a measurement in dBm.
It is an absolute value (because it is measured with respect to a a fixed reference).
dB = 10 log(Power2/1mW)
Power in RF measurements is most commonly represented in dBm.
So what’s the power in dBm at 1mW? Of course 0 dBm!
3. dBW or Decibel relative to 1W power level
dBW is not very different from dBm the only difference being that we simply use 1 W as a reference in place of 1mW used in dBm.
dB = 10 log(Power2/1W)
It is an absolute value.
4. dBc or Decibel relative to carrier power level
dBc is the power measured relative to the carrier power level.
This is typically used to specify parameters such as SFDR (Spurious Free Dynamic Range), Harmonics, etc. which are more meaningful when main carrier power is kept in perspective
0 dBm = 1 mW = -30 dBW
+30 dBm = 1 W = 0 dBW