dB, dBm, dBW, dBc Basics: What's the Difference between them?

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 —— a function that operates on a unitless parameter

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)（Power2/Power1  is unitless! ）

It is a relative value between two power levels.

A: Many values are unitless, for example ratios andcoefficients. 

True! But look at how power is expressed; not in dB,but in dBm or dBw. 

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!

Therefore dBw and dBm express Prelative to 1 Watt and 1 mWatt, respectively！ 

Note: The argument of the log10 function is a ratio(i.e., unitless). 

For example, 
20 dBm means 100 x 1mW = 100 mW
3 dBw means 2 x 1W = 2W

Make sure you are careful when doing math with decibels!

Standard dB Values
Note that 10 log10(10) = 10.
Therefore an amplifier with a gain G = 10 is said to have again of 10 dB. 


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


In conclusion:
0 dBm = 1 mW = -30 dBW

+30 dBm = 1 W = 0 dBW

I believe you guys can tell the difference between dB, dBm, dBW, dBc.

Q1.    Input 10 mW to an RF amplifier and the measured output is 150mW, what is the gain in dB of this amplifier?

Q2.    The gain of an RF amplifier is 18 dB, if the measured output power is 230 mW, then what is the input power?

Q3.    How much is 12 dB over 30 dBm?

Ans.1
Since Pin=10 mW   and Pout=150 mW
Gain (dB)=10log(150/10)=11.8 dB

Ans.2
*Since Pout=230 mW  and   Gain=18 dB
*10log(230/Pin)=18 dB
*log(230/Pin)=(18/10)=1.8
*101.8=230/Pin
*Therefore, Pin=230/(101.8)=230/63.1=3.65 mW

Ans3.
12 dB over 30 dBm is 42 dBm.

And we can conveniently say

30 dBm (1W) + 12 dB (x 16) = 42 dBm (16W)

Did you get it right？ Waiting for your comment!

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