# dB vs. dBm

Discussion in 'Electronic Basics' started by ssylee, Oct 24, 2008.

1. ### ssyleeGuest

Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal
measured on a spectrum analyzer, which has the units of dBm. From
reading some search results on google, I'm aware the dB is a measure
of relative power where dBm is a measure of power relative to 1 mW. In
order to convert between the two units, do I have to know about one of
the power involved in the dB unit, which is the attenuator?

2. ### terrySGuest

I agree. dBm is a measurement related to Zero dBm being one milliwatt.
Decibels are a logarithmic ratio.

So 100 milliwatts compared to 1 milliwatt.
The logarithm of 100 = 2.0; so 100 milliwatts would be 2 Bels larger/
louder/stronger etc.

Since we are talking 'decibels', 2 bels = 20 decibels (tenths of a
Bel).

Btw a quick way to think about dBs is as follows.
Every time a power ratio doubles. For example from one m.watt to two
m.watts, or from 50 to 100 m.watts it is aratio of approx. 3 decibels.
Derived from logarithm of two = 0.3xxx (approx).

So power ratio:
1 to 2 = 3 dB.
2 to 4 = 6 dB.
4 to 8 = 9 dB.
8 to 16 12 dB.
16 to 32 15 dB.
64 18 dB.(Getting there!)
20 dB.*This 20 dB is pwr. ratio 100 times more/less.
128 21dB.
256 24 dB.
512 27 dB.
& 1024 = 30 dB.

3. ### EeyoreGuest

You also need to know the impedance of the circuit. Typically 50, 75 or
600 ohms.

dB is a RATIO (either power or voltage or SPL)

dBm is a measure of POWER

I think wikipedia covers it quite well.
http://en.wikipedia.org/wiki/Decibel

Graham

4. ### EeyoreGuest

One thing puzzles me here. Attenuators REDUCE signal level NOT add to it.
http://en.wikipedia.org/wiki/Attenuator_(electronics)

Can you clarify what's going on here please ?

Graham

5. ### SURENGuest

same as the friends aa toldd
a c

6. ### EeyoreGuest

Let's say it's advisable to know that the impedances have to be the same.

Graham

7. ### Jasen BettsGuest

me guessing:
OP wants to know how much to add to the readings his device gives to
get the unattentuated value.

Bye.
Jasen

8. ### EeyoreGuest

Could be. It wasn't clear to me.

Graham

9. ### neon

1,325
0
Oct 21, 2006
DB Is a ratio of units. To me DBm could mean modulation as opposed to mw ma mv or whatever.

10. ### Guest

He's measuring something on a spectrum analyser through a 20dB
attenuator and now has
to add the 20dB to the values seen on the analyser to get the true
values

-Lasse

11. ### neon

1,325
0
Oct 21, 2006
Take that signal and amplfy it by A=100 =20 db  