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dB vs. dBm

S

ssylee

Jan 1, 1970
0
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?

Thanks in advance!
 
T

terryS

Jan 1, 1970
0
---
You just add them and keep the dBm unit.

For example, if your spectrum analyzer shows a 0dBm signal coming out of
the attenuator, then the signal at the input of the attenuator will be
+20dBm, or 100 milliwatts

JF

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.
 
E

Eeyore

Jan 1, 1970
0
ssylee said:
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?

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
 
E

Eeyore

Jan 1, 1970
0
John said:
---
Nope.

As long as the impedances are the same, throughout, it doesn't matter
unless you're concerned about the voltages and/or currents, which the OP
clearly wasn't.

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

Graham
 
E

Eeyore

Jan 1, 1970
0
Jasen said:
me guessing:
OP wants to know how much to add to the readings his device gives to
get the unattentuated value.

Could be. It wasn't clear to me.

Graham
 

neon

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

neon

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