# Converting dBs into dBms?

Discussion in 'Electronic Basics' started by Marcos Martinez Sancho, Apr 26, 2006.

1. ### Marcos Martinez SanchoGuest

Hi there,

how can I convert a value in dBs into dBms (and other way around)?

As far as I remember, it was something like that:

db = dBm + 10

but I am not sure. It's been almost 10 years since I don't use this stuff.

Any input is appreciated.

Thanks.

Marcos

2. ### Phil AllisonGuest

"Marcos Martinez Sancho"

** Bit like converting chalk into cheese - it cannot be done.

"dBs" express a power * ratio * in logarithmic format.

But "dBm" expresses signal (power) level referenced to 1mW in log format.

......... Phil

3. ### ChrisGuest

Hi, Marcos. Phil's right. Try starting by looking at the Wikipedia
article:

http://en.wikipedia.org/wiki/Decibel

Read it all, but you're going to specifically look about halfway down,
at the Electronics subheading. It's all there.

dB is generally a relative measure like gain, and has no units. For
beginners, rules of thumb are:

Power: 10dB per decade (X10) -- approx. 3dB per octave (X2)

Voltage: 20dB per decade -- approx. 6 dB per octave

dBm (or dBmW) , as Phil says, is a specific power measurement,
referenced to 1mW of power. The calculation ratios expressed in the
Wiki article apply.

Good luck
Chris

4. ### Marcos Martinez SanchoGuest

OK. I get that. All this stuff has become very rusty in my mind, sorry.

Therefore, dBs may be applied to ratios, such as SNR.

And dBm may be a power meaasurement.

Let's say,

S = 20 dBm

N = 5 dBm

SNR = 10 dB

Am I correct?

Marcos

5. ### ChrisGuest

Nope. To find the difference, you would just subtract. The difference
is 15dB (it's now a relative, unitless power measurement).

This is one of the big advantages of using dB as a measure -- you keep
the slide rule off the desk. ;-)

By the way, occasionally some of the basic stuff gets lost in a corner
if you haven't used it in a while. Sci.electronics.basics is
anonymous, but you may wait a while for a correct answer, and there's
always a chance that someone will throw a fly in the ointment. If you
can't ask a co-worker, or if you're in a hurry, try the Wikipedia:

http://en.wikipedia.org/wiki/Signal-to-noise_ratio

Although the articles are done by voluntary contributors, they're
usually well-done, and if someone finds a problem, there's an easy way
to resolve it by recommending an edit.

Good luck
Chris

6. ### Marcos Martinez SanchoGuest

Yeah, yeah, 20 - 5 = 15 db. That's what I meant.

Thanks.

Marcos