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dBm and watt

E

eeh

Jan 1, 1970
0
Hi,

Could anyone tell me how to calculate 20W in dbm? Is it equal to
10log(20/0.001)?

Thanks!
 
T

Tim Wescott

Jan 1, 1970
0
Ted said:
Shouldn't that be 10*10log(20/0.001)?

Ted

"ten times log to the base ten of twenty over zero point zero zero one".

So 10 log(20/0.001) is correct if you assume that it's log to the base
10. "10log" will usually be taken to be ten times log to the base 10.
 
R

RST Engineering \(jw\)

Jan 1, 1970
0
You could do it that way, or you could do it in your head if you can
remember that a watt is +30 dBm (*). Ten times that (10 watts) is +10 dB,
or 40 dBm. Double that to twenty watts is another 3 dB, or 43 dBm.


(*) Or from the definition, a milliwatt is 0 dBm, ten milliwatts is +10, a
hundred is +20, a thousand is +30...

Jim
 
W

Walter Harley

Jan 1, 1970
0
eeh said:
Hi,

Could anyone tell me how to calculate 20W in dbm? Is it equal to
10log(20/0.001)?

Although the answers you've gotten are fine, this seems like kind of an odd
question to be asking in the first place. The sort of systems (line-level
audio) that are specified in dBm don't usually involve power levels anywhere
near 20W. Indeed, dBm is kind of an obsolete measurement, even for audio;
more commonly these days, we see dBu, which is essentially a voltage
measurement rather than a power measurement. (0dBu = 0.7748V, which happens
to be the voltage that would produce 1mW into a 600 ohm load.)

Are you sure you're asking the right question? What is the context?
 
P

Paul

Jan 1, 1970
0
Walter said:
Although the answers you've gotten are fine, this seems like kind of
an odd question to be asking in the first place. The sort of systems
(line-level audio) that are specified in dBm don't usually involve
power levels anywhere near 20W.

It's quite common in RF technology also, and 20W is not exactly super
power...

Paul
 
R

Rich Grise

Jan 1, 1970
0
Shouldn't that be 10*10log(20/0.001)?

I think this could be a transcription error - it is
ten times the log to the base ten of power one over power two.
I surmise that 10log looked like log10. :)

Cheers!
Rich
 
K

Ken Taylor

Jan 1, 1970
0
Paul said:
It's quite common in RF technology also, and 20W is not exactly super
power...

Paul
What he said....we care always using dbm and dbW in satellite comm's.

Ken
 
P

Pooh Bear

Jan 1, 1970
0
Ted said:
Shouldn't that be 10*10log(20/0.001)?

More like 10*log10(20/0.001) if you like but since ln is normally used
for natural logs then log without a suffix may be reasonably taken to
mean base10.

Graham
 
M

Mac

Jan 1, 1970
0
Although the answers you've gotten are fine, this seems like kind of an odd
question to be asking in the first place. The sort of systems (line-level
audio) that are specified in dBm don't usually involve power levels anywhere
near 20W. Indeed, dBm is kind of an obsolete measurement, even for audio;

[snip]

Let's not start this again. dBm is very commonly used for 50 Ohm systems
and equipment, including signal generators, amplifiers, spectrum
analyzers, etc.

--Mac
 
D

Dave

Jan 1, 1970
0
Walter said:
Although the answers you've gotten are fine, this seems like kind of an odd
question to be asking in the first place. The sort of systems (line-level
audio) that are specified in dBm don't usually involve power levels anywhere
near 20W. Indeed, dBm is kind of an obsolete measurement, even for audio;
more commonly these days, we see dBu, which is essentially a voltage
measurement rather than a power measurement. (0dBu = 0.7748V, which happens
to be the voltage that would produce 1mW into a 600 ohm load.)

Are you sure you're asking the right question? What is the context?

In "low power" RF engineering, dBm is *much* more commonly used to
indicate power than any other unit - including the pW, nW, uW, mW or any
other multiplier in front of the W.

However, once you get to 1~10 W or so, normally Watts would start being
used. So 20 W would be close to the upper limit where dBm would be used
- the use of W would be more common then.

I've also known dBW to be used (dB relative to 1 W) for higher power
things, but usually people use W, kW or MW then.
 
W

Walter Harley

Jan 1, 1970
0
Dave said:
[...]
In "low power" RF engineering, dBm is *much* more commonly used to
indicate power than any other unit [...]

Sorry, my bad - I'm not an RF guy, didn't realize it was common there. Now
I know better!

Though I still do wonder what the context of the OP's question was.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Pooh Bear
More like 10*log10(20/0.001) if you like but since ln is normally used
for natural logs then log without a suffix may be reasonably taken to
mean base10.

Those nice people on IEC TC25, who gave us 'kibis' and 'mebis', also
gave us 'ld' for logs to base 10 and 'lb' for logs to base 2, to go with
'ln' for logs to base n, well, no, to base e actually (why not 'le',
then?)..

The log abbreviations are useful. Does ANYONE use the binary multiplier
prefixes?
 
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