ON 20090313@1:49:45 PM at page:
On a web page you were interested in at:
http://www.piclist.com/techref/member/djl-DJL-P3/index.htm#39885.5762152778
dave j lagzdin[djl-DJL-P3] Says
Virchanza wrote:
>> Say I tell you that a particular transmitter transmits with a power
>> of 500 mW, well you can express that as decibel milliwatts as follows:
>>
>> 10 * log10(500 milliwatts) = 27 decibel milliwatts
>>
>> What I'm curious about though, is why so many people write 27 dBm
>> instead of 27 dBmW. Why does everyone leave out the underlying unit of
>> "watt"?
Gokhan SEVER responded to me as follows:
"P is defined as 10*log10(p / 1mW) with a unit of dBm. And here m means
decibels relative to a power of one milliwat (1 mW). Therefore we don't
need
to use an extra milliwatt in our final calculation. "
I don't agree with this. For instance, let's say I take the following
four sentences:
The current is 1 amp.
The current is 10 amps.
The current is 100 amps.
The current is 1000 amps.
Now let's say I convert them to milliamps, giving:
The current is 1000 milliamps.
The current is 10000 milliamps.
The current is 100000 milliamps.
The current is 1000000 milliamps.
Then I convert to bels:
The current is 3 bel milliamps.
The current is 4 bel milliamps.
The current is 5 bel milliamps.
The current is 6 bel milliamps.
Using "tenths of a bel" instead of bels, I get:
The current is 30 dB milliamps.
The current is 40 dB milliamps.
The current is 50 dB milliamps.
The current is 60 dB milliamps.
So the unit I'm left with is "decibel milliamps", or in shorthand: "db
mA". Note the presence of "A" to indicate amps.
Why one earth do people write "decibel milliwatts" as "dbm", i.e.
without the W to indicate watts? I remember when I was back in college,
we used to get docked marks if we left the units out; for instance if
you gave an answer as "5" instead of "5 amps", you'd probably only get
90% instead of 100%. So why is it OK to leave out the "watt" in "dBm"???
Where did this practise start, and why do so many people perpetuate it?
..............
Virchanza wrote:
>>Richard Seriani, Sr. wrote:
>> Your disagreement seems to be based on your misunderstanding of what
>> decibels serve to represent. Where did you get the idea that you could
>> convert amps directly to dB?
>>
I can convert whatever I want to decibels. (Why wouldn't be able to?)
I have 100 dogs = I have 20 decibel dogs
I have 1 million dollars = I have 60 decibel dollars
The universe is 6 billion years old = The universe is 98 decibel years old
The diameter of Jupiter is 142,984,000 m = The diameter of Jupiter is 82
dB m
.............
Olin wrote:
That's wrong several ways. First, there is no ratio here. 20 dogs compared
to what? You haven't specified what the 0 dB reference is. Second, you are
implicitly saying that the number of dogs is proportional to power. That is
rediculous, certainly without context. How do you know they're not related
to the power squared like volts are, or some other relationship?
To stretch things and make this example correct you could use dB to compare
the motive power of sleds. You could rate a 15 dog sled team some dB above
or below a particular snow mobile, for example. But you couldn't give your
sled team a absolute dB rating without specifying the 0 dB reference, like 1
Watt, 1 Horsepower, the equivalent of 10 standard sled dogs, etc.
Jinx wrote:
Those are quite industrial units and impractical for many applications.
Most people would commonly use the smaller unit milli-Dog (mD),
which replaced the old imperial unit, Pomeranian (1D = 3.874Po). A
notable exception is in use specifically and uniquely at Da Kine Bail
Bonds, the "Dwayne Chapman". One Dog is claimed to be applicable
and all you need for every situation
A new unit, the Hb (handbag), is being promoted by Prada and Gucci.
Roughly equivalent to a hecto-chihuahua, dBHb would be a useful
measurement of vacuousness
But there's also a lot of Shitzu around of course