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'[EE] Ubiquitous use of dBm instead of dBmW'
2009\01\26@012603 by

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"?

Hello,

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.

Thanks goes to my textbook "Radar Meteorology".

2009/1/26 Virchanza <virtuallavabit.com>

>
>    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"?
>
> -
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"?

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

2009/1/26 Virchanza <virtuallavabit.com>

{Quote hidden}

> -

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"?

Sorry for posting this twice, I forgot that I hadn't subscribed to [EE]
and so I was wondering what happened when my first post didn't show up.

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?

----- Original Message -----
From: "Virchanza" <virtuallavabit.com>
To: "Microcontroller discussion list - Public." <piclistmit.edu>
Sent: Tuesday, January 27, 2009 3:48 AM
Subject: Re: [EE] Ubiquitous use of dBm instead of dBmW

{Quote hidden}

decibels serve to represent. Where did you get the idea that you could
convert amps directly to dB?

Decibels represent the RELATIONSHIP between two values of power. The letter
appended represents the reference level. Some common references are 1
millivolt (dBj), 1 volt (dBv), and 1 milliwatt into a 600 ohms (resistive)
load (dBm). There are also references used by acoustitians, dB SPL
(referenced to the lower threshold of human hearing), and music, dBVU
(represented to 100% modulation).

So, if you want to use amps as your reference, the calculation should be 10
log (I1/I2), assuming base 10 logarithm. However, dBA is already taken
(frequency filtered SPL measurements), making dBmA confusing (1/1000 of
dBA??)

> Why one earth do people write "decibel milliwatts" as "dbm", i.e.
> without the W to indicate watts?

They don't. They write dBm as decibels IN REFERENCE TO 1 milliwatt and 600

>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"???

Apples and oranges. If you left off the dBm your answer, you'd still lose
points. Actually, it you wrote it as dBmW, you'd lose points because that
reference is not defined. See above.

> Where did this practise start, and why do so many people perpetuate it?
>
Does it really matter? Isn't that sort of like asking why we drive on
parkways and park in driveways. Where did that practice start and why do so
many people perpetuate it (the naming, not the driving and parking - though
that might be a good question, also)?

Richard
Note: Errors in this response are purposely retained for the pleasure of
those who enjoy pointing out such.

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

> Decibels represent the RELATIONSHIP between two values of power.

The bel is a handy way of dealing with really big numbers, nothing more.

1 bel = 10
2 bels = 100
3 bels = 1000
4 bels = 10000

Yes, people seem to use it a lot for describing a ratio, but that's
because the ratio might be a really big number. For instance, instead of
saying "The gain of this amplifier is 60 dB", there's nothing stopping
you from saying "The gain of this amplifier is 1 million".

There's nothing about the bel that forces it to be used as a ratio.

{Quote hidden}

I had a space between "dB" and "mA".

>> Why one earth do people write "decibel milliwatts" as "dbm", i.e.
>> without the W to indicate watts?
>>
>
> They don't. They write dBm as decibels IN REFERENCE TO 1 milliwatt and 600
>

I've heard people say that the power output of a particular transmitter
is X dBm's.

{Quote hidden}

"dB mW", with a space.

>> Where did this practise start, and why do so many people perpetuate it?
>>
>>
> Does it really matter?

The reason I ask is that I'm in a position where I have to teach some of
this stuff, so I want to be able to explain why they'll hear that a
transmitter has a transmittal power of X amount of "dBm".

Have you taken a look at:

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

I DO think that Decibels are specific to power and not a generic
logarhithmic scale. decibels were derived from a Bel by the Bell
Telephone Company for quantifying telephone line attenuation. They
were not intended as a general logarithmic ratio unit. In fact, other
such units exist, such as the standard "p" prefix in chemistry (e.g.,
pH = -log_base10(concentration of H+ ions in moles per liter))

For example, when you have a power ratio, you use the relationship:

ratio in dB=10*log_base10(P1/P2)

but when you have a ratio of two voltages, you use:

ratio in dB=20*log_base10(V1/V2)

This is a kludge, in a way, since it assumes that the two voltages are
being applied to the same load impedance and that their ratio can be
converted to a power ratio by simply squaring.

This holds for anything else which relates to power in a quadratic
relationship. For example, sound pressure level:

dBSPL=20*log_base10(Pressure1/Pressure2)

Also, the above wiki page says that dBmW and dBm are both allowed. If
I were to explain this to someone in a class, I would just say that by
convention, dB relative to 1mW is often abbreviated dBm, probably
because it is so common that writing one less letter is useful.

Sean

On Tue, Jan 27, 2009 at 10:29 AM, Virchanza <virtuallavabit.com> wrote:
{Quote hidden}

> -
2009/1/27 Virchanza <virtuallavabit.com>

>
>
> Yes, people seem to use it a lot for describing a ratio, but that's
> because the ratio might be a really big number. For instance, instead of
> saying "The gain of this amplifier is 60 dB", there's nothing stopping
> you from saying "The gain of this amplifier is 1 million".
>
>
I think you still overcomplicate the definition. In your above statement,
the gain of the amplifier can be voltage, current or power. By saying dB you
are not saying on which parameter your amplifier have that gain, right?
If you indicate dBm, that will immediately and by definition, mean Power
Gain in dB on mW. That's all.

--
Ariel Rocholl
On Tue, Jan 27, 2009 at 10:55 AM, Ariel Rocholl <forosarocholl.com> wrote:
> 2009/1/27 Virchanza <virtuallavabit.com>
>
>>
>>
>> Yes, people seem to use it a lot for describing a ratio, but that's
>> because the ratio might be a really big number. For instance, instead of
>> saying "The gain of this amplifier is 60 dB", there's nothing stopping
>> you from saying "The gain of this amplifier is 1 million".
>>
>>
> I think you still overcomplicate the definition. In your above statement,
> the gain of the amplifier can be voltage, current or power. By saying dB you
> are not saying on which parameter your amplifier have that gain, right?
> If you indicate dBm, that will immediately and by definition, mean Power
> Gain in dB on mW. That's all.
>

No.

dB by itself always refers to a _ratio_.  But dBm is an _absolute_
measurement of power, that is, the ratio to 1 mW.

So it never makes sense to talk about gain in dBm, because gain is a
ratio by definition.

Regards,
Mark
markrages@gmail
--
Mark Rages, Engineer
Midwest Telecine LLC
markragesmidwesttelecine.com
Virchanza wrote:
> I can convert whatever I want to decibels. (Why wouldn't be able to?)

No you can't.  Decibels are just a way of expressing a ratio of powers.

> I have 100 dogs = I have 20 decibel dogs

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.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
>There's nothing about the bel that forces it to be used as a ratio.

Oh yes there is, because that is how it is defined, as the ratio between two
numbers, in electronics normally between input and output of an amplifier or
attenuator. It is not just an arbitrary number.

So your examples may be usable by determining what the starting point is for
the numbers you give.

At 10:29 AM 1/27/2009, Virchanza wrote:
>The reason I ask is that I'm in a position where I have to teach some of
>this stuff, so I want to be able to explain why they'll hear that a
>transmitter has a transmittal power of X amount of "dBm".

I am not sure when the use of dBm started, but it is common back to
at least the end of WWII in my library. The first piece of equipment
I had that directly showed dBm was the HP 430 power meter introduced
in 1948. More for laboratory use than for general use and more
commonly in the microwave area than elsewhere in the early days. Some
quantities are best expressed with a logarithmic relationship, and
power is one of those. In laboratory work you often need combinations
of components such as attenuators, directional couplers, samplers or
amplifiers, all of which have some characteristic expressed in dB,
whether it is a gain, coupling, insertion loss or reflection
coefficient. If we express power in dBm then we can very simply
calculate new or resulting power levels using simple addition and subtraction.

Simply put, if we have a signal source at +13dBm, followed by an
attenuator of 20dB and a 7dB gain amplifier then we have an output of
13 - 20 + 7 = 0dBm. Engineers are inherently lazy folk, so we took to
that simplicity like ducks to water!

The Decibel became more commonly use than the Bel because it gave us
usable increments without having to insert a decimal point. Decimal
points are analogous to a speck of dust and should generally be
avoided! I haven't seen a reference to Bels since Bell System
Technical Journals of the 30's and 40's.

10dB SINAD in 50 ohms. But in time gone by it was common to express
then in dBu, or dB relative 1 microvolt in whatever the specified
system impedance was.

In some fields, especially higher power RF, it is not unusual to see
dBW, dB relative to 1 Watt, but we should always be careful to
specify the system impedance.

Using dBm to specify higher power levels outside the lab seems to
have only become common since the late 80's. The trend started in the
microwave field in my experience, but seems to have spread over the
years and seems to be pretty much universally understood in the field
now. This seems to parallel the use of laboratory type equipment such
as spectrum analysers in such areas as cable TV, cellular
infrastructure and the like.

So why don't we use dB mA? Well, how often are we dealing with
current over wide dynamic ranges? It is more likely that we would
deal with power, hence dBm or dBW. Also, in the RF world it is most
unlikely we would be measuring or specifying current anyway. In my
younger days I recall we had RF ammeters especially for use in the
antenna field, and even then only at low frequencies, power is much
easier to measure.

I hope that helps.

John

> -----Original Message-----
> From: piclist-bouncesmit.edu On Behalf Of Virchanza
> Sent: Tuesday, January 27, 2009 10:30 AM
>
> I can convert whatever I want to decibels. (Why wouldn't be able to?)

Because the IEC 60027-3 standard and the SI system (by incorporating the IEC
standard) define the neper (Np), bel(B) and decibel(dB) as logarithmic
_ratio_ quantities (dimensionless quantities or more precisely, a quantity
with the dimensions of 1).
http://physics.nist.gov/Pubs/SP811/sec05.html
physics.nist.gov/Pubs/SP811/sec08.html#8.7
http://en.wikipedia.org/wiki/Dimensionless_unit

You must follow standard definitions for units of measurement or you will
not be interoperable with the rest of the science and engineering
communities, as shown by this thread.

> I have 100 dogs = I have 20 decibel dogs

No you have 1 hectodog or 10 dekadogs :-). Seriously though, since "dog" is
not an SI unit, the only correct alternative ways to express this would be
using standard scientific notation like: 1*10^2 dogs.

Paul Hutch

This is BY FAR the main reason most RF "people" learn and love dBm...
calculating total system gain/loss, and or path losses, etc... becomes
simple math, when expressed in dBm.

I see two common measurements in my experience with radio systems,  dBm and
uV.  I far prefer converting uV measurements to dBm before calculating
system performance.  It just makes life simpler.

But... I still memorize both numbers for "common performance" on certain
receivers I use -- and my Service Monitor's output attenuator has a
dual-ringed scale with both on it.

For some reason, during bench work, I find myself saying, "This receiver
should show 12 dB SINAD on the test gear at .25 uV if it's performing to
factory spec".

But, at the tower site when hooked to the entire antenna system, I say
things like "We're losing 3dB somewhere... that's not good.  What's broken?"

Maybe my usage isn't 100% accurate, but I usually can get my point across
and we can fix the broken thing, whatever it is...

Nate

{Original Message removed}

2009/1/28 Nate Duehr <natenatetech.com>:
> This is BY FAR the main reason most RF "people" learn and love dBm...
> calculating total system gain/loss, and or path losses, etc... becomes
> simple math, when expressed in dBm.
>
----snip
>
> Nate
>

Nate
I can also remember having to convert between output voltage in "emf"
(older Marconi instruments ?) and output voltage "pd" (HP & others).
The difference being the emf measurement is the open circuit voltage,
while the pd voltage is when terminated. i.e emf = 2 x pd in correctly
terminated systems or 6dB difference. Quite significant when measuring

RP

2009\01\27@165437 by
At 02:48 PM 1/27/2009, you wrote:
>This is BY FAR the main reason most RF "people" learn and love dBm...
>calculating total system gain/loss, and or path losses, etc... becomes
>simple math, when expressed in dBm.
>
>I see two common measurements in my experience with radio systems,  dBm and
>uV.  I far prefer converting uV measurements to dBm before calculating
>system performance.  It just makes life simpler.

For sure. I think I am used to seeing sensitivities expressed as uV
with the dBm value in parenthesis. When dealing with very sensitive
low noise devices I think uV is perhaps a common way to talk, but
measured inb dBm.

>But... I still memorize both numbers for "common performance" on certain
>receivers I use -- and my Service Monitor's output attenuator has a
>dual-ringed scale with both on it.

I think I gave up using uV back in the late 60's because I was
measuring other characteristics of receivers as well, such as
blocking, intermod and adjacent channel measurements where everything
was in dBm. Even the old HP 608 generators had a dual scale with dBm
outermost, same as the 8640B.  The last generators I owned were R&S
SM series and although they had the capability of reading out in uV
or dBu I don't recall one of them EVER being set to anything other than dBm.

>For some reason, during bench work, I find myself saying, "This receiver
>should show 12 dB SINAD on the test gear at .25 uV if it's performing to
>factory spec".
>
>But, at the tower site when hooked to the entire antenna system, I say
>things like "We're losing 3dB somewhere... that's not good.  What's broken?"
>
>Maybe my usage isn't 100% accurate, but I usually can get my point across
>and we can fix the broken thing, whatever it is...

I suspect Nate, that you are right in line with the majority of PMR
types. The only difference being that lab people are perhps less
liekly to think in uV at all.

John

2009/1/27 Mark Rages markragesgmail.com

>
> So it never makes sense to talk about gain in dBm, because gain is a
> ratio by definition.
>
>

You are of course right. Thanks for the correction.

--
Ariel Rocholl
At 10:29 AM 1/27/2009, Virchanza wrote:
>The reason I ask is that I'm in a position where I have to teach some of
>this stuff, so I want to be able to explain why they'll hear that a
>transmitter has a transmittal power of X amount of "dBm".

I am not sure when the use of dBm started, but it is common back to
at least the end of WWII in my library. The first piece of equipment
I had that directly showed dBm was the HP 430 power meter introduced
in 1948. More for laboratory use than for general use and more
commonly in the microwave area than elsewhere in the early days. Some
quantities are best expressed with a logarithmic relationship, and
power is one of those. In laboratory work you often need combinations
of components such as attenuators, directional couplers, samplers or
amplifiers, all of which have some characteristic expressed in dB,
whether it is a gain, coupling, insertion loss or reflection
coefficient. If we express power in dBm then we can very simply
calculate new or resulting power levels using simple addition and subtraction.

Simply put, if we have a signal source at +13dBm, followed by an
attenuator of 20dB and a 7dB gain amplifier then we have an output of
13 - 20 + 7 = 0dBm. Engineers are inherently lazy folk, so we took to
that simplicity like ducks to water!

The Decibel became more commonly use than the Bel because it gave us
usable increments without having to insert a decimal point. Decimal
points are analogous to a speck of dust and should generally be
avoided! I haven't seen a reference to Bels since Bell System
Technical Journals of the 30's and 40's.

10dB SINAD in 50 ohms. But in time gone by it was common to express
then in dBu, or dB relative 1 microvolt in whatever the specified
system impedance was.

In some fields, especially higher power RF, it is not unusual to see
dBW, dB relative to 1 Watt, but we should always be careful to
specify the system impedance.

Using dBm to specify higher power levels outside the lab seems to
have only become common since the late 80's. The trend started in the
microwave field in my experience, but seems to have spread over the
years and seems to be pretty much universally understood in the field
now. This seems to parallel the use of laboratory type equipment such
as spectrum analysers in such areas as cable TV, cellular
infrastructure and the like.

So why don't we use dB mA? Well, how often are we dealing with
current over wide dynamic ranges? It is more likely that we would
deal with power, hence dBm or dBW. Also, in the RF world it is most
unlikely we would be measuring or specifying current anyway. In my
younger days I recall we had RF ammeters especially for use in the
antenna field, and even then only at low frequencies, power is much
easier to measure.

I hope that helps.

John

> 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

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

Olin Lathrop wrote:
> Virchanza wrote:
>
>> I can convert whatever I want to decibels. (Why wouldn't be able to?)
>>
>
> No you can't.  Decibels are just a way of expressing a ratio of powers.

Do you mean power as in "2 to the power of 4 is 16", or power as in
"current multiplied by voltage gives power"?

Virchanza wrote:
>>> I can convert whatever I want to decibels. (Why wouldn't be able
>>> to?)
>>
>> No you can't.  Decibels are just a way of expressing a ratio of
>> powers.
>
> Do you mean power as in "2 to the power of 4 is 16", or power as in
> "current multiplied by voltage gives power"?

This is getting silly.  I went to Wikipedia, typed in "decibel", and got a
nice description in a few seconds.  So can you.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

Olin Lathrop wrote:
> This is getting silly.  I went to Wikipedia, typed in "decibel", and got a
> nice description in a few seconds.  So can you.

The bel is simply 10 to the power of X. There's no reason to restrict it
to the usage of expressing ratios of power (where power = energy per
second).

Virchanza wrote:
> The bel is simply 10 to the power of X.

Actually it's the log base 10 of X.

> There's no reason to restrict it
> to the usage of expressing ratios of power (where power = energy per
> second).

Other than its definition, of course.  If you just want Log10, then say
Log10.  Bel means something more specific.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
Perhaps there is a language problem here. Here we mean power as in
current times voltage or RPM times torque or force times velocity,
etc.

Bel is NOT just log10. It was a term coined to describe, in a
numerically compact way, the several orders of magnitude of signal
attenuation one could see over long lengths of cable. deciBel is just
1/10th of that, but has come to dominate since it is a more convenient
unit (it gives a more appropriate level of resolution with needing
digits to the right of the decimal point)

Sean

On Wed, Jan 28, 2009 at 9:18 AM, Virchanza <virtuallavabit.com> wrote:
> Do you mean power as in "2 to the power of 4 is 16", or power as in
> "current multiplied by voltage gives power"?
>
> -
Olin Lathrop wrote:
> Virchanza wrote:
>
>> The bel is simply 10 to the power of X.
>>
>
> Actually it's the log base 10 of X.

X bels = 10 to the power of X

The power gain of this amp is 6 bels = The power gain of this amp is 10
to the power of 6

Virchanza wrote:
> The power gain of this amp is 6 bels = The power gain of this amp is
> 10 to the power of 6

I think you're catching on.  While this statement is correct, the first
"power" is redundant and could cause someone to wonder how much other parts
of the statment can be trusted.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

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