What's the "m" mean in dBm?

[throbscottle]

Title says it all. I was reading this page http://www.marcspages.co.uk/tech/5101.htm about analogue meters and dB measurement, very interesting and informative.

But I come across dB and dBm used almost interchangeably. I got the idea from somewhere that the "m" is for "metres" - is this correct, and if so, why?

TIA

 

[ericgibbs]

hi,
As often used in engineering 'm' means milli.

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

E

 

[dougy83]

The m means milliwatt in the case of dBm, i.e. 1mW reference, as you'll see in Eric's link

 

[MikeMl]

It expresses power ratios relative to 1mW. For Example, +30 dbm is 1W (1000mW).

10 log (1000/1) = 30

 

[Ramussons]

dB is a Logarithmic way of expressing ratios.

10 log (P1/P2) = k dB
If the Reference is 1 milli watt (P2), then the Ratio is denoted as dBm.
. . . . . . . . . . . . . . .1 Watt, . . . . . . . . . . . . . . . . . . . . . .. . . . dBW

Ramesh

 

[steveB]

Should I repeat it again, in case he didnt' understand it the first, second, third and fourth time? LoL

 

[dougy83]

Should I repeat it again, in case he didnt' understand it the first, second, third and fourth time? LoL

Better safe than sorry.

 

[dr pepper]

You see different references:

dbm as explained

dba means the value has been a-weighted or scaled to the average person hearing response.

dbd used by hams for comparing aerial gain versus a basic dipole, interesting as manufacturers often bull up theor ratings by quoting dbi (isotropic), which for us on earth is only theoretical.

 

[throbscottle]

So much more fun asking you lot than looking it up (and good link Eric) Being of very little brain, like a certain bear, I can only absorb a small amount of this mathematical stuff at a time, so every time I look into it I learn a little bit more. Today I learnt quite a lot more, so thanks for the repetition, it is actually appreciated!

 

[Willen]

I found a post by Trash like this, is it correct?

The formula is _dB = 10 log (p1/p2)

where p1 and p2 = power 1 and power 2 respectively.
For example if
p1 = 5mW and
p2 = 10mW

5/10 = 0.5
LOG(0.5) = 0.3
10 x 0.3 = 3dB (gain)

if we reverse the values.
10/5 = 2
LOG(2) = -0.3
10 x 0.3 = -3dB (loss)

To convert dB back to gain.
16db / 10 = 1.6
10^(1.6) = 39.8 (close enough to 40 times)

These examples are for power.
If voltage (or current) is used, then the formula is dBV = 20 log (v1/v2)

I calculated myself but got opposite result than Trash's calculation. (got -3dB instead of +3dB and got +3dB instead of -3dB).

 

[crutschow]

A dB can be used with many different suffixes as show near the bottom of this reference.

 

[rumpfy]

The above is all true; BUT, the power level is usually measured with a voltmeter, where the voltage is measured across a KNOWN VALUE of resistance.
In the case of a dBm power level measurement, the reference resistance is 600 ohm and ' 0 dBm' is actually 1 milliwatt in 600 ohm. So a dBm power meter will be measuring 0.775 Volt RMS when it displays 0 dbm. If we use such a 'power meter' to measure power levels in a circuit where the load resistance is say 30 ohm, at 0.775 volt ( 0 dBm), the power level will actually be 20 milli watt.

 

[JimB]

In the case of a dBm power level measurement, the reference resistance is 600 ohm and ' 0 dBm' is actually 1 milliwatt in 600 ohm. So a dBm power meter will be measuring 0.775 Volt RMS when it displays 0 dbm.

The above is all true; BUT...
This is correct for audio circuits, but for RF circuits the resistance is 50 Ohm.
So on a typical RF signal generator 0dBm is 0.2236volts across 50 Ohm.

JimB

 

[throbscottle]

Okay, so next question. You know those dB meters used to measure sound levels? Well, what is the 0dB reference point for those? Only I always wondered...

 

[MikeMl]

Wiki is your friend.

 

[throbscottle]

Oh yes, so it is

 

[rumpfy]

There is 2 parts to the above.
1. The meter calibrated in dBm will give a result as 'x dB' with respect to the reference power level of 0 dBm. If the meter is specified as a having a dBm calibration, the 0 dBm will be 1 miilwatt in 600 ohm. It does not matter whether the meter is measuring 'audio' or 'telephony transmission levels between 120 kHz and 100 Mhz or any thing else; if it is calibrated as dBm, it will need 0.775 volt input at the terminals to give a reading of 0 dBm. There are other standards for 0 dB and it is possible that RF generators with 50 ohm output impedance may use another dB x system. It may be that dBv is used for these instruments and I stand to be corrected. What is the central idea is that these 'standardisations' have been around for ever an a day and in using these meters to measure absolute power, one needs to understand that a voltmeter can only give a power value if it is referred to a standardised value of load impedance.
2. Throbscottle asks about 'these dB meters used to measure sound levels'. I presume TB is referring to 'Sound Level Meters' used in accoustic work. if so, these meters use a reference level of 0 dB equals;
10 *-10 microwatt per cm 2,

The reference power is an accoustic power level.

Data values taken from "Reference data for Radio Engineers" published by ITT (1954)

Hope this helps,
rumpfy

 

[throbscottle]

Prize goes to rumpfy for the most concise answer (Oh I was thinking the of the HSE rather than "acoustic work", but it's the same thing).

Thanks folks, this is one thread I will keep turning back to!

 

[JimB]

Rumpfy said:

The meter calibrated in dBm will give a result as 'x dB' with respect to the reference power level of 0 dBm. If the meter is specified as a having a dBm calibration, the 0 dBm will be 1 miilwatt in 600 ohm.

The first time I asked "what is dBm?", sometime around 1971, that is the answer I received and it made sense in terms of all the equipment around me at the time.

Rumpfy also said:

There are other standards for 0 dB and it is possible that RF generators with 50 ohm output impedance may use another dB x system. It may be that dBv is used for these instruments and I stand to be corrected.

I beg to differ and state that most RF equipment uses 0dBm as 1mW in 50 ohm.
I support of this statement, I offer examples from an RF Signal generator type Hewlett Packard HP8640B.

Look at the attachments, a scan of a part page from the specifications in the maintenance manual and a photo of the meter on the front panel of the generator.

JimB

 

[steveB]

I agree with JimB about the RF.

My experience is that dBm is just dB referenced to 1 mW irrespective of the details. Obviously, measurement equipment designed for a particular purpose might have additional requirements, but the definition is a simple power reference.

For example, I used to work in the optical communications field and dBm is used extensively there. Optical power measurements is often based on photodiode or thermopile measurements and impedance is not relevant in the measurement process. Still, the power is just the power and dBm readings tell you the power.