# Please solve the decibel problem

1. Apr 21, 2015

### taoz

10dB+10dB=20dB
10dB+1dB=11dB ??
10dBm+10dB=20dBm
10dBm+1dB=11dBm ??
Is all correct? and what is the difference with dB+dB and dBm+dB???

2. Apr 21, 2015

### phinds

How about you look up the definitions and how to convert from one to the other.

3. Apr 22, 2015

### Averagesupernova

I believe this has been covered on PF already. A search should bring results. dB represents gain or loss. dB with a suffix is an absolute. For instance, 0 dBm is .001 watts into a specified impedance. Often it is 50 ohms. I believe the telephone system my have a similar 600 ohm standard.

4. Apr 22, 2015

### jim hardy

indeed, the world famous and much revered Simpson 260 has a db scale referred to 1mw/600 ohms, see lower left

image from http://simpson260.com

5. Apr 22, 2015

### davenn

so @taoz, did you do as phinds suggested and look up the difference between dB and dBm ?
what did you discover ?

Dave

6. Apr 22, 2015

### JakeBrodskyPE

When you see a decibel reference in the wild, there are TWO numbers to consider, the number of decibels, and whatever reference it compares to. Thus the letter m after the dB refers to one milliwatt. That milliwatt could be at any impedance. Old signal generators used to refer to voltages at 50 ohms. Then someone figured out that these numbers made a hell of a lot more sense if you listed them in terms of dBm. Analog phone systems often derive dBm from RMS voltage measurements at 600 ohms.

When dealing with transmitters you sometimes see references to dBW (one watt). When dealing with antennas you often see gain references to an isotropic radiator (radiates equally in all directions). Among ham radio operators you often see reference to antenna gain compared to a dipole, which has a theoretical gain of 2.2 dB more than an isotropic radiator.

There are other references. I used to work on analog frequency division multiplex systems for phone trunks. There is dBrn (dB referenced to the noise floor), dBrnC (C-message wieghting on a 3400 Hz Channel referenced to the noise floor), dBrnC0 (this was dynamic range of the channel from the Test Level Point, or maximum power, to the noise floor with C-message weighting). You can get wild with all these references and they are commonly used in many system designs. And don't get me started about psophometric versus C-message weighting...

So when someone casually tosses the word decibel around your inner engineer should scream Decibels over WHAT????

I taught this to my teen-age daughter and now she taunts her high school teachers with that question every time they discuss decibels. If she gets it, so can you.

7. Apr 23, 2015

### meBigGuy

A signal that is 10dB greater than 5 dBm happens to be 15 dBm.

dBm is a measurement of power. It is the ratio of a power level to a reference power level, but that makes it a measurement of power. It has no units because it IS a unit.
Asking for the units of dBm is like asking what are the units for an inch. dBm is a unit for expressing power. You can convert it to watts if you like.

dB expresses a ratio between power levels, with no information as to what those levels were. It has no units because it is simply a ratio. You can not convert it to anything, but it is a useful ratio if you apply it to a known power level.