The decibel is a relative power unit. At audio frequencies a change of one decibel, abbreviated dB, is just detectable as a change in loudness under ideal conditions.

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DECIBEL


 

Authored by Ian C. Purdie VK2TIP

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What is a Decibel?

This is a relative power unit. At audio frequencies a change of one decibel (abbreviated dB) is just detectable as a change in loudness under ideal conditions.

For a given power ratio the decibel change is calculated as:

dB = 10 log P2/P1

If we used voltage or current ratios instead then our formula becomes:

dB = 20 log V2/V1

Examples of using the Decibel

The decibel units add and subtract. For example, if we had an amplifier stage with a voltage gain of 22 which from above is 26.85 dB gain, followed by a further amplifier stage which has a voltage gain of 17 (24.6 dB) then the total overall voltage gain is 22 * 17 = 374 (51.46 dB).

Adding together the 26.85 dB plus the 24.6 dB = 51.45 dB, The minor difference was caused by my rounding to the nearesting second decimal place.

What is dBm for example

I received this email question:

"could you please explain me the difference between db, dbmV and dbmicroV?"

My reply:

I assume you understand decibels, if not:

see http://www.electronics-tutorials.com/basics/decibel.htm [this page]

dBm for example simply is referenced to milli-watts where one milli-watt = 0dBm.

A very common dBm figure is +7dBm where following the decibel rules and dividing +7 by 10 we get 0.7 and the anti-log of that is 5.0118 or five as the nearest whole number.

So +7dBm is another way of saying 5 milli-watts.

The same applies to the other values you mentioned, just different reference levels.

Why use this system? Instead of saying +7dBm why not say 5 milli-watts? There are several reasons:

a) In systems with gains and losses it is far easier to add and subtract the dBm's.
b) With different impedance's throughout circuits, power levels in dBm's remain constant, only the rf voltages and impedance's change.

Hope this helps.

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Updated 25th July, 2000

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