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What is a Decibel?

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發表於 14-9-2007 01:00:07 | 顯示全部樓層 |閱讀模式
DECIBEL
Whatis a Decibel?
This is a relative power unit. At audio frequencies achange of one decibel (abbreviated dB) is just detectable as a change inloudness under ideal conditions.
For a given power ratio the decibel change is calculatedas:
dB = 10 log P2/P1
If we used voltage or current ratios instead then ourformula becomes:
dB = 20 log V2/V1
Examplesof using the Decibel
The decibel units add and subtract. For example, if wehad an amplifier stage with a voltage gain of 22 which from above is 26.85 dBgain, 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 decimalplace.
Whatis dBm for example
question:
"could you please explainme the difference between db, dbmV and dbmicroV?"
reply:
assume you understand decibels,
dBm for example simply is referenced to milli-watts whereone milli-watt = 0dBm.
A very common dBm figure is +7dBm where following thedecibel rules and dividing +7 by 10 we get 0.7 and the anti-log of that is5.0118 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, justdifferent reference levels.
Why use this system? Instead of saying +7dBm why not say5 milli-watts? There are several reasons:
a) In systems with gains and losses it is far easier toadd and subtract the dBm's.
b) With different impedance's throughout circuits, power levels in dBm's remainconstant, only the rf voltages and impedance's change.

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