Expert: Dave Nyce - 7/1/2002

Question
Sir ,

    I am a 22 boy , an assistant Engineer of Audio Engg... Would you mind answering my simple questions ?

   Q : 1 dBm = ? Watt

  and what is " dBm , dBu , dBV , dBr , dBg " ?

  Please answer me ASAP , THANKS !!!!!!!!  

                               Regards  

Answer
When the range of measurements is too wide to use a linear scale, a logarithmic scale based on the decibel can be used. The decibel is a ratio between two levels.

When the two levels are powers, the formula is:

gain or loss in dB = 10 log10 P1/P2

When the two levels are voltage or current, the formula is:

gain or loss in dB = 20 log10 V1/V2

So, if a power ratio is 3 dB, that means a power ratio of 2 times.

If a voltage ratio is 6 dB, that means a voltage ratio of 2 times.

Often, one of the values in the ratio is a standard value, the other being compared to it. For dBm, the m stands for 1 milliwatt, so the value is compared to 1 milliwatt. So, 0 dBm is 1 milliwatt.

dBu is relative to .775V, because it is the voltage developed by 1 mW when appled to 600 ohms.

dbV  is relative to 1 Volt

dBr  (relative) is used to specify various levels in a circuit relative to a specified zero level point.

I don't think I've heard of dBg.

Hope this helps!

Dave