VU question

[Magnus Danielson]

From: Tim Ressel <Tim_R1 at verifone.com>
Subject: VU question
Date: Wed, 23 Feb 2000 08:43:55 -0800

> To all,

Hi Tim!

> Pop quiz: Do VU meters display voltage or power? In other words, what is half: 3
> dB or 6 dB?

-3 dB is allways half the power
-6 dB is allways half the voltage

Recall that you have

          P
L = log  (----) [B]
       10 Pref

or by popular use

               P
L = 10 * log  (----) [dB]
            10 Pref

and you have the nice little relation

     2
    U
P = -
    R

giving

                  2
                 U
L = 10 * log  (-------) [dB]
            10 R * Pref

                      U
L = 20 * log  (--------------) [dB]
            10 sqrt(R * Pref)

where Pref migth be converted into Uref:

Uref = sqrt(R * Pref)

and thus:

                U
L = 20 * log  (----) [dB]
            10 Uref

dBm => Pref = 1 mW
dBu => Pref = 1 mW & R = 600 Ohm ~=> Uref = 0.775 V
dBV => Uref = 1 V
dBuV => Uref = 1 uV

So, voltage and power relate to each other in a strict manor.

Did that clear things up?

Here, U is the effective voltage, thus, the RMS... really the POWER value of
voltage.

> Tim Ressel--Compliance Engineer

I thougth the dBuV would make you feel right at home ;)

Cheers,
Magnus