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Technical question: dBm vs dBv in service notes

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Laurie
Double Platinum Member

Canada
4854 Posts

Posted - 02/13/2010 : 15:43:50

G'day! Many older Boss service notes specify the adjustments in "dBm". I've always assumed that they meant "dBv" and that when they say "0dBm" they mean 0.775V RMS, and that you need to use the 20LOG(X/Y) calculation to work out your actual voltage.

For example, using this approach, -20dBm as stated in a service note is really 0.0775 VRMS. (or 77.5mV RMS)

Is this the understanding everyone else has?

cctsim
Silver Member

United Kingdom
418 Posts

Posted - 02/13/2010 : 19:19:23

Hi Laurie,

I think they mean 0 dm on a 600 Ohm resistor.

0 dBm corresponds to 1mW of power, which is ~ 0.775 V on a 600 Ohm resistor.

The formula is P=V^2/R which solved for the voltage gives:

V=sqrt(1 mW * 600 Ohm)=0.7746 V

600 Ohm is the standard reference for audio applications.

Edited by - cctsim on 02/13/2010 19:51:09