[Amps] conversion scale from dbu to microvolts

[Karl-Arne Markström]

In a radio context, the proper use of dBu is as a field strength value, where 0 dBu corresponds to a field
strength of 1 microvolt/meter

It appears that dBu and dBuV are used interchangeably in at least Japanese receiver literature and specifications
as elaborated in the "Decibels" section of  
http://www.radiomods.co.nz/kenwood/kenwoodgeneral.html
where a conversion table is provided together with some more background.

The use of the "EMF" qualifier is one way to avoiding the problem of
a non-matched input in a receiving system, as it pertains to the available signal voltage
from the source, not the actual voltage across the receiver terminals. 
More often than not this is very different from EMF/2.

73/

Karl-Arne
SM0AOM


----- Original Message ----- 
From: "Jim Brown" <jim at audiosystemsgroup.com>
To: <amps at contesting.com>
Sent: Saturday, July 29, 2006 5:19 PM
Subject: Re: [Amps] conversion scale from dbu to microvolts


On Sat, 29 Jul 2006 16:12:01 +0200, Karl-Arne Markstr"m wrote:

>dBu is a measure of voltage, expressed as dB relative to 1
>microvolt.

Perhaps you're thinking dBuV?  dBu is a measure of voltage 
referenced to 0.775v, and is widely used in pro audio, where 
output stages are low impedance (typically 50-100 ohms) and 
intended to drive high-Z loads (typically >10K). For this sort of 
application, where the output stage does not deliver power, a 
voltage referenced measurement is appropriate, and a power-
referenced measurement is entirely inappropriate. 

In the VERY old days (more than 40 years ago), pro audio systems 
WERE power-matched, usually to 600 ohms, but sometimes to 150 
ohms. For these systems, dBm and VU were meaningful expressions. 

>There is one major problem with this expression, as it also has 
to
>be referred to the termination. 

There's no problem with it at all in a system that is not run 
terminated, or as part of a specification for an RF input stage. 
There are many reference levels tied to dB expressions of voltage 
and power, and all of them have their place as long as you 
understand what you are measuring or specifying.

73, 

Jim Brown K9YC





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