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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