In article ,
"Tim Williams" wrote:

"Michael A. Terrell" wrote in message
...
You need to do a lot of studying on how to use the dB.

I know full well what a logarithm is; don't patronize me.

My point was that some otherwise ambiguous dB scales (at least one) have a
defined absolute basis.

Like Michael stated dB is a logarithmic reference-less ratio value.

You can use dB for things like amplifiers that have a gain or
attenuators that have a loss for example. The gain of an amplifier can
be expressed in dB because the reference value is the input value of
the amplifier, which will allow you to calculate the output value but
if you are speaking of a value of power or voltage by itself then you
need an absolute scale with a reference quantity.

Absolute scales would be dBV, dBuV, and dBm. In those terms the
reference is 1 volt, 1 micro-volt (0.000001 volt), and 1 milliwatt
(.001 watt).

The reason you need a reference value is noise prevents you from
measuring 0 Volts and 0 watts accurately so you need to use a small
reference value in its place and so everyone agreed to use these
values.

Definitions a
dBV = 20 * log (volts / 1 )
dBuV = 20 * log ( volts / 0.000001 )
dBm = 10 * log ( power / .001 )

So for example:
30 dBm = 1.0 watt
0 dBm = 0.001 watt (the reference value)
-30 dBm = 0.000001 watt

Most of the time in radio dBm, dBuV, and watts are used.

--
Telamon
Ventura, California