Re: 3 dB bandwidth

Pooh Bear <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:

> Richard Henry wrote:
>
> > "Don Bowey" <dbowey@xxxxxxxxxxx> wrote in message
> > news:BEE1E224.7486%dbowey@xxxxxxxxxxxxxx
> > > On 6/24/05 3:48 PM, in article 42BC8DA1.1CFA0EA3@xxxxxxxxxxx, "Pooh Bear"
> > > <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:
> > >
> > > > Adrian Tuddenham wrote:
> > > >
> > > >> Decibels are, as you say, dimensionless; but they are specifically a
> > way
> > > >> of expressing the ratio of two power levels - only power levels, not
> > > >> voltages or currents or anything else.
> > > >
> > > > No no no no *NO* ! dBs are simply ratios. They can however be referred
> > to
> > > > units such as dBV ( decibel Volt ) , dBm ( decibel miliwatt ) and so on
> > and
> > > > are *widely* used in this way throughout the electronics industry !
> > > >
> > > > What the heck has got into this thread ? God Almighty !
> > >
> > > A little problem with dBm; it isn't a ratio. dBm expresses a reference
> > to
> > > 1 mW. 30 dBm= 1W.
> >
> > It's a ratio to 1 milliwatt.
>
> Somoene else finally brings some sense to this thread !

You can *measure* the voltage but the calculations are based on power.

You can't have different definitions of a decibel, depending on the
units you use to measure it. That's like saying you have different
definitions for Ohms Law, depending on whether you are measuring volts
and amps or deriving it from Watts. The formulae work all right, but
the definition of an Ohm does not include Watts.

The other formulae are useful derivatives (under certain conditions
which should be specifically stated), but they are not the definition.
(They can also be very confusing to a beginner who thought he had just
got a grasp on what a decibel was.)



If you add a suffix '$' to denote some reference level, then dB$ becomes
an absolute unit.


Hope that makes sense.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
.

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