Re: 3 dB bandwidth

On Sat, 25 Jun 2005 00:58:11 +0100, Pooh Bear wrote:

> Don Bowey wrote:
>
>> 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.
>
> Yes. so 0dBm is 1 mW.
>
>> 30 dBm= 1W.
>
> Yes too.
>
> and -30dBm is 1uW. And it would be 60dB less ( 1 million times ) than +30dBm.
> I.e. the dB difference between the numbers yields the ratio.
>
> How is that *not* a ratiometric measurement ? What is your point ?

I think his point was that he wanted to say something that was right.

A 'dB' is a certain ratio of powers. A 'dBm' is an expression of the
ratio of the expressed power to a milliwatt, therefore, it is a
dimension, if you will. It's a number of milliwatts, expressed in
a log scale. A dBV, like he said, is the ratio of the applied voltage
to one volt, expressed as a log, but because watts are proportional
to volts squared, dB Volts are 20 log, and dB milliwatts are 10 log.

Hope This Helps!
Rich


.

Relevant Pages

  • Re: 3 dB bandwidth
    ... God Almighty! ... >> A little problem with dBm; ... > I.e. the dB difference between the numbers yields the ratio. ... It conveys no absolute info without being given a reference. ...
    (sci.electronics.design)
  • Re: 3 dB bandwidth
    ... >> of expressing the ratio of two power levels - only power levels, ... A little problem with dBm; it isn't a ratio. ...
    (sci.electronics.design)
  • Re: 3 dB bandwidth
    ... Richard Henry wrote: ... >> A little problem with dBm; it isn't a ratio. ... dBm expresses a reference ...
    (sci.electronics.design)
  • Re: 3 dB bandwidth
    ... >>> voltages or currents or anything else. ... > A little problem with dBm; ... It's a ratio to 1 milliwatt. ...
    (sci.electronics.design)
  • Re: Mystery OPT
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