At 06:50 PM 8/10/97 +0100, Bob Duckworth wrote:
>
>
>>From 1 to 9 S units his is 25db (power ratio) = 50db voltage ratio
>which is pretty darn close to 48db or 6db/S unit.
Oh, oh, here we go again. When I was in engineering school, the
definition of dB was that it is 10 times the log(base 10) of the
power ratio. For voltage change ratio in the circuit, it is
20 times the log(base 10) of the voltage ratio, and the answer is
the same numeric number of dB's. Because, by definition a dB is
a power ratio change with respect to whereever P1 is measured.
If so, then the above statement that 25 dB power ratio is the
same (in the same circuit) as a 50 dB voltage ratio is not true.
A dB is a dB is a dB (as is a rose a rose).
You can, of course, express voltage ratios in logarithmic terms,
to make big ratios smaller, and small ratios bigger which is part of
the point of logarithmic arithmetic( to eliminate lots of zeros both before
or after the decimal point), but you must specify Dbv, not just dB. And
the arithmetic of dBv must also be specified, that is dBv= what times
the log of the voltage ratio?
At least, I think that is what I learned! Please help me if I
am wrong.
73, Jim, KH7M
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|