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Old February 26th 07, 01:42 AM posted to rec.radio.amateur.moderated
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Default Formula for Q of resonant circuit?

M J Dowden wrote:
I am trying to find the formula used to calculate the Q of a
parallel R-L-C resonant circuit. There are a number of questions in
the Amateur Extra test pool[1] that ask this and the formula that I
have does not produce the correct answers.
I have been using Q = (2 * Pi * f * L) / R, but the questions in
section E5G of the test pool don't seem to work with that. The
answers that are produced by that formula are among the
"distracters," suggesting that I am making an obvious error that they
expect some people to make.


It's obvious that this formula for Q is not valid for a parallel
RLC-Circuit. If you let R go towards infinity, by this formula,
the Q goes to zero, although it should obviously be infinite,
since the circuit has no losses.

The correct formula is Q = R / (2*pi*f*L) = R * 2*pi*f*C

73,
--
Fabian Kurz, DJ1YFK * Dresden, Germany * http://fkurz.net/
Online Log: http://dl0tud.tu-dresden.de/~dj1yfk/log.html

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