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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?

On 25 Feb 2007 16:40:59 -0500, 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.


The formula you used for Q is for a series resonant circuit - call it Qs.
For a parallel resonant circuit, you should calculate Q as Qp = 1/Qs.

That is, at the end of your calculation, hit the "1/x" key on your
calculator.

(See also http://en.wikipedia.org/wiki/RLC_circuit)

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