From: "dave.harper" on Thurs 14 Jul 2005 18:55

Joel Kolstad wrote:

No, although for low Q resonators, it's often pretty close.

Generally speaking, most relatively simple AM receivers are really only
intended to pick up relatively strong, nearby transmitters. As such, you can
get away with an awful lot of "cut and try" when it comes to designing the
circuit (largely ignore matching impedances, Q's, etc.) and still obtain
acceptable results.

Thanks for the response Joel. So if I understand correctly, Q is
basically an indicator of how well the LC circuit resonates? Could you
think of Q as the inverse of a dampening coefficient?

In a way, you might think that. For going to more advanced things
besides "simple AM receivers," I'd suggest thinking of Q as a
built-in LOSS element.

For parallel-tuned circuits, the loss can be modeled as a resistor
in parallel with the L and C. This equivalent resistor value is
the reactance of either L or C (they are equal at resonance)
multiplied by Q. A high Q indicates least loss in a parallel
circuit, a high value of equivalent parallel resistance.

But, for series-tuned circuits, the loss is equal to a resistor
in series with L and C. That resistor value is equal to the
reactance of either L or C divided by Q. A high Q in a series-
tuned resonance would have the lesser value of series resistance.

If so, I guess ideally you'd get the best Q with an iron-core inductor,
thick windings, and as few windings as possible?

Yes and NO. Q will vary by MANY things. Generally, physically
big coils will have higher Q, physically big windings will have
higher Q. Shape factor, like length versus diameter of a
solenoidal winding has an optimum value. Nearby shielding will
tend to reduce Q; one reason why toroidal forms have higher Q
than solenoidal or cylindrical windings.

CORE MATERIAL IS FREQUENCY SENSITIVE! "Iron core" has to be
defined. Power transformer laminations are okay at up to
about 10 KHz and then become more lossy with increasing
frequency. Special iron (tape shape, usually) is used for
higher frequencies in the supersonic range. At LF and higher,
various kinds of iron POWDER are used to enhance Q (within
their specified frequency range).

Q applies to capacitors also...and is affected by things like
plate area, plate material, dielectric if other than air, and
(to some degree) physical shape factors. Generally, though,
the Q of most resonating capacitors is 10 to 100 times larger
than inductors and can usually be neglected in most
calculations of tuned circuits. Inductor Q rules! :-)

For self-education, I'd suggest spending some time with a good
Q Meter and trying out measurements on various kinds of
inductors. That will probably give you the best Q picture in
your mind.