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# Re: Now, How does a coil really work??

```At 12:43 PM 1/22/99 -0700, you wrote:
>Original Poster: "danial stocks" <diode-at-hotmail-dot-com>
>
>
>
>Hi folks...
>One of the important parameters of a tuned circuit is it's 'Q'
>The Q value can mean a lot of things - the ratio of 3db bandwidth to
>centre freqency is the normal one, but it also means the length of time
>for which the tuned circuit, when set into oscillation by a transient as
>opposed to being continually pumped at a res. freq, will continue to
>oscillate for before the oscillations die away due to system losses. I
>cant remember this bit perfectly so maybe someone out there with a book
>might be able to supply the appropriate info, but the Q of the tuned cct
>= the number of oscillations before pretty much dying out ? 5%? 10%? of
>original value.

Hello Danial (and list)

The quality or Q factor of a damped simple harmonic oscillator is the rate
at which energy decays. It is usually defined as the number of radians
through which the damped system oscillates as its energy decays to E =
Eo*exp(-1).

Since Q is usually very large and the damping resistance is very small, we
can do some mathematical trickery and say that to a very close
approximation Q = Wo*L/r which is a constant of the damped system.

The fact that Q is constant implies that the ratio (energy lost in
system)/(energy lost per cycle) is also a constant, with Q/2*Pi the number
of cycles (or complete oscillations) through which the system moves in
decaying to E = Eo*exp(-1).

Thus the ratio (energy lost in system)/(energy lost per cycle) = Q/2*Pi.

Safe coiling,

Gavin Hubbard

```