When I feed a squarewave into a resonant tank circuit (LC network)
what component determines the _frequency_ of the resulting ringing?
I know the Q, or persistence, relates to how well the network is
tuned. But what about the actual ringing frequency?
Stuart Hall
If you grab hold and force a garden swing to move at some frequency,
slow or fast then it will. Likewise if you connect a "powerful" (low
impedance) squarewave generator to an LC then it will do that square
wave - the harder the drive, the more slavishly the copying.
Alternately, if you "ping" a garden swing, it will, as soon as the
push disconnects, start to move "freely" at its natural frequency.
Likewise the LC, you have to "ping" it and then let go to allow it to
move freely.
If you know the frequency of the swing then you can adjust your
pushing in sympathy for best effect. Likewise the LC if you connect a
sinusoidal generator to it but it *would* have to be just right it
there was a "direct" (low impedance) connection.
To give the LC a bit of freedom, to release it a little from the hard
grip, put a resistor in series with the sinusoidal generator,
(increase the driving impedance) now, as you swing the generator's
frequency through the LC's resonance, it is allowed to build up an
amplitude of its own. If you get it just right, the amplitude will
keep increasing (just like the garden swing) to greater than the
driving force! The greater this effect is, the greater must be the Q
i.e. the bigger the series resistance the bigger the Q.
I.e. as the resistance gets greater so the generator is less connected
to the LC and the less the generator *damps" the swing, the greater
the Q.
As the other poster says, if you use a square wave instead, then you
will be using a whole bunch of sinusoids simultaneously (because a
square wave is ~ the sum of all the odd harmonics of its fundamental
i.e. f + 3f/3 + 5f/5...) so it is likely one of these harmonics will
"rattle" the LC at the its resonant frequency - by chance.
But only if that square wave it (appreciably) below the LC's
resonance.
Robin