RESONANCE

Resonance is a characteristic of AC Circuits consisting of an Inductor and a Capacitor.

When the values of inductance, capacitance and the applied frequency are such that the 
inductive reactance and the capacitive reactance cancel each other out, we have a state called RESONANCE and the circuit is said to be TUNED or resonant.  

Reactance is the opposition to the change of current flow in an AC circuit.

The value of an inductor (in Henries) and a capacitor (in Farads) produce a resonance at only ONE frequency.

IN A SERIES L and C circuit

At resonance, the difference between the inductive reactance and the capacitive reactance is 0.
This means that the resistance or impedance (if a resistor is included in the circuit) is 0. This is because the capacitive inductance is inverse to the inductive inductance at the resonant frequency.
Therefore, at resonance, the reactances of the two (L and C) cancel each other out and hence there is no (or very little) impedance (or resistance), hence the current is VERY HIGH

Now since impedence/reactance is very low in an resonant circuit, we can use OHMS law to calcualte Current

I = V / R
I = 10 / 1 = 10amps for example -

Main thing to understand is that in RESONANT series circuits (tuned circuits), IMPEDANCE is very low.

Now, because impedance is very low, current is high and hence the voltage drop across the devices is high since voltage and current are directly proportional.

IN A CIRCUIT THAT DOES HAVE SOME RESISTANCE

Now, in a RESONANT series circuit, we know that the REACTANCE is zero.  Add a resistor then we have IMPEDANCE (impedance is reactance of the Inductor and Capacitor and the Resistor).  Now since Reactance is 0, then impedance must equal the resistance or Z = R

Increasing the capactance, and given a fixed frequency, will decrease the resonant frequency.  Why?  Because the smaller the area of the charge plates on the capacitor, the greater the opposition to the flow of the alternating source signal - likewise, the greater the area of the charge plates on the capacitor (larger the capactance value), the smaller the reactance and hence, the lower the resonant frequency. Because the charge plates on the larger capacitor is larger, the more of a charge can be taken causing the opposition to be less and hence the lower the resonant frequency.

Remember the formulae for Capactive Reactance =
Xc = 1 / 2piFC
where C is in Farads

IN A PARALLEL L C Circuit

At Resonance, in a Parallel LC Circuit (also know as a TANK Circuit) - current is flowing into one device whilst it is flowing out of another.  There is an inverse relationship between inductive reactance and capacitive reactance.
Now, in a parallel circuit, the voltage is the same across all branches, but the current is different. Now, since the impedance is the same in a tuned parallel circuit, using OHMS law we can deduce that the current is the same across the capacitor and the inductor, but they are inverse in relation.  Hence, the net affect is that there is 0 Amps in the parallel resonant circuit.  This implies, the IMPEDANCE IS VERY HIGH IN A PARALLEL RESONANT CIRCUIT.

For a tank circuit, the source needs to be fedback to keep the tank oscillating - this is acheived by feedback through an amplifier device.