A tank circuit will not convert a direct current to AC. If you connected a tank circuit to a DC source, the inductor, if no resistor was in series with it, would short the DC source. If you did have a resistor to limit the current through the coil, then you would simply have a direct current flowing through the coil. No current would flow through the capacitive branch when the capacitor is fully charged. Effectively, with direct current, an inductor has 0 reactance and a capacitor has infinite reactance.
Inductive Reactance
[tex]X_l=2\pi fl[/tex]
Capacitive Reactance
[tex]X_c=\frac{1}{2 \pi fc}[/tex]
Both of these are measured in ohms.
When dealing with RCL circuits, the total impedance is given by:
[tex]Z=\sqrt{R^2+X_t^2}[/tex]
[tex]X_t=X_lX_c[/tex]
At the resonant frequency,
[tex]f_r=\frac{1}{2\pi\sqrt{LC}}[/tex]
the total reactance is 0 and this means in a series RCL circuit that the total impedance is at it's lowest.
A parallel RCL circuit operating at the resonant frequency will have maximum impedance.
The frequency of a DC signal is 0 Hz so reactance doesn't come into play. Now if you're talking about pulsating DC, that's a different matter all together.
