Electronic – The reason for using air coils in LC tank circuits


I noticed that in LC tank circuit designs very often people use air coils as the L element as opposed to an axial inductor. I understand that the problem with the axial ones is that they tend to self-resonate. But I also thought that using an air core coil allows for much smaller inductance (on a scale of nH), as opposed to the usual uH scale of axial inductors. This in turn allows for a larger capacitance of the capacitor in the tank so that the other capacitances in the circuit don't influence the resonant frequency as much.

Would it be reasonable to conclude that using larger capacitors and smaller inductances (air coils) is a more robust approach to designing LC tank circuits, or should the effect be negligible and the main concern be reducing self-resonance of the inductor?

Best Answer

All inductors are self resonant so choose an inductor that has a self resonant frequency sufficiently higher than the operating frequency of your circuit. Or, make use of the self resonance if appropriate to do so.

For a tank circuit in the collector or drain of a transistor, higher Q occurs when the inductor/capacitor ratio is bigger and this may be desirable in some applications. However, for a tank circuit in a quadrature detector (used in FM demodulation), higher Q occurs with a higher capacitance/inductance ratio and this is usually desirable for FM demodulation.

In other words, the target application dictates whether you want a higher L/C ratio or a higher C/L ratio.