# Tune L C resonant circuit with semiconductors

I have an air coupled transformer consisting of a primary L C resonant circuit fed with feedback transistors and a secondary L C circuit feeding a bridge recitfier to power a small circuit over an air gap. It is almost identical to this circuit: https://en.wikipedia.org/wiki/Royer_oscillator

I also use a FET shorting the rectifier on the secondary side that is triggered occasionally to pass a signal back to the primary circuit. It looks like this example, the secondary coil and resonance capcitor is on the left, in this case a forked light barrier is used as source of the signal to be transmitted: The lowered resistance leads to a small change (about 2%) in frequency which can be sensed on the primary side. That worked well in several places, but if the constant power consumtion is higher, it sometimes fails: While the signal is triggered, it tends to overload the primary circuit, giving an trashed non-sine waveform.

However, by changing the capacitance C4 mounted to the secondary resonator, the frequency can be changed easily over a broad range of about 30% without much additional power consumption.

So I wonder if there any way to change the AC capacitance with the FET or some other simple circuit?

• A varicap maybe? – Arsenal Nov 16 '15 at 16:11
• It seems varicaps are only useful for small AC voltages with almost no power, that are then amplified. I need to tune the high power secondary oscilator (about 30V peak-to-peak). – dronus Nov 16 '15 at 16:56
• What range of capacitance change do you need? Is driving a classic air-spaced tuning capacitor with an RC servo an option? – Brian Drummond Nov 16 '15 at 17:41
• You could put in a bank of 8 capacitors each 2x the previous one, and switch them on and off similar to a DAC. So, 2.2, 4.7, 10, 22, 47, 100, 220, 470. This would give you 256 possible capacitance in more or less uniform steps. This is just a wild idea. I don't really understand your circuit. So I haven't put in any values for the caps. – mkeith Nov 16 '15 at 18:50
• I just need to capacitance levels (binary signal). Using a motor driven caps is not an option, there is not enough space (flat circuit board) and I don't like worrying about mechanical reliability. – dronus Nov 16 '15 at 22:19