I'm trying to design an oscillator for use in a theremin. It requires a large L value and a small C value in the tank to maximize the circuit's sensitivity to antenna capacitance. I'm pretty new to this stuff, so I used the following as a starting point:
I wound my own air coil inductor (estimated at about 1.5 milliHenries) and upped the capacitor values to start with. I slowly swapped out caps until I got down to C1=560pF (ceramic), C2=10nF (poly film), C3=100nF (poly film), resulting frequency 110KHz. At this point, trying to reduce the value of C1 or C2 makes the circuit stop oscillating, and adjusting C3 has a negligible effect on the frequency. I've tried adjusting resistor values, but haven't found anything that works by trial and error. What can I do to make this circuit work with lower capacitance, without reducing inductance? I've tried setting the forward bias on the transistor using a variable resistor for R1, and even when the bias is identical to a working configuration, it just won't oscillate at lower frequencies. Do I need to adjust other resistor values? Change the voltage? Or could the problem be with my inductor? (4 inches long, 3 layers of 22 gauge magnet wire around a 1.25" OD PVC pipe) I had similar problems with an smaller inductor too, though (2 inches, 1 layer, 3/4" pipe).
In other words, I don't really know what I'm doing. Please help.
I wound my own air coil inductor (estimated at about 1.5 milliHenries) and upped the capacitor values to start with. I slowly swapped out caps until I got down to C1=560pF (ceramic), C2=10nF (poly film), C3=100nF (poly film), resulting frequency 110KHz. At this point, trying to reduce the value of C1 or C2 makes the circuit stop oscillating, and adjusting C3 has a negligible effect on the frequency. I've tried adjusting resistor values, but haven't found anything that works by trial and error. What can I do to make this circuit work with lower capacitance, without reducing inductance? I've tried setting the forward bias on the transistor using a variable resistor for R1, and even when the bias is identical to a working configuration, it just won't oscillate at lower frequencies. Do I need to adjust other resistor values? Change the voltage? Or could the problem be with my inductor? (4 inches long, 3 layers of 22 gauge magnet wire around a 1.25" OD PVC pipe) I had similar problems with an smaller inductor too, though (2 inches, 1 layer, 3/4" pipe).
In other words, I don't really know what I'm doing. Please help.