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.

 

The frequency is determined primarily by the inductance and the values of C1 and C2.

You can probably get this to oscillate with lower capacitance by increasing the circuit's gain. The ratio of C2 to C1 should be closer to 5:1 than the 1:1 shown in the schematic.

Also, make sure that the transistor is operating at the collector current that gives the highest Ft or HFE. At 100 kHz, go for best HFE. If you really intend to run this at 50 Mhz, then go by the Ft curve. The 2N2222 is not (NOT) an RF transistor, and thus does not come with an Ft vs Collector current curve.

By the way, be aware that your inductor has some capacitance of its own, and that will limit the maximum frequency of oscillation. You can probably reduce your total capacitance by adding a tap to your inductor and making it into a Hartley oscillator. But that's just a guess.

Any other ideas?

 

Hello,

On these pages you will find a lot of information about the Theremin, including some schematics:
http://www.thereminworld.com/schematics.asp
http://www.thereminvox.com/

Greetings,
Bertus

PS if a link does not work you can paste it in the search field of the Wayback Machine:
http://www.archive.org/web/web.php
This will give a list of dates the page still was there.

 

I plan to run it closer to 500KHz. Thanks for the cap ratio/gain advice. And I'll work on optimizing collector current.

Cheers.

 

Good advice all around....and your trial and error approach is pretty much how ALL us old geezers learned about oscillators! I wouldn't despise a day of that. Not too many "kids" get that sort of hands on twiddling these days!

Keep in mind a BJT draws considerable power from the tank circuit....seriously reducing the Q and the ability to keep oscillating over a wide range. An FET will make things much more forgiving....it will remove all or almost all of the unknown loading from the tank.

If you MUST use a BJT, you might try "tapping down" on the coil a bit farther. I think you'll still like the FET better.


Eric