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unknown oscillator type is this a Hartley oscillator?

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Sep 13, 2009
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unknown oscillator type

Hello everyone,
I recently bought elenco's basic electronics experiments kits and intend to build some of the circuits in this kit. One circuit topology that crops up regularly is the transformer center tap of the primary coil connected to the supply with a cap and a resistor connected in parallel with the complete primary coil of the transformer.

I've attached the schematics.

you can also find this schematic on page 43 of the following manual**broken link removed**

My question is this a Hartley oscillator? How so? the center tap is not connected to ground but VDD. In the manual it says that the oscillation frequency can be altered by changing the capacitor value or the resistor value (one in parallel with the capacitor and primary transformer coil). I thought that only changing the cap (inductor coil is fixed and cannot be changed) value would change the frequency any ideas on this?

Finally I realize that this is some sort of LC oscillator but if anyone can give me some detailed info on how it works exactly, I'd greatly appreciate it.

Re: unknown oscillator type

Explaining as simply as I can:
The transistor is connected in a configuration that inverts the signal fed into it. If the base pin goes higher in voltage, the collector voltage will reduce, so the signal is turned upside down.

To make an oscillator you need positive feedback, meaning some of the output signal is fed back to the input in such a way that it reinforces it, causing an instability that changes the output cyclically. Those cyclic signals are the oscillations you want. If the feedback is negative, meaning it cancels rather than reinforces the input, you get a stable amplifier instead of an oscillator.

If you took the feedback straight from the collector of the transistor, because it is inverted, you get negative feedback which is no use at all. This is where the center tapped transformer comes into play. The transformer primary winding is all in the same direction, think of it as a single coil with a tap in the middle. This means that if you take the tap as a reference point and inject a signal at one end, the other end will have the same signal but turned upside down. This is how the oscillator signal gets inverted before going back to the transistor. The transistor inverts, the transformer inverts again so it is now the right polarity to achieve positive feedback.

Hope that helps.


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