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Dual-frequency (simultaneous dual resonance) analog sine wave gen with 1 op-amp?

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741:
Just speculation...

I just wondered:
(a) If a mechanical object could resonate at 2 frequencies (seems no problem)
(b) how about a Wien-bridge type of circuit? The output would contain 2 distinct sine waves.

However it would have to be "interesting" - not just 2 separate Wien-bridges with the outputs 'mixed'. I imagine it would use 2 f/b circuits, but can it be done with say 1 (single) op-amp?

OM222O:
Any object can only resonate at one specific frequency  afaik. It will also resonate at other harmonics of that frequency too, but not at two different frquencies. I.e: it can oscillate at 100Hz,200Hz,300Hz, etc. But not at 100Hz, 170Hz etc.

Maybe there is some specific device made to be bale to do that, but I'm not aware of such thing. Besides op amps are so cheap, it would be pointless to use one rather than 3 (combine outputs of 2 using a 3rd one) especially sine quad op amps exist for the same price of the single packag3 for the most part  :-//

chris_leyson:
It's an interesting concept. I can't think of a feedback network that would gererate two feedback signals at the same phase let alone the same amplitude. If your amplitude is slightly off in one direction you either have no oscillation or too much. I think to get this to work you would have to use a super linear 100% AM modulator after the oscillator and maybe you could put the modulator in the feedback path. I think a lot of engineers must have looked into generating squeaky clean two tone signals for distortion measurements and I would opt for the linear AM modulator approach.

soldar:
To oscillate you need loop gain >1 and feedback with 180ยบ phase change.

The amplifier will provide gain at any frequency.

The feedback network will only provide the correct phase shift at a single frequency.

But what happens is you have two feedback loops? Do they work separately to provide two separate frequency oscillations? Or do they combine and provide a single oscillation at a different frequency?

SiliconWizard:
I don't know of any kind of oscillator that can oscillate simultaneously at two independent frequencies. Obviously if they are harmonics, yes. Most actual oscillators generate harmonics!

The only weird kind of oscillators I know: so-called chaotic oscillators. Chua's circuit is an example of this: https://en.wikipedia.org/wiki/Chua%27s_circuit
(Attached is one possible implementation with a single op-amp.)
It certainly generates several frequencies but those are not fixed.

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