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

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Wolfgang:
Having fun and to learn something is the best of all reasons.  >:D

Zero999:
Yes, an oscillator can output two frequencies. Often this is unintentional, such as a crystal oscillator exciting two modes or the crystal or an op-amp being unstable and generating a much higher frequency, as well as the intended oscillator frequency.

David Hess:

--- Quote from: Wolfgang on June 17, 2019, 12:08:16 am ---If you look for useful dual resonances there are SC cut crystal oscillators where the secondary (b Mode) resonance is tracked for exact crystal temperature measurement.
--- End quote ---

I remember those now.  But didn't the other mode use a separate oscillator?

Wolfgang:
... normally yes. But it can also happen (unintentionally) in the same Colpitts type oscillator.

rfeecs:

--- Quote from: T3sl4co1l on June 16, 2019, 04:41:08 pm ---The history goes back to I think early Western Electric DTMF sets, which used a single transistor (they were expensive back then!) and a keypad matrixed selection of LC tanks, carefully tuned for the frequencies required.

--- End quote ---

That's a great example.  Here's a schematic of the Western Electric 2500 Touch-tone phone:
http://www.repeater-builder.com/tech-info/dtmf/bell-dtmf.html


Only one transistor to generate two tones.  Lots of DIACs, it looks like.

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