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

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T3sl4co1l:
Modern power MOSFETs exhibit a similar symptom when switching, if a low impedance is present on the gate terminal (effectively making a grounded-gate oscillator with device capacitances and package parasitics).  Though the physical mechanism differs.

Tim

Wolfgang:

--- Quote from: 741 on June 16, 2019, 03:17:35 pm ---Re "...can only resonate at one specific frequency..."

I was imagining say a "dual pendulum" (one tied to 'bob' of the other), or a 'spring + weight' linked to a 'spring + weight'.

There are also some simple set-ups where you can observe resonance passing from one part to another and then back again. Something like tie two pendulums with different time constants to one freely moving but rigid support. You see one pedulum go fully resonant, then quieten, then the other starts, then back again.

Maybe it depends on the definition of 'object' though. Is a guitar or a car an object for the purposes of this discussion for example?

--- End quote ---

wanna see a crystal oscillator running at several frequencies simultaneously ?
https://electronicprojectsforfun.wordpress.com/silly-circuits/silly-circuts-some-unusual-behaviour-of-crystal-oscillators/

soldar:

--- Quote from: duak on June 16, 2019, 06:42:47 pm --- There is similar prior art with the Reflex Receiver: https://en.wikipedia.org/wiki/Reflex_receiver 
--- End quote ---
Thanks. That is very neat. I had never seen that before.

emece67:
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David Hess:

--- Quote from: duak on June 16, 2019, 06:42:47 pm ---Sometimes the amplifier in an oscillator will have unintended oscillations at high frequency that are not harmonically related to the intended frequency.  Does anyone remember snivets or Barkhausen oscillation in vacuum state devices?
--- End quote ---

Those are effectively the same thing as emitter/source follower oscillations when driving a low impedance load.  The input series inductance and base/gate/grid shunt capacitance form a Colpitts oscillator if the tank resistance is lower than the negative input resistance.  Tetrodes have a negative resistance kink in their transfer function at low anode voltage due to secondary emission.

The solution for either is the same; add a lossy element in series with the base/gate/grid or emitter/source/cathode which is greater than the negative resistance.

I would not really count this as dual resonance since it does not involve the feedback network.  It is just parasitic oscillation.


I could see making a feedback network which satisfies the criteria for oscillation at more than one frequency but the gain also needs to be controlled at each frequency.  That is an awful lot of work compared to just using two or more separate oscillators.  I think I have seen some RF oscillator designs which used duplexers to support multiple tank circuits with one active device but it was not to operate at more than one frequency simultaneously.

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