Electronics > Projects, Designs, and Technical Stuff
Dual-frequency (simultaneous dual resonance) analog sine wave gen with 1 op-amp?
<< < (4/6) > >>
Wolfgang:

--- Quote from: David Hess on June 16, 2019, 11:49:08 pm ---
--- 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.

--- End quote ---

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.
vk6zgo:

--- 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  To minimize cost one active device amplfied both RF and AF.  There was a design for one in my Handy-Dandy 101 electronic projects breadboard that I got when I was a kid.

I'd bet it'd be easier to design one if the two frequencies are far apart.  If the frequencies are close together, I'd think the Intermodulation Distortion performance of the amplifier could have a big effect.

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 ---

A superegenerative receiver comes closest, (the "self quenching type), but the very purpose of
the quench signal is to prevent sustained RF oscillation, whilst retaining the "loss cancellation"
properties of positive feedback.
741:
Part of the reason for this post was to see if an elegant solution can be found, for instance adding a handful of discrete components to a Wien bridge.

Even if the solution is complicated though, it might well inspire some new circuit design 'shape'. It would be nice if both frequencies were prime, eg 11KHz and 17kHz.

I appreciate the comments about [a seeming need for] separately controlling the 2 amplitudes. I speculate maybe their sum (the circuit output) can be controlled as one. However, it may be that this allows one tone to dominate or something.
Wolfgang:

--- Quote from: 741 on June 17, 2019, 11:27:09 am ---Part of the reason for this post was to see if an elegant solution can be found, for instance adding a handful of discrete components to a Wien bridge.

Even if the solution is complicated though, it might well inspire some new circuit design 'shape'. It would be nice if both frequencies were prime, eg 11KHz and 17kHz.

I appreciate the comments about [a seeming need for] separately controlling the 2 amplitudes. I speculate maybe their sum (the circuit output) can be controlled as one. However, it may be that this allows one tone to dominate or something.

--- End quote ---

It *is* possible to control the two amplitudes separately, by using a diplexer approach and separate gain control.
What do you do with this circuit ?
741:

--- Quote from: Wolfgang on June 17, 2019, 12:03:38 pm ---What do you do with this circuit ?

--- End quote ---

I do not have an application in mind - this was just an idea that interested me "for its own sake".
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod