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Is this a suitable sine wave oscillator? how would I control the frequency?

Discussion in 'General Electronics Discussion' started by crystal please, Sep 19, 2016.

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  1. crystal please

    crystal please

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    Jul 5, 2016
    I'm making a bank of analog sine wave oscillators and I'm interested in keeping the design compact.
    I've started by making a Wien Bridge oscillator. As I'm working from a single power supply and I'm trying to keep the design small, I've been using the LM386n-1 IC, which was bought from http://www.kynix.com/Detail/174669/LM386N-1.html
    Click image for larger version

    [​IMG]
    I'll be using the oscillators to do some additive audio synthesis, maybe some FM and I'll probably need some of the oscillators as LFOs.
    My questions are-
    Is this a suitable approach for making a compact analog sine wave oscillator?
    Also, how would you suggest I control the frequency, which component value should be changed to achieve this? - I've tried messing with all the resistors to change the pitch, but they all seem to be interdependent.
    Here is the circuit I've been working from, it's from the National Semiconductor LM386 Low Voltage Audio Power Amplifier datasheet-
    Click image for larger version

    [​IMG]
    And this is what my implementation looks like, I don't have a suitable incandescent bulb so I've just replaced it with a 1KΩ pot. I'm still not sure what I'll use in the final circuit as AGC, suggestions are welcome.
    Click image for larger version

    [​IMG]
     
  2. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Umm... the images don't appear using my browser. And the incandescent bulb is a critical part of the feedback loop for a Wien Bridge oscillator... you cannot replace it with a fixed resistor and expect a stable sinusoidal output. See this Texas Instruments application note for design methodology.
     
    Last edited: Sep 19, 2016
    Harald Kapp likes this.
  3. Alec_t

    Alec_t

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    Use the 'Upload a file' button to upload images. Note the image size and types allowable.
     
    hevans1944 likes this.
  4. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Unless you are driving a speaker directly with the output, an LM386 is not a good part for a general purpose signal source. It is not a true opamp, has relatively low input impedances, and an intentional DC offset at the output that you cannot control.

    Separate from that, a Wein bridge circuit is great for low distortion, but difficult to control and extra difficult to vary over any kind of reasonable frequency range. A phase shift oscillator is more simple, has better (ok, easier) amplitude stability, has distortion in the 1%-2% range, and can be varied over a small range by changing one resistor.

    Besides the link in post #2, Linear Technology, the original National Semiconductor, and Analog Devices also have excellent app notes on sine wave generation.

    ak
     
    hevans1944 likes this.
  5. hevans1944

    hevans1944 Hop - AC8NS

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    OBW0549 likes this.
  6. OBW0549

    OBW0549

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    Jul 5, 2016
    A triangle wave generator is also a lot easier to change frequency: just change a single resistor.
     
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  7. hevans1944

    hevans1944 Hop - AC8NS

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    And you can get everything you need in just one integrated circuit... the XR2206.
     
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  8. AnalogKid

    AnalogKid

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    I thought about mentioning the 8038 as a sinewave source chip, but post #1 sounded to me like he wanted a native sinewave osc circuit.

    Also, a diode breakpoint shaper is much more difficult to do in reality than in a schematic. Getting THD below 5% requires precision resistors, hand matched diodes, and thermal tracking, things that none of those zillion schematics mention.

    ak
     
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  9. hevans1944

    hevans1944 Hop - AC8NS

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    The ICL8038 is an obsolete part, no longer manufactured (at least by Intersil... there may be Asian clones) with no recommended replacements. See this datasheet.

    I hesitated to recommend diode break-point generators because the "corners" cause considerable harmonic distortion, even with LOTS of diodes and precision resistors, etc. However, I used this method in the 1970s to generate audio tones over about two octaves of an equally tempered music scale. The result was pleasant enough to my ears, but would be probably be unacceptable to a trained musician. All depends on what the OP wants to do. The XR2206 does appear to be a viable substitute for the ICL8038, although I have never used one. Perhaps someone else here has experience with them? Feel free to chime in with comments.
     
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