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Tri-level composite signal from microcontroller

Discussion in 'Microcontrollers, Programming and IoT' started by epicedium, Jun 29, 2017.

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  1. epicedium

    epicedium

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    Jun 29, 2017
    What is the easiest way to generate this pulse from a microcontroller running at 3v3? I have IO pins and PWMs available.

    https://en.wikipedia.org/wiki/Tri-level_sync

    I haven't dealt with negative voltages before, and I'm not that familiar with analogue electronics (other than simple opamp circuits). My first instinct is to generate -3v3 (somehow?) then use 2 or 3 microcontroller pins to switch 2/3 mosfets or transistors with resistors dividing voltage, but I do wonder if there's a better/ simpler way.

    It doesn't seem possible/appropriate to create a virtual ground unfortunately, as the composite signals go out over BNC using chassis ground.
     
  2. duke37

    duke37

    5,364
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    Jan 9, 2011
    The wikipedia article says there is no DC, perhaps a capacitor in the line would be sufficient.
    There are integrated circuits which can give a low current negative supply, I have never used one.
     
  3. epicedium

    epicedium

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    Jun 29, 2017
  4. Alec_t

    Alec_t

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    Jul 7, 2015
    The opamp would need a dual-polarity supply if you want it to put out a negative-going pulse.
     
  5. Alec_t

    Alec_t

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    Jul 7, 2015
    Duke suggested a cap. Here's a sim. Good enough for your needs?
    3-level.PNG
     
  6. epicedium

    epicedium

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    Jun 29, 2017
    Alec_t, Duke- the cap, it may well be acceptable, yes! I'll simulate it into a 75Ohm load and experiment- my experience with AC is pretty minimal so I don't really grasp how it works. I assume that I could clamp the spike with diodes and that the magnitude and duration of the spike are a product of the capacitance and load. I'll report back once I've had a play around. Thanks!
     
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