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4046 VCO

Discussion in 'Electronic Basics' started by Andrew Holme, Mar 22, 2008.

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  1. Andrew Holme

    Andrew Holme Guest

    Is there an easy way (e.g. diode and resistor) to change the duty
    cycle of the CD4046 VCO? I would try on breadboard; but I'm away from
    home. The datasheet says 50% duty; but I want to make it variable;
    and I'd rather not use an auxilliary monostable.

  2. Jamie

    Jamie Guest

    Not really,.
    The best you can hope for is to simply lock with the signal coming
    in to generate an out put ref.
    You could how ever, use that output ref to drive a simple PWM via
    a dual op-amp that is composed of a triangle wave form, driving a
    voltage comparator input while your ref is driving the other input
    with some tailoring of components to scale it of course. This can
    yield a 0..100% duty cycle..
    You could get tempted to use the OSC from the PLL as the PWM ref
    how ever, this can cause a problem in the PWM filters unless the
    PLL shift is very little of course.

    Not knowing your full intend of the circuit makes it very hard to
    visualize your ideas."
  3. I don't see the original question, so I will answer the O.P.
    here. Yes, there is an easy and predictable way to change
    the output duty cycle (if you are willing to swap out parts).

    Since each half cycle charges the timing capacitor with
    current supplied to its opposite end, while grounding the
    other one, you can replace the timing capacitor with two
    capacitors of different values, each connected between one
    of the timing pins and ground. Now, this is changeable,
    rather than variable, but if you use variable capacitors or
    a butterfly tuning capacitor (center movable grounded plate
    that interacts with two isolated sets of plates, one
    increasing capacitance and one decreasing capacitance as you
    turn the shaft, you would have a variable duty cycle. It
    could also be done with a pair of varactor diodes in the
    proper circuit, but I wouldn't call it simple.
  4. Tim Wescott

    Tim Wescott Guest

    That would imply that you could also modulate the VCO command voltage at
    the VCO output frequency -- with an analog multiplexer you could (maybe)
    select the + and - times independently.

    By the time you're done you may have even managed to use less than 10x
    the board space of a 555.


    Tim Wescott
    Wescott Design Services

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at
  5. Andrew Holme

    Andrew Holme Guest

    I'm having to post from Google Groups because I'm away from home, so I
    don't know if you'll see this or not; but thanks to all for the
    replies so far. I was wondering about a diode and pot in series
    between pins 4 and 12 i.e. to shorten the VCO Out low period by a
    variable amount; but I really want independent control of centre
    frequency and duty cycle, which that is not.
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