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Why a small cap at oscillator output?

Discussion in 'Electronic Design' started by Mark, May 13, 2004.

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  1. Mark

    Mark Guest


    In some design I use a crystal oscillator. The oscillator works with
    two nands. The second nand is a buffer/driver. In some schematic you
    see a small capacitor on the output of the buffer (47pf/68pF). What is
    the function of this capacitor?

  2. Fred Bloggs

    Fred Bloggs Guest introduce Vcc spikes that keep the oscillator section going....
  3. Mark

    Mark Guest

    I mean that the capacitor is connected between the output of the
    buffer to ground. Is this to have slower rising/falling slope for EMC?
    Filter the high harmonics of a block signal?
  4. Well what does the stage it's driving consist of? Can you post a
  5. Dave VanHorn

    Dave VanHorn Guest

    Maybe this will help.
    The crystals are there to provide loading capacitance that isn't practical
    to include inside the crystal.
  6. Dave VanHorn

    Dave VanHorn Guest introduce Vcc spikes that keep the oscillator section going....

    Now that's funny.

    Like when a senior VP from sperry told me that flyback converters need
    "noise currents" to start up. He was trying to explain intermittent
    flameouts in a production design of a 5V-30V/10V converter. The problem
    turned out not to be "lack of suficient noise current", but a reverse
    polarized screw.

  7. From the description given, this isn't a loading cap. It's not even in
    the osc. stage. That's why I would have liked to have seen a circuit.
  8. Fred

    Fred Guest

    In a parallel resonant circuit using an inverting amplifier, the capacitors
    provide a central tap much in the same way a centre tapped coil provides
    anti-phase voltages on each end of the winding.

    The crystal is designed to resonate at the correct frequency with the design
    capacitive load.
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