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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

    Hi,

    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?

    Regards,
    Mark
     
  2. Fred Bloggs

    Fred Bloggs Guest

    ....to 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
    schematic?
     
  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.

    http://www.dvanhorn.org/Micros/All/Crystals.php
     
  6. Dave VanHorn

    Dave VanHorn Guest

    ...to 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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