Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Crystal oscillator load caps

  1. Nov 5, 2011 #1
    Why does a crystal oscillator need load capacitors?
    Is it because there will be some capacitive load across the xtal pins?
    Last edited by a moderator: Nov 6, 2011
  2. jcsd
  3. Nov 5, 2011 #2


    User Avatar
    Science Advisor
    Gold Member

    Here's the equivalent circuit of a crystal.


    http://en.wikipedia.org/wiki/Crystal_oscillator" [Broken]
    Last edited by a moderator: May 5, 2017
  4. Nov 8, 2011 #3
    Why should the electronic circuit provide a total parallel capacitance as specified by the manufacturer?
    I'm assuming this is referring to the load capacitors.
    Last edited by a moderator: May 5, 2017
  5. Nov 8, 2011 #4


    User Avatar
    Science Advisor

    Load capacitance of a crystal in an oscillator affects the frequency of the oscillator.

    So, the maker may say that with a load of 30 pF the crystal will be within 50 parts per million of the frequency marked on the can. This sounds pretty good, but this is actually an error of 500 Hz at 10 MHz. This would be unacceptable for many applications. It is equivalent to an error of 4 seconds a day if it was controlling a clock.

    So, if you have suitable calibrating equipment, you can adjust the frequency to some extent to get better accuracy. You may be able to get closer by adjusting the capacitors across the crystal.

    There are limits to this, though, as the capacitors have to be there for the oscillator circuit to work and the oscillator will stop working if these capacitors are not in the right ratio to each other..
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook