Crystal oscillator load caps

  • #1
634
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Main Question or Discussion Point

Why does a crystal oscillator need load capacitors?
Is it because there will be some capacitive load across the xtal pins?
 
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Answers and Replies

  • #2
dlgoff
Science Advisor
Gold Member
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Here's the equivalent circuit of a crystal.

200px-Crystal_oscillator.svg.png


A quartz crystal provides both series and parallel resonance. The series resonance is a few kilohertz lower than the parallel one. Crystals below 30 MHz are generally operated between series and parallel resonance, which means that the crystal appears as an inductive reactance in operation. Any additional circuit capacitance will thus pull the frequency down. For a parallel resonance crystal to operate at its specified frequency, the electronic circuit has to provide a total parallel capacitance as specified by the crystal manufacturer.
http://en.wikipedia.org/wiki/Crystal_oscillator" [Broken]
 
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  • #3
634
1
Here's the equivalent circuit of a crystal.

200px-Crystal_oscillator.svg.png




http://en.wikipedia.org/wiki/Crystal_oscillator" [Broken]
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.
 
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  • #4
vk6kro
Science Advisor
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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..
 

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