Oscillator Load Capacitance

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Hi guys, I'm looking to get some 10MHz and 20MHz Oscillators form Digikey. However, when I do not understand what the Load Capacitance rating means, some are listed in pF and some are listed as series.

Also could anyone suggest a nice Oscillator for the ATTINY2313 and ATMEGA32?

I'm not a cartoon character but I play one in real life.

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Read Atmel's application note 42 about using crystals and ceramic resonators: http://www.atmel.com/dyn/resourc...

The main issue with the load capacitance of the crystal is that you need to have a compatible set of capacitors on the crystal to make that specified load capacitance. The application note tells you how to size those capacitors. For example, for DigiKey part 300-8487-ND, those 10MHz crystals have a specified load capacitance of 18pF. Assuming your parasitic capacitance in 5pF, then your capacitors would be 2 * (18pF - 5pF) = 26pF each.

Last Edited: Wed. May 30, 2007 - 03:12 AM
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kevinrosenberg wrote:
Read Atmel's application note 42 about using crystals and ceramic resonators: http://www.atmel.com/dyn/resourc...

The main issue with the load capacitance of the crystal is that you need to have a compatible set of capacitors on the crystal to make that specified load capacitance. The application note tells you how to size those capacitors. For example, for DigiKey part 300-8487-ND, those 10MHz crystals have a specified load capacitance of 18pF. Assuming your parasitic capacitance in 5pF, then your capacitors would be 2 * (18pF - 5pF) = 23pF each.

Thank you very much Kevin, thats exactly the info that I was looking for =).

I'm not a cartoon character but I play one in real life.

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There are two kinds of crystal oscillators. One operates at what is called the "series resonance" of the crystal. This resonance is the frequency at which the (AC) impedance between the pins of the crystal is almost zero. The frequency is independent of how much capacitance happens to be in parallel with the crystal. There will always be parallel capacitance - its inside the oscillator and part of the circuit board, etc. But, even though its there, it has almost no effect on the frequency that the oscillator runs at.

The other kind of oscillator oscillates at the parallel resonance of the crystal. At this frequency, the impedance from pin to pin of the crystal is almost infinite. This frequency depends on how much capacitance is connected in parallel with the crystal. This parallel capacitance is called "load capacitance". Your generic single-inverter oscillator is this kind of oscailator.

The common oscillator connection is for the crystal to be connected from the inverter output to the input. And, there is a capacitor at each end of the crystal to ground. The NET load capacitance is the SERIES eqivalent value of these two capacitors (that is how they appear to the crystal) PLUS stray capacitance from the circuit board and the guts of the oscillator. Suppose that the crystal is rated for 22pf load capcitance. The stray capacitance is about 7pf. So, that leave 15pf to be made up from discrete external caps. If the external caps are equal, then their equivalent is half their individual values. Thus, in this case, we would want a pair of 30pf caps.

It should be made clear that the same crystal exhibits BOTH series and parallel resonances. If a crystal is ground so that it oscillates at 8.000MHz in an oscillator that runs in series resonance, this crystal is called an "8.000MHz series resonant" crystal. But this same crystal, with maybe 22pf of parallel capacitance, might have a parallel resonance of 8.1MHz. Same crystal, different oscillator circuit. This SAME crystal could, if the manufacturer wanted, be sold as an 8.1MHz/22pf parallel resonance crystal. Note: I may be a bit off on the relationship between the series and parallel resonant frequencies but the principle is correct.

Most microprocessor oscillators run in parallel resonance mode.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Nicely explained, Jim!

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Hello,

Here is another link from 'Freaks' on crystal specifications. Colin added some nice links / crystal tutorials.

http://www.avrfreaks.net/index.p...

John

You may only be one person in the world but, you may be the world to one person! "Life! Life, do you hear me? Give my creation LIFE!" Gene Wilder SKYPE Name: JonRobrt