Crystal load capacitance?

FlyingFishFinger

Hi all-
I'm confused on what "load capacitance" means. Part of a circuit diagram I have is shown below. The two caps are labeled as being 22pF. I am confused as to whether I need to purchase a crystal that has a load capacitance of 22pF or a series crystal and add the caps on my own?
Thanks

Raf

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
You've got to play the game.
You can't win.
You can't break even, except on a very cold day.
It doesn't get that cold.
~Laws of Thermodynamics~

Mike Green

Any crystal is "cut" for a specific frequency under specified conditions. The main condition is the "load capacitance" which is the total capacitance across the crystal. The most common load capacitance that I've seen for most fundamental crystals these days is 22pF. In this case, you have two identical capacitors in series and the total is (22 + 22)/2 = 22pF. You will need to add the caps yourself unless the oscillator has them built-in. For example, the Propeller's oscillator has several (selectable) load capacitance values built-in and you don't need to add your own.

FlyingFishFinger

So if I purchase a Mouser crystal labeled as having a load capacitance of 22 pF will I be ok?

Raf

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
You've got to play the game.
You can't win.
You can't break even, except on a very cold day.
It doesn't get that cold.
~Laws of Thermodynamics~

Post Edited (FlyingFishFinger) : 11/18/2007 3:40:20 AM GMT

FlyingFishFinger

Wait, they don't have SMD crystals of the frequency I want with 22pF...so I guess I'll add them myself.
Thanks for the clarification anyway.

Raf

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
You've got to play the game.
You can't win.
You can't break even, except on a very cold day.
It doesn't get that cold.
~Laws of Thermodynamics~

Post Edited (FlyingFishFinger) : 11/18/2007 5:49:55 AM GMT