Help fix my soundcard! What values for crystal's load capacitors?

[cuibono]

I 'improved' my soundcard one too many times, and now it doesn't work.

The soundcard is a Delta 1010lt, and uses a Via Envy chip (vt1712). I was messing around with different crystal oscillators, and now I want to go back to how things originally were. I removed both crystals, which I still have, but I destroyed the four load capacitors associated with them. So I need to figure out what those capacitors were. I've included a picture with the capacitors circled.

The crystal's values are 24.576MHz and 22.5792MHz, and are made by NSK. Typical values for Cload are 18pF (but can be 20pF, and actually over a wide range). I looked as NSK's website. I looked over the Via datasheet. No indications. All I can do is guess values for the crystals.

Also correct me if I'm wrong, but with a Cload rated as 18pF, this would mean using two capacitors of 36pF. I'll probably order a number of caps of various values, and just plug them in.

I could also buy new crystals, but the 22.5792 is seemingly difficult to find.

Any help?

[dmills]

Stick a pair of 22pf ceramic disks in there, it will be close enough (got to allow for a little trace capacitance anyway).

Regards, Dan.

[cuibono]

Thanks for your input! Did you get that value from using the following formula? Wouldn't 27pF be closer?

"4. The load capacitance (CL) of my parallel resonant crystal is rated at 20pF. How do I calculate the value of the load capacitors used in my parallel resonant oscillator circuit?

Use this formula to approximate the value of capacitors needed:

CL=((C1 x C2) / (C1 + C2)) + Cstray

Cstray is the stray capacitance in the circuit, typically 2-5pF. If the oscillation frequency is high, the capacitor values should be increased to lower the frequency. If the frequency is low, the capacitor values should be decreased, thus raising the oscillation frequency. When CL =20pF, C1 and C2 will be approximately 27-33pF each, depending on the amount of stray capacitance."

from FAQ Crystals

[kipman725]

as long as it's within about 1/2 an order of magnitude it dosen't matter. The absolute acuracy of the clock frequancy isn't that stringent (for comunication). 22pF is a normal value.

[BWRX]

Using 20pF caps should work fine. Don't use ceramic disk ones though. Use 0805/0603 size NP0 type surface mount ceramics, just like the kind that were originally there.