Electronic – choosing crystal when tracks are very close together


This is a circuit I am having trouble with. I made 20 units with the exact same setup.

Three units with this setup managed to work with 33pF ceramic caps with 20% tolerance and a crystal from Ebay.

The remaining units are failing yet nothing is smoking or showing discoloration. I even replaced the 20% ceramics with 5% npo caps and still nothing.

I think now the lucky units were working because the crystal load capacitance and ESR happened to be spot-on.

What I want to try to do since people here consider ebay as a flea market is buy crystals from digikey. But there are two types I could possibly choose from.

One with 18pF load capacitance and one with 20pF load capacitance. The ideal frequency I'm looking for is 22.1184Mhz.

When I look at my tracks between the crystal leads they're probably too close together and I should probably increase the parasitic capacitor value when calculating my crystal load capacitance value.

One source on the internet claims this math for crystal load:

 CL = C/2 + StrayC

Because the caps I will use are 5% 33pF, its actual value would be between 31.35pF and 34.65pF

Now diving those capacitor numbers by two, we get 15.675pF and 17.325pF respectively.

People say the normal stray capacitance value is between 2 and 5pF but isn't mine more due to tight track spacing? I don't understand. So if it was 2pF then values would be:

  17.675pF and 19.325pF 

Which means an 18pF load capacitance crystal is my best bet, but my stray capacitance is probably higher since I'm using a socket for my micro controller.

If its 5pf stray, then values would be:

20.675pF and 22.325pF

Which means a 20pF load capacitance crystal is my best bet.

Somehow I think the 20pF load crystal is better for my situation but I could be wrong.

Am I best to choose a crystal with 20pF load capacitance or 18pF load capacitance? and yes I will still use a standard IC socket for the micro (AT89S52) that uses this crystal.


Best Answer

If you follow the recommended oscillator circuit, a good quality active crystal should work with a load capacitance that's off a bit. The problem isn't that it won't oscillate at all, the problem is that it'll oscillate at the wrong frequency.

Looking at your layout your oscillation frequency may be extra sensitive to temperature variations because FR-4 is a crappy dielectric, but it should work.