# How much to increase crystal load capacitance based on this inaccuracy?

The frequency of my 40MHz crystal is 10.6 ppm high, how much should I increase my load capacitance? I am using 12pF crystal with 15pF load caps. As I understand if caps need to be unbalanced the higher value goes to the output cap, but how many pF total increase needed do you think?

Schematic is here:

UPDATE:

I just added 1 pF at a time and here is the results. I didn't use a trimmer at all, I just replaced the capacitors one by one using hand soldering:

15pF input / 15pF output 4.0000415MHz

15pF input / 16pF output 4.0000288MHz - 0.0000127 drop

16pF input / 16pF output 4.0000183MHz - 0.0000105 drop

16pF input / 17pF output 39.999968MHZ - 0.0000215 drop

• Pullability of crystals will vary, but how did you calculate the 15pF? It should be more like 20 or 22pF. Oct 30, 2021 at 12:17
• Do you have some way to maintain the temperature of the crystal and oscillator constant? If not, temperature variation may swamp the accuracy you get by trimming the capacitance. Nov 9, 2021 at 21:40
• @SpehroPefhany I just used the standard formula of: CL = ((CX1 x CX2) / (CX1 + CX2)) + Cstray ... And most of the time people online would say that the usual stray capacitance is 4 or 5pF so I just figured 4.5 ... And simplified: CX1 = 2(CL - Cstray) ---> 15pF = 2(12pF - 4.5pF) ... Simplification I got from here: community.nxp.com/t5/LPC-FAQs/… Dec 1, 2021 at 23:30
• @ElliotAlderson I didn't use a trimmer at all, I just replaced the capacitors one by one using hand soldering in increments of 1pF as I noted in the update to my question. I didn't mention not using a trimmer though, that was probably confusing so I amended it, it’s good you mentioned that. Dec 2, 2021 at 3:37

Get hold of the trimmer capacitor like this one (just an example): -

Picture from here.

And, solder it in place - adjust until you have the right value of frequency and estimate how much capacitance you have added using the trimmer (or measure if you have the right equipment) and add fixed values of capacitance that replace the trimmer (or just live with the trimmer in place).

Or just buy a few small value capacitors and try them out by adding them to your circuit board.

I'm giving you this answer because, in order to calculate the required extra value requires you to have technical knowledge of the equivalent circuit of the crystal and, hardly any crystal manufacturer supplies that information. If you did have the data, then you still have to guess what tolerance your currently fitted loading capacitors have and, at the end of the day, you might as well do it by trial and error.

If you want the theory about how much you can lower the frequency of a 10 MHz crystal having approximately 63 mH effective inductance, try my site: -

If you go through that website page you will see that how much you can shift the frequency with extra capacitance is determined by the effective inductance of the crystal. Also be aware that too much loading capacitance may cause the crystal to dissipate too much power: -