Crystal/Resonator Noob question

Answer #4 explains it:

It is 18pF according to the datasheet you pointed us to. ;-) Oddly enough though they don't bother stating whether the crystal is cut for parallel or series resonance.

It is the capacitance that the crystal expects the circuit to provide so that it runs on its stated frequency.

Look at Section 3 of the User Manual. You will hook it up as shown in figure 7b. I suggest two 33pF caps to start.

It'll usually be less than that. The OP needs to take into account other capacitances: Traces to ground and to each other, pins to ground, etc. For the LPC2106 the chip capacitances might be found in a family spec, not necessarily in the datasheet.

The notion of load capacitance is meaningless in the context of a series resonant crystal -- either you're operating it at series resonance, where it's impedance is purely resistive and minimal, or you're operating it at "parallel resonance" where the crystal is inductive and working against the load capacitance.

You can select the values of capacitors C1 and C2 to match the crystal C_L using: C_L = (C1 * C2)/(C1 + C2) + CS. Here,CS is the Stray Capacitance (Typically 2-5pF) and CL is the Circuit capacitance required by the crystal for operating at the desired frequency.

Take care while selecting values of C1 and C2. Large values will increase frequency stability but decrease the loop gain, And so, that may cause problems during oscillator startup. Take care w.r.t C1 and C2.

Lot of tutorials are available in the internet. And yout develpment environment manual will surely have clear explanations for this .

Karthik Balaguru

I am sorry, but which user manual are you referring to? All I could find is the datasheet and a few application notes.

Thank you,

I was thinking that Cstray wouldn't be all that high given the smd parts, and that a pair of 22pF might not always start reliably.

Thanks for the info Tim. :-)

I don't know what it is for the LPC because I pretty much don't use NXP parts anymore these days. But it has to be checked. Sometimes it is surprisingly high, on some uC families the burden caps can even be programmed.

Page 39.

I guess I should have looked at figure 8. Unless I'm missing something, they're speccing even more C than I suggested (2 38pF caps). Hmmm....by my calculations then Cstray is ~1pF. That would mean that the OP should use a pair of 34pF caps to get the extra 17pF he needs, I stand corrected. Good luck finding them. ;-)

Interesting. But somehow 38pF doesn't feel right for a 15MHz crystal.

If I change the crystal to have a load capacitance of 20pF as the manual says, I need a 38pF C1 and C2. It seems that these are difficult to find. Can I use a 39pF instead? What would be the consequences if I do?

Thank you,

Slight changes in capacitance will cause slight changes in frequency. However, note that your circuit layout also causes slight changes in capacitance. If you need higher precision than these variances allow, design in a small variable cap to tune the resonant frequency.

Also, the two caps don't *have* to be the same value. You can do some testing with different values to try to get closer to the desired frequency.

In fact, if the caps are exactly the same, oscillation may not occur. There needs to be a miniscule amount of difference in order to get the crystal going. It's the physics of the thing. Fortunately caps of the same value are never, ever precisely the same.

-- Ray

Really ?! You do realise this is electronics, not physics ?

Are you able to prove that in Spice ? (which will allow you to have identical Cap values)

Tip: Oscillation is all about phase. Try it yourself, use a trimmer.

-jg

And make sure the fung shui is ok. Might have to place some mirrors if not ...

Theoretically what I stated is true. If both caps are exactly (and I do mean exactly) the same value, there will be no oscillation. The first oscillation requires an imbalance. Spice takes that into account and simply allows oscillation to occur so forget proving it in a Spice model. You do have to go back to basic physics to get to the truth. But, it's a minor point. I mean that. It's not worth arguing about because there can never, ever be two capacitors with the exact same value. Period.

-- Ray