Capacitance value for PIC crystal

If its a genuine AT cut crystal then I would have hoped for a quite good performance for just a small temperature change.

--
Mike Perkins 
Video Solutions Ltd 
www.videosolutions.ltd.uk

good

The ECS crystals I'm using are 100 PPM over -40 to +85 C, with initial tolerance of 30 PPM. Not bad for less than a dollar:

You can get them with as little as 10 PPM tolerance and stability, for about $3 each:

And these are impressive for about $0.40:

Paul

What is "a lot of difference?"

The crystal frequency as well as the parasitic capacitance of both board and micro will change with temperature. You'll do better with an external oscillator that's rated as being way accurate (the best ones are ovenized, which means they're big, expensive, and power hungry -- but then, for timing you pays your money and you gets what you paid for).

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

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Another consideration is the phase delay introduced by the gates internal t o the PIC. If these are running at say 10ns, that is 10/50 x 360=72o in a ddition to the 180o inversion. The Pierce allows for 90o from the crystal a nd points forward, so with the additional 72o, that leaves just 18o shift f rom the crystal. As the PIC gate Tpd moves around with temperature ( a minu scule amount), so does the phase shift across the crystal, and so does the loop frequency. If you model the crystal as series LC , all paralleled with load C, with assumed Q and look at phase versus delta-f/fo, that is chnage in phase as a function of ratio of frequency perturbation to resonant freq uency ( the most popular plot), that will give an idea of how the oscillato r loop frequency pulls with its phase shift. I think Co in that manufacturer's pulling equation is also called header ca pacitance, or the net capacitance between the metalization of the crystal a nd the conductive housing.

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Right , IIRC the frequency shift relative capacitance shift is desensitized by sqrt(Q)- not going to do a bunch of algebra right now.

On a sunny day (Fri, 7 Jun 2013 17:19:06 -0400) it happened "P E Schoen" wrote in :

I was actually referring to the temperature coefficient of the caps, not the crystal. And not sure how stable input capacitance of PIC is versus temperature. Try heating and lowering your setup 10 degrees and see if it still meets spec.

On a sunny day (Fri, 07 Jun 2013 20:59:27 +0100) it happened John Devereux wrote in :

standard by adding caps,

difference.

No no, the small disk ceramics of a few pF drift a lot.

standard by adding caps,

difference.

Oh, you mean those things with the wires, poking through holes in the board! Like they used in olden times?

--

John Devereux

On a sunny day (Sat, 08 Jun 2013 10:19:11 +0100) it happened John Devereux wrote in :

Rubidium standard by adding caps,

difference.

Yep, as I used here:

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is it possible to 'pull' those crystals 1kHz with only 8pF? probably not. What does LTspice show?

NP0, or C0G specced capacitors should have a capacitance virtually independent of temperature.

I would have assumed that drift as function of temperature would be more likely be down to the PIC itself, where the speed, gain and threshold of the semiconductor would be far more sensitive on temperature, and far outweigh any change in capacitance or crystal frequency.

--
Mike Perkins 
Video Solutions Ltd 
www.videosolutions.ltd.uk

On a sunny day (Sat, 08 Jun 2013 18:45:49 +0100) it happened Mike Perkins wrote in :

Yes, possible. I noticed the drift when soldering different caps to the combination crystal+PIC+caps:

Had to wait several minutes for it to stabilize. It is actually a really nice frequency counter that takes up very little space, but only calibrated at room temperature. IIRC have seen temperature drift in other applications with these sort of caps too.

Neat - it is cheating to measure its own frequency!!

doesn't seem to work?

--
Mike Perkins 
Video Solutions Ltd 
www.videosolutions.ltd.uk

On a sunny day (Sat, 08 Jun 2013 20:27:30 +0100) it happened Mike Perkins wrote in :

Interesting is why it does not measure 20.000... but one lower :-)

Yes, links change, this project is already a few years old. I do not always keep up with those changes... I think if you google dl4yhf (callsign) it will show up, lemme try:

Moved to qsl.net...

the PIC. If these are running at say 10ns, that is 10/50 x 360=72o in addition to the 180o inversion. The Pierce allows for 90o from the crystal and points forward, so with the additional 72o, that leaves just 18o shift from the crystal. As the PIC gate Tpd moves around with temperature ( a minuscule amount), so does the phase shift across the crystal, and so does the loop frequency. If you model the crystal as series LC , all paralleled with load C, with assumed Q and look at phase versus delta-f/fo, that is chnage in phase as a function of ratio of frequency perturbation to resonant frequency ( the most popular plot), that will give an idea of how the oscillator loop frequency pulls with its phase shift.

capacitance, or the net capacitance between the metalization of the crystal and the conductive housing.

They often claim C0 is due to the case, etc, but the source is irrelevant. The values are part of a model that seems to work fairly well regardless of exactly where it comes from.

I seem to recall that one of the two capacitances, the "motional" capacitance IIRC, is very small, typically in the fF range. So that might be C1. Assuming it is 27 pF is likely a major source of error. Some crystal makers give all the data needed to verify that you are designing the oscillator correctly, but many don't.

--

Rick

Just remember that the various frequency errors are additive including the aging spec which is often the worst one of the bunch considered over a few years.

--

Rick

standard by adding caps,

difference.

That's like saying, cars go really slow, you know, the ones with wheels on all four corners. Do you have any idea of the type of cap on the inside?

--

Rick

Good luck on that. I don't think there are any good LTspice models for crystals... at least not the ones I wanted to simulate. I think they just tell you to construct a cap model using the crystal parameters which most crystal makers don't provide. A model is only as good as the data.

I'd say a test is worth a lot more than a simulation in this case.

--

Rick

Roll your own. A combination of capacitance, inductance, and resistance. What else do you need?

If you have insufficient data - measure...

--
"For a successful technology, reality must take precedence  
over public relations, for nature cannot be fooled." 
                                       (Richard Feynman)