Crystal/Resonator Noob question

Hmmm, A hypothesis that can't be proven ? You have theorised a notch of 'infinite' Q, and a width narrower than spice can model ?. So, it's unobservable ?

Sorry, no:

Oscillation requires not an imbalance, but a gain above unity, and that gain includes a phase shift component.

So, you can find a small Series OP R helps lower the power of the Osc, as well as non equal caps can also lower the Osc Power.

In the presence of a crystal, all that the CAPs do, is determine the voltage ratio on the OP and IP nodes, and if you lower the OP C, you raise the IP voltage swing. Those nodes are out of phase, so equal caps are immaterial. Try it on the bench, and feel free to use a trimmer.

Or use spice - properly set up, you can model 'Hi Q crystal' oscillators and show the startup.

-jg

On Mon, 19 Nov 2007 19:53:41 +1300, I said, "Pick a card, any card" and Jim Granville instead replied:

Sorry, yes.

Read your own sentence and see that you just agreed with me.

Show me any two capacitors that are exactly equal and I'll eat them both. I mean equal to an infinite number of decimal places not just the ink printed on them. Get busy. We'll both have to live another few millennia to get two that come to within 500 places.

You know those oscillator cans? Inside those are capacitors that come as close to being exactly equal as any on earth. They're grown on the substrate normally. Now, in testing, if they don't begin to oscillate, one of the capacitors is scratched to change the value.

Take a crystal and make it oscillate without caps.

No, you can't. Spice takes into account that capacitors have unequal value even if they are numbered alike. It's the nature of imperfection in manufacture. Are you of the opinion that a simple ink marking makes the values the same because the ink states they are the same? Please tell me you know better.

-- Ray

Ah, so you think Equal Caps somehow nulls the gain ? :)

How unequal do you think they need to be, to magically recover the gain ? Got a formula for that ?

I'm not sure what you are claiming - but I have used spice for just that. Have you ?

If what you claimed were true, designers would avoid equal caps, because thermal and age drifts would mean field failures, whenever the caps hit your magic value. Same with VCOs, with trimmer caps : Oh NO!! Ci = Co = Drop dead time !.

You have confused equality with a null.

-jg

On Mon, 19 Nov 2007 20:39:43 +1300, I said, "Pick a card, any card" and Jim Granville instead replied:

How do you suggest that? Unequal is necessary to start oscillation. Once it's going, it keeps going as long as power is applied and no components fail. Remember, this is a theoretical model. Unequal capacitance is needed to start the oscillation. Period.

Baloney. There's no possible way to do it. You rely too much on the tool. Think theory of oscillation. Forget Spice. It has flaws in it to make it work. If you really did use a formula for oscillation with exactly equal capacitance it would fail. That's why no formulae out there use it. Instead they rely on crystal cut to determine frequency. We're discussing oscillation here. What it is that makes the crystal resonate to start. Not what it is that is required in a circuit. Look, the power is applied and one cap MUST charge up faster than the other. That's an absolute for making the crystal resonate. There's no choice or no Spice model to show you that. It really is down to the physics of it all.

The printing on the caps doesn't make them equal. Just because they both read 38pf or 100uf doesn't make the exactly equal. There are enough differences to allow them to be used without concern to create an oscillator.

-- Ray

I don't accept your premise that "oscillation requires an imbalance". But even if true, the two pins of a microcontroller crystal oscillator circuit are usually connected internally to the input and output of an amplifier. So one pin is at very high impedance, and one at very low. Does this not constitute an enormous "imbalance"?

Oscillation can be started by the amplification of thermal noise (in the absence of other disturbances). You could think of it as a very high gain tuned circuit - noise at the tuned frequency is amplified. No "imbalance" required.

On Mon, 19 Nov 2007 09:30:58 +0000, I said, "Pick a card, any card" and John Devereux instead replied:

You don't have to "accept it." Just ignore it. Because there can never, ever be two capacitors of exactly equal value you needn't worry your pretty head about it. See how that works?

Yes, but to start it on power up in a circuit does require a different set of circumstances. You're moving into irrelevance.

-- Ray

and you believe Cap matching is all that determines that ?

So I'll take that as a no, you have not used Spice to model crystal oscillators amd startup.

Still waiting on just how precisely matched the caps must be, for this effect to show, and the formula.

I'm thinking here I should have taken Joerg's offer of feng shui! :)

-jg

On Mon, 19 Nov 2007 23:00:13 +1300, I said, "Pick a card, any card" and Jim Granville instead replied:

I have other things to do. Believe what you wish.

-- Ray

I see you have no answer.

Not at all irrelevant, this is exactly how many crystal circuits start up. You can easily see it on a scope for microcontrollers that use watch crystal oscillators (32768Hz). It can take several hundred milliseconds for the oscillations to build up to full amplitude. For the first few tens of milliseconds the oscillations are undetectable since they are buried in thermal (and other) noise. The oscillations grow exponentially until they reach circuit-limited normal operating amplitude. The exact time taken to complete this process can vary each time the circuit starts - it is noise that kicks things off.

This can lead to unreliable microcontroller startup if you are not careful. I got bitten in just this way recently with the ADUC7000 series, switching to the external crystal oscillator before it was ready.

You don't tend to notice this so much with MHz range crystals since everything happens so much faster.

I'm more than happy with my understanding. I see you have not been able to reply to any of my questions, so noob's reading this can makeup their own minds. I was more worried about their (mis)understanding than yours.

Tip for Noobs: Good engineers always work with numbers.

-jg

On Mon, 19 Nov 2007 10:26:03 +0000, I said, "Pick a card, any card" and John Devereux instead replied:

We are clearly not discussing the same thing. Believe what you wish. I have other things to do.

-- Ray

Exactamente. Noise is what starts an oscillation. No noise = no oscillation, regardless of capacitors or whatever. That is why younger EEs sometimes bang their heads on the table when their fancy new circuit doesn't work in SPICE.

Ray: The typical connection for a crystal or other frequency determining component in the case of a uC is from an output to an input, often called XOUT and XIN. Then (ideally) the crystal is the only feedback path between them and the capacitors only matter with respect to frequency and stability of oscillation. You can test it: Make one of the caps variable via a really fine vernier, for example using an old UHF tuner trim-cap in parallel with a cap that's a pF smaller than the fixed cap on the other side. Now tune it slowly. The oscillation won't stop.

And really good ones work with numbers and soldering irons :-)

Seriously, it is quite possible that an oscillator does not start up in SPICE or a noiseless mathematical environment.

On Mon, 19 Nov 2007 23:33:26 +1300, I said, "Pick a card, any card" and Jim Granville instead replied:

Your questions don't relate to what I was discussing. They relate only to some unrelated point you were making.

-- Ray

Yes, I nearly added that. You certainly should reality-check your spice results, against the real world. Seen some clangers out there, from the '..but it worked in spice?!' novices....

True, but in practise it's easy to avoid. With the very hi Q's of crystals, you can get good results from spice with care and patience. You do need to watch the step size, and being able to tell a flakey run resulting from wrong settings, helps as well :)

So I should add :

Tip for Noobs: Craftsmen know their tools, and what they can, and cannot do.

Or "But I've clearly demonstrated in SPICE that this doesn't work!". One of those was a PIN diode circuit of the esoteric kind which then rolled off the fab at 30k/year for a few years. They all worked :-)

Nowadays you can be pretty liberal with step size, reltol and all that. Back in the DOS days that was different. It could mean that the sims ran all night.

And make some of your own tools, too. Like near field sniffers, FET probes etc. Just think about it: A few hundred years ago many folks had to make all their tools because the next Home Depot was, well, it didn't exist.

... snip ...

That showed up in a crossword the other day, and I still have no idea what it is or means. Kindly provide a clue.

feng = wind shui = water

It's all about the qi.

Best regards, Spehro Pefhany

-jg