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A bit of help with a crystal oscillator

As I have shown in other posts, a Schmitt has no stable state and must oscillate. The trick is to channel the energy through the crystal instead of around it, as in a plain RC oscillator.

Time domain oscillators start on numerical instability. There are numerous examples in Oscillators.zip where the oscillation starts at zero amplitude and builds up to strength.

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It is extremely difficult to couple a pertubation into the oscillator tank to kick start an oscillator. The tank simply ignores the energy in an external pulse. You have to somehow inject the pulse in series with the tank but I have not found a practical way to do that. A simpler method may be to simply increase the loop gain by increasing the drive level.

Reply to
Steve Wilson
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A current source pulse usually works. Or series, a voltage pulse. Either disappears after the pulse.

I've simulated oscillators where every node parked at exactly 0 volts forever. More loop gain wouldn't help. A tiny injected pulse, or some initial condition setting, got them going.

You can make pretty good time-domain noise generators too.

XOs are hard to sim in time domain because the Q is so high. Sims can run for days. I like to force the initial conditions near the expected operating values, and zoom up to see where it wants to go.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
jlarkin

I have tried both. I found them ineffective.

I'd like to see some of your syms.

I show how to start and settle high Q XO's in several cycles. I call the technique "Fast Start". I show how to use it Oscillators.zip

Reply to
Steve Wilson

Use a bigger one! Spice is cool because you can make a thousand-amp pulse.

I'll see if I can find one that parks at zero.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
jlarkin

That would be great. Thanks.

Reply to
Steve Wilson

We got the 25MHz oscillator running fine, and easy enough to probe with the 0.9pF probe, but the 32768Hz one is very delicate.

I realised why any probing kills it: the probe is 1Mohm and the feedback resistor (inside the CPU) is 18Mohms. Clearly one needs to do any probing via a capacitor, of say 10pF.

LM wrote

Reply to
Peter

But you can pay $1000 for a good one.

And you can get into trouble in production, if 20% of them don't oscillate.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

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