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Visjnoe Senior Member Offline Posts: 233	high-Q crystal oscillator simulation time Dec 03^rd, 2006, 6:20am Hey, it is generally stated that high-Q (crystal) oscillators take a long time to simulate due to their high Q. Could anyone please explain this from the simulator's (SPICE) internal working/algorithms? Kind Regards Peter
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makelo

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Hillsboro, OR

Re: high-Q crystal oscillator simulation time
Reply #1 - Dec 11^th, 2006, 7:41am

The lumped resonant model for the crystal increases the size of the inductor and decreases the size of the capacitor as the Q increases. It is typical to have an inductor with several milliHenries with a fempto-farad capacitor. This creates some very slow time constants before achieving steady state.
One strategy is to adjust your crystal model (smaller L, larger C) to a lower Q ~100 for initial simulations and raise it back later.

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simon2

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Southampton, United Kingdom.

Re: high-Q crystal oscillator simulation time
Reply #2 - Sep 20^th, 2007, 5:03pm

Hi Visjnoe,
it actually has nothing to do with the simulator, but everything to do with the real-world circuit (which the simulator should accurately reflect): in simple terms, the build up of the envelope is directly related to the damping (and hence "Q") of the circuit - if it is a "high Q" circuit it takes a long time ....

The converse of this is that by examining two points of the oscillation's envelope before limiting, you can directly calculate "loaded Q" for that oscillator.

Cheers,
SimonH.

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Simon.Harpham@ieee.org
http://www.SiliconDevices.com

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sheldon

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Re: high-Q crystal oscillator simulation time
Reply #3 - Sep 21^st, 2007, 5:58am

Peter,

Simulating a crystal oscillator can take a long time to simulate in the
time domain. As Simon pointed out due to the high Q, it can take a long
time to reach steady state. However, frequency domain simulation of
crystal oscillators is very fast and this is how they are generally simulated.

Best Regards,

Sheldon

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