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Dec 5^th, 2024, 6:06pm

The Designer's Guide Community Forum › Simulators › Circuit Simulators › Cystal osc which satisfies start up condition cannot sustain oscillation

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Cystal osc which satisfies start up condition cannot sustain oscillation (Read 1168 times)

unaffected

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Cystal osc which satisfies start up condition cannot sustain oscillation
Dec 30^th, 2021, 12:58am

I designed a pierce oscillator oscillating at 25MHz, the figure attached is the stb analysis result, the loop gain is 20dB at 0deg.

I tried everything I can do to start the oscillator. I set the maxstep to 10ps or 100ps, set initial condition of Lm to 1uA or 10uA， errpreset=conservative, integration method is trap or traponly. tighten reltol, vabstol, iabstol, using transisent analysis or hb analysis, at t=0+ the oscillations starts, but then its amplitude becomes smaller and smaller, and the oscillation stops.

My question is:
1. From the loop gain and phase plot in the attached figure, I think this oscillator can oscillate at about 25MHz. Am I right?
2. Why this cystal oscillator cannot sustain oscillation? Is there anything I can do to simulate it correctly?

Thank you very much.

stb_analysis.png

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Ken Kundert Global Moderator Online Posts: 2386 Silicon Valley	Re: Cystal osc which satisfies start up condition cannot sustain oscillation Reply #1 - Dec 30^th, 2021, 10:59pm Seems like you are doing the right things from a simulation perspective. It might be helpful to have the schematic and the transient waveforms.
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unaffected Junior Member Offline Posts: 15	Re: Cystal osc which satisfies start up condition cannot sustain oscillation Reply #2 - Dec 30^th, 2021, 11:23pm Thank you very much. I attached the schematic of the oscillator
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unaffected Junior Member Offline Posts: 15	Re: Cystal osc which satisfies start up condition cannot sustain oscillation Reply #3 - Dec 30^th, 2021, 11:30pm I set reltol = 1e-6, vabstol = 1e-8, iabstol =1e-14, maxstep=10ps, set initial condion of Lm (inductor of crystal) to 10uA, reran the simulation. please look at the attached transient results(VDD=3.3V).
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Ken Kundert Global Moderator Online Posts: 2386 Silicon Valley	Re: Cystal osc which satisfies start up condition cannot sustain oscillation Reply #4 - Jan 4^th, 2022, 9:40pm What happens if you keep on simulating? Does the amplitude go to zero, or does it settle to a stable level? -Kenk
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unaffected Junior Member Offline Posts: 15	Re: Cystal osc which satisfies start up condition cannot sustain oscillation Reply #5 - Jan 4^th, 2022, 10:51pm The attached file is the transient result of 1ms-2ms, at 2ms the peak to peak amplitude of X2 is 127.6mV.
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unaffected Junior Member Offline Posts: 15	Re: Cystal osc which satisfies start up condition cannot sustain oscillation Reply #6 - Jan 6^th, 2022, 5:21pm The attached file is the transient result of 2ms-3ms,
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unaffected Junior Member Offline Posts: 15	Re: Cystal osc which satisfies start up condition cannot sustain oscillation Reply #7 - Jan 22^nd, 2022, 5:27am The data sheet of crystal says the CL=9pF, in former simulation, I add 6pF capacitor to each crystal node(CL=3pF) , �and the cystal osc can not sustain oscilation. After I changed the �cap at eash cystal node to 17pF, the cystal oscillate oscillate �steadily.
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unaffected Junior Member Offline Posts: 15	Re: Cystal osc which satisfies start up condition cannot sustain oscillation Reply #8 - Jan 22^nd, 2022, 5:29am This is the stb plot of cystal osc with 6pF capacitor at each node
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unaffected Junior Member Offline Posts: 15	Re: Cystal osc which satisfies start up condition cannot sustain oscillation Reply #9 - Jan 22^nd, 2022, 5:32am This is the stb plot of cystal osc with 17pF capacitor at each node
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unaffected

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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #10 - Jan 22^nd, 2022, 5:47am

1. In the stb plot of crystal osc with small load cap, the lowest phase is -15 deg.
2. In the stb plot of crystal osc with 17pf cap at eash crystal node, the lowest phase is -53 deg.

As far as I'm concerned，in 1, the lowest phase -15 deg do not provide enough margin，perhaps this is the reason the crystal osc can not sustain oscillation.

I am not sure if my explanation is correct, any comments and suggestion are always welcome.

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Ken Kundert

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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #11 - Jan 26^th, 2022, 4:55pm

It is an interesting question. I don't know what is going on. Presumably you are running the stability analysis about the DC operating point. I noticed that in the first case you have only 15 degrees of surplus phase shift. It is conceivable that as the signal grows the you lose all of your surplus phase shift and so the Barkhausen criteria cannot be satisfied because the phase shift around the loop cannot be 0 degrees. In your second simulation you have more than 50 degrees of surplus phase shift. Presumably in this case that extra room is sufficient to allow you to reach 0 degrees of phase shift around the loop even with the variation in the effective loop gain as you transiton to the large signal operating point.

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unaffected

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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #12 - Feb 22^nd, 2022, 12:19am

Thank you. I am running the stability analysis about the DC operating point

Recently, I drew the nyquist diagram of crystal osc with small load cap, nyquist criterion shows this osc can not oscillate.
I also drew the nyquist diagram of crystal osc with 17pf cap at eash crystal node, nyquist criterion shows this osc can oscillate.

The above results are in accordance with the simulation results.

Backhausen criterion is necessary not sufficient condition for oscillation, for some oscillators, nyquist criterion must be used to check the stability.

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Ken Kundert

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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #13 - Feb 22^nd, 2022, 10:57pm

The stb analysis performs a Nyquist analysis. A Nyquist analysis is very tricky to perform on your own because of loading issues. If you attempted do a Nyquist analysis without an stb analysis, the results are probably incorrect. The stb analysis was created specifically to overcome the near impossibility of doing an accurate Nyquist analysis on non-ideal circuits.

Furthermore, the Barkhausen criterion and the Nyquist criterion are equivalent. The only difference is that when we want the circuit to oscillate we reference the Barkhausen criterion and when we don't want the circuit to oscillate we reference the Nyquist criterion, but they are both the same criterion.

-Ken

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