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Jan 23rd, 2022, 10:46am
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Cystal osc which satisfies start up condition cannot sustain oscillation (Read 178 times)
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Cystal osc which satisfies start up condition cannot sustain oscillation
Dec 30th, 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.
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stb_analysis.png
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Ken Kundert
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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #1 - Dec 30th, 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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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #2 - Dec 30th, 2021, 11:23pm
 
Thank you very much.
I attached the schematic of the oscillator
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cystal_osc.jpg
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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #3 - Dec 30th, 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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« Last Edit: Dec 31st, 2021, 6:34pm by unaffected »  

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Ken Kundert
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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #4 - Jan 4th, 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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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #5 - Jan 4th, 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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transient_1m2m.png
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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #6 - Jan 6th, 2022, 5:21pm
 
The attached file is the transient result of 2ms-3ms,
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transient_2m3m.png
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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #7 - Yesterday at 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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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #8 - Yesterday at 5:29am
 
This is the stb plot of cystal osc with 6pF capacitor at each node
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small_CL.jpg
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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #9 - Yesterday at 5:32am
 
This is the stb plot of cystal osc with 17pF capacitor at each node
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17pF_cap_at_each_node.jpg
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Re: Cystal osc which satisfies start up condition cannot sustain oscillation
Reply #10 - Yesterday at 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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