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crystal oscillator (Read 11989 times)
Anna
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crystal oscillator
Sep 05th, 2007, 12:58pm
 
Hello,
I try to find out very precise measurement(+/-1 ppm) of the output frequency of the crystal oscillator , I simulate the LC tank model of the Crystal with related oscillator ( both provided by TSMC)
But the dft result of the output frequency is 15ppm different from PSS.
which one is more accurate measurement ?
I simulate 20ms trans with option analog ( retol=10-6,vabstol=10-9,iabstol=10-13).    
Thanks
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vivkr
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Re: crystal oscillator
Reply #1 - Sep 6th, 2007, 11:06pm
 
Anna wrote on Sep 5th, 2007, 12:58pm:
Hello,
I try to find out very precise measurement(+/-1 ppm) of the output frequency of the crystal oscillator , I simulate the LC tank model of the Crystal with related oscillator ( both provided by TSMC)
But the dft result of the output frequency is 15ppm different from PSS.
which one is more accurate measurement ?
I simulate 20ms trans with option analog ( retol=10-6,vabstol=10-9,iabstol=10-13).    
Thanks


Hi Anna,

If I understand correctly, you are using a DFT to find the frequency of your oscillator. If you want an accuracy of 1 ppm in estimating this, you would need an enormous number of points to guarantee that your FFT bin width is smaller than the target resolution. Also, it would be necessary to guarantee that you take exactly an integer number of cycles (-1 FFT point) while doing FFT because you cannot use windowing if the accuracy must be that high. But to do this, you would need to know the frequency a priori : ) However, if you are measuring the frequency with an accuracy of 15 ppm, then you probably are already following most such guidelines to begin with.

If you are doing a DFT on the time domain simulation output, I would suggest instead to try and find the frequency directly from this output (also non trivial).

One way to simulate this correctly would be to make the simulator timestep < 1ppm of the timeperiod but this will slow down the simulation. I would also set the integration method to "traponly". However, I do not know an easy method for estimating the frequency to such a high level of precision.

Please let us know if you find a good method.

Regards
Vivek
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ywguo
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Re: crystal oscillator
Reply #2 - Oct 9th, 2007, 8:59pm
 
Hi Ana,

It's difficult to measure such precise freuquency/period in simulation. Furthermore, I think it is not necessary to measure precise frequency/period when simulating crystal oscillator. The precise frequency is guaranteed by the very high Q value of the crystal and the circuit design. If the circuit proves start oscillation in simulation, it is OK. Don't worry about the frequency accuracy.


Yawei
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Ken Kundert
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Re: crystal oscillator
Reply #3 - Oct 9th, 2007, 9:34pm
 
Crystal oscillators, being very high Q, take a very long time to settle. So with a normal transient, you may be looking at the results while still being far from steady-state. As such, I would tend to trust the PSS results more than the transient results.

You can measure the period of your signal very accurately using the "period measurement" module that you find on the verilog-ams page (http://www.designers-guide.org/VerilogAMS/).

-Ken
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Visjnoe
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Re: crystal oscillator
Reply #4 - Oct 10th, 2007, 8:14am
 
Dear Anna,

I faced the same problem some time ago. Don't try to find the exact oscillation frequency using transient analysis (for all the reasons stated above).

When you consider that the oscillation frequency equals the resonant frequency of your oscillator load (XTAL + (parasitic) capacitors/inductors), you should just model the LC-load correctly. Then you can resort to a straightforward AC analysis from which you extract the frequency at which resonance occurs.  This will be your oscillation frequency.

However, to ensure the ppm accuracy during AC analysis, keep your frequency range small (around your expected oscillation frequency) and use a (very) large amount of points (linear/logarithmic sweep) per decade.

I was able to simulate the oscillation frequency accurately within <5min AC runs, obtaining exact silicon correlation.


Regards

Peter
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Anna
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Re: crystal oscillator
Reply #5 - Oct 23rd, 2007, 4:25pm
 
Thank all of you for your help.
I measured the period by applying long transeint as intial condition for shorter second transeint which has maxstep=.1ps( which is 1ppm).

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David Lee
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Re: crystal oscillator
Reply #6 - Oct 31st, 2007, 1:06am
 
I would recommend using a simulator that supports harmonic balance.
Extra care is needed in a time-domain PSS analysis to yield accurate frequency for crystal oscillators.
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- David
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rf-design
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Re: crystal oscillator
Reply #7 - Dec 14th, 2007, 6:23am
 
I also have seen differences between TR and PSS for high Q circuits. It seems that for tough convergence setting for PSS the initial transient  time approaches the initial settling time of the normal transient simulation.

There is always a difference between AC and TR frequency because of the frequency shift effects of the nonlinear caps in the circuit. TR and PSS should give the right value. This discrepancy is also a good indication that your oscillation frequency is sensitive to amplitude variations.

To extract 1ppm frequency you do not need 1 million cycles. If the set the FFT you can extract the frequency of the oscillation by bilinear interpolation between two neighborhood FFT bins taking the simply the amplitude. The interpolation is accurate up to about the harmonic level. So with -30dB to can extract up to 3% bin size.
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hiSkill_11
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Re: crystal oscillator
Reply #8 - Mar 15th, 2016, 11:27pm
 
I am now in a very delusive case. I simulated startup behavior from zero state and found that it shown oscillation with 10% less frequency, and could you help me understand that? when I performed stability simulation, it shown correct oscillation frequency as crystal specified. I also wanted to pss simulation, but always failed convergence. There are zero diagnal at output log file, but it's very difficult for me to find the root cause. Attached is startup sim results, when I magnified it, it shown wrong freqency sin waves.


Ken Kundert wrote on Oct 9th, 2007, 9:34pm:
Crystal oscillators, being very high Q, take a very long time to settle. So with a normal transient, you may be looking at the results while still being far from steady-state. As such, I would tend to trust the PSS results more than the transient results.

You can measure the period of your signal very accurately using the "period measurement" module that you find on the verilog-ams page (http://www.designers-guide.org/VerilogAMS/).

-Ken

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