**Anna wrote on Sep 5**^{th}, 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