tuza2000,

Here is an example.

.tran 1ns 12.8us
.fft v(op,on) from=1ns to=12.8us NP=512 window=Kaiser-Bessel

where the sampling rate is 40MS/s, the period is 25ns. At least I figure out which harmonic is larger using that method.


Best regards,
Yawei

Tuza2000,

First , there  has been a detailed discussion of this issue in the thread,

http://www.designers-guide.org/Forum/?board=ms_design;action=display;num=1118555... 245;start=7#7

For a pipeline ADC, the methodology for simulating an ADC is identical to the
methodology a S/H. In the case of the S/H, the noise is not limited by the
quantization noise of the ADC, or the thermal/shot/... noise of the S/H. So the
FFT noise floor is only limited by the numerical noise of the simulation and the
FFT. The largest noise source in the FFT is usually the interpolation noise. Interpolation
noise occurs because FFTs have very strict requirements about how the signal is
sampled, 2^N equally spaced samples. Since SPICE-type simulators do not usually
stop at the correct time points without help, the FFT noise floor is often limited by the
error in estimating the actual value from the available time points. The easiest
solution is to set the maximum step size to be small to minimize the error. However,
this increases the simulation unacceptably so Spectre provides the strobe option.
It forces a time step at the instant required for the FFT. One other option to consider
is using a Spectre's zvcvs source as an ideal sample and hold. It has the same effect
as strobe and allows the use of Spectre's built-in Fourier integral calculation.

  Also, as mentioned in the thread above, the rectangular window is sufficient. Using
windowing functions introduces error. You might want to leave some additional time
in the simulation at the start.Usually there is some delay required to reach sinusiodal
steady-state. For example, common-mode feedback loops usually require time to
settle. So you might want to add an additional 100-200ns to the simulation.
                                                                           Best Regards,

                                                                              Sheldon

Hi,

Again, I found the answer myself after looking at Ken's book "The designer's guide to Spice and Spectre" carefully. On page, 240, in the paragraph right before it enumerates the useful characteristics of strobing, it says that in contrast to SPICE, Spectre actually places a time point at the sample time and interpolation will not be a used."