Hi, Yawei,
ywguo wrote on Jun 11th, 2007, 7:22pm:Do you mean that you run a DFT function in the calculator?
Yes.
ywguo wrote on Jun 11th, 2007, 7:22pm:I think that means the parameter strobeperiod improves the accuracy of DFT function.
The DFT is sensitive to the accuracy of the samples used - so you use strobeperiod to force the simulator to produce exactly the time points you need and therefore eliminating the interpolation error.
ywguo wrote on Jun 11th, 2007, 7:22pm:Now the FOUR (fourier integral) result is much larger than that of DFT function at high frequency. I want to know whether the FOUR result have large error due to intepolation error. Then I run a simulation with maxstep = 1ps. Now the simulation speed is unacceptable slow.
Spectre uses a quadratic numerical integration algorithm to calculate the Fourier integrals for the requested frequency points (cf.
Spectre User Guide). To get reasonable results for high frequencies, you will probably need small time steps. Unfortunately, Cadence tells you hardly anything about the algorithm (which is why I prefer the DFT method), but I suggest a minimum of 10 or 20 points per period (of your harmonic), i.e. maxstep = 0.05/f ... 0.1/f.
Did you try the suggestion of the
Spectre User Guide ("If you are concerned about accuracy, perform an additional Fourier transform on a pure sinusoid generated by an independent source")? If you want to check harmonics, you might want to use a triangle wave (with you fundamental frequency).
BOE