What is the difference between the .four and FFT anlysis

Hi,
I am doing the THD analysis for my circuit with HSPICE. But I found the .four and FFT in HSPICE both can perforn this functiion. I am wondering what's is the difference between them and which one I should select to perform my THD analysis.

Thanks

it's the same difference between fourier analysis and FFT. It will not affect the THD results.

one is fourier transfor(.four)
and FFT=Fast Fourier Transform
so ,the result bwill be same..its just that the Alog. used to carry out fourier transform is different from FFT !

FFT is for sampled signals , Four is for continuous signals

I don't think ANYONE computes fourier transforms without the Fast Fourier Transform (FFT) algorithm!

I don't have HSPICE docs in front of me, but if I remember well in Spice2 the command is .FOUR , and it'll take the transient analysis points at whatever time steps that the simulator computed, which may not be equally spaced.

The problem is that they must be equally spaced for the FFT to work correctly.

i think that in Spice3 it'll compute interpolations, so that the points it uses for FFT are equally spaced. This helps, but it worsens the resolution of the FFT.

Hspice (and Spectre, or Eldo) have a specific version of the .FOUR analysis that forces the transient simulation engine to compute the solution at every time point that the FFT engine needs it, so you'll have top notch results without worrying about setting the max time steps. It's better to use this than to post-process the transient simulation results, in my opinion.

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