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| LTSpice pulse options |
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| aandrew:
--- Quote from: StillTrying on May 05, 2019, 04:25:33 pm ---I tried 1.5ps, I always use near prime or odd numbers, don't know if it makes any difference. --- End quote --- Why do you always use near prime (what's a near prime number?) or odd numbers? |
| iMo:
--- Quote from: Simon on May 06, 2019, 12:52:05 pm --- --- Quote from: SiliconWizard on May 06, 2019, 10:33:23 am ---I am looking at the FFT of RC filtered steps, i would think that 10 points of those curves is a good idea? Questin is to do a proper FFT how many cycles do i need? I am pretty sure that the resulting graph from 2 and 14 cycle was different. --- End quote --- --- End quote --- What do you mean by "point of a curve"? LTspice is not a DSO with a limited sampling rate.. The more periods of a signal you put into the FFT the better. The FFT can do up to 16mil "samples" of the "signal trace" (you have 3 choices what time range to include). |
| Simon:
Well the time step is the resolution of the simulation is it not? the RC charge/discharge is not a rise/fall ramp, it's a curve so you cannot just take a couple of points and draw a line between them. |
| iMo:
I think not. The FFT works such you set the number of "FFT points" (up to 16mil) and the TIME INTERVAL (3 choices you get). So the lowest freq in the spectra is given by the time interval (ie 100ns). The highest is related to the number of FFT points you selected. There is not such thing like "number of points in the signal trace". The trace you see is a "continual curve", and as such is taken into the FFT (with up to 16mil "sampling points"). It does not matter whether the time scale is fs, ps, ns - those are just "numbers" for LTspice. If you see a square wave 50/50 in the trace it takes "the smooth picture" of the trace, cuts it into up to 16mil "sampling points" and creates the spectra. Then it recalculates the freq axis. |
| Simon:
No, I am not talking about the FFT but the initial simulation. Getting the data that the FFT is to calculate from. |
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