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iMo:
The both are the same, but you see different details.
See below.
A 500GHz square wave from my above example - upper is 100ps long transient analysis, lower is 5000ps long one.
No windowing, 260k samples.
The spectra are the same afaik.. :)
Simon:
So if I do FFT over a smaller period it will give a larger frequency sweep.
iMo:
Below the same with 100ps and 5000ps long waveform.
16.7mil points FFT, B-H window.
Again, the spectra are the "same", where the bottom one (5000ps long) has got much lower min frequency in the spectra, AND, it is much more detailed.

Fundamental is 500GHz, square-wave where the rise is 0.1ps and fall is 0.3ps.
Simon:
i am getting the reverse, 1s of data gets an FFT from 1Hz to 8MHz, but for 20ms it goes to about 300MHz from 50Hz. But below 1KHz there is no useful data and there is data well beyound the 100MHz.

Is this all predictable? or do I have to guess and play around when I want to look at specific frequencies?
iMo:

--- Quote from: Simon on May 06, 2019, 08:22:26 pm ---So if I do FFT over a smaller period it will give a larger frequency sweep.

--- End quote ---

With constant N FFT points a shorter transient analysis duration gets "smaller FFT sampling time steps" - therefore the max spectral freq is higher.

Again, the time domain waveform coming from transient analysis is "smooth" with none "time steps".
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