Electronics > Projects, Designs, and Technical Stuff
LTSpice pulse options
iMo:
--- Quote from: Simon on May 06, 2019, 08:34:57 pm ---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?
--- End quote ---
Sure it is predictable.
Let the transient analysis is "ta" seconds long.
Let the number of the FFT points is N.
Then you will get N/2 spectral lines and* the highest spectral line is
fmax = (N/2)/ta
and the lowest spectral line is
fmin = 1/ta
or something like that :)
PS: An example:
ta = 20ms
N = 16.7mil
fmax = 417MHz
fmin = 50Hz
ta = 1s
N = 16.7mil
fmax = 8.35MHz
fmin = 1Hz
* the spectra is again "smooth" with theoretical max N/2 spectral lines.
iMo:
And as always you may zoom in into the spectra:
PS: in order to get the best spectra you have to apply a Windowing function before you do the FFT.
There are dozens of W functions, each suitable for a specific case.
LTSpice offers many.
Otherwise it is a rocket science..
Simon:
OK, thanks, if the shorter data still yields results then it makes sense with the shorter data. I thought the idea of FFT was to assume that the portion of signal supplied repeats itself over and over to infinity.
pwlps:
--- Quote from: Simon on May 09, 2019, 06:44:02 am ---OK, thanks, if the shorter data still yields results then it makes sense with the shorter data. I thought the idea of FFT was to assume that the portion of signal supplied repeats itself over and over to infinity.
--- End quote ---
Yes exactly, it repeats itself, this is why windowing or at least padding with zeros is recommended to attenuate the artifacts generated by the wraparound. The artifacts are even more dramatic when you try to restore the original signal from its FFT spectrum, you might have a look on this : https://www.eevblog.com/forum/beginners/fft-and-signals/msg2321997/#msg2321997
Simon:
So I assume the period of time i analyse should correspond to the signal period so that when it wraps it's a carrect signal.
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