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DFT parameters in spectrum emasurements

IcoNem

Hi all,

I am trying to get the DFT of the output waveform to get SINAD and ENOB. There are some things I am not sure about when reading at the help section about spectrum:

At a certain point it says: "The Sample Count value must be greater than zero. By default, this field displays the number of data points in the selected signal." but if I am looking at a waveform that lasts 20.48 us and time step during simulation was 1 ns means that the trace has 20480 points, but by default I get 1024 in the Sample Count field, why is that?

At another point it says: "The number of input periods in the period of the Fourier transform needs to be an integer and a prime number, where the period of the Fourier transform is 1/ the fundamental frequency of the Fourier transform." So it is saying that the interval stop time-start time should be an integer number of periods of my waveform, and that is ok, but why should it also be a prime number?

Also I am not sure  about how the signal first harmonic is chosen, is it just the one with bigger amplitude?

Thanks a lot for your help

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  • Andrew Beckett

    I think the first is a mistake in the documentation. I don't think it's related to the number of points in the signal - there's just a default value which happens to be 1024

    The prime number part could also be better described. This is to ensure coherent sampling - see https://en.wikipedia.org/wiki/Coherent_sampling 

    The first harmonic would be 1/(stopTime-startTime) - in other words the period of the FFT defines the fundamental. For things like THD you can choose how many harmonics are used to compute the desired signal - which might be more than 1 if you have more than one period of your signal in the interval being FFT'd.

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    I think the first is a mistake in the documentation. I don't think it's related to the number of points in the signal - there's just a default value which happens to be 1024

    The prime number part could also be better described. This is to ensure coherent sampling - see https://en.wikipedia.org/wiki/Coherent_sampling 

    The first harmonic would be 1/(stopTime-startTime) - in other words the period of the FFT defines the fundamental. For things like THD you can choose how many harmonics are used to compute the desired signal - which might be more than 1 if you have more than one period of your signal in the interval being FFT'd.

    Regards,

    Andrew.

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