DSO203 GCC APP - Community Edition (2.51+SmTech1.8+Fixes)

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Hello



I don’t understand the behaviour of Pedro’s FFT.



I use a 10kHz “sine” from the internal DAC, channel A with 0.5V/div, 100uS/div as an example.

[list]

  • [*]when I vary the amplitude of the sine, the spectrum changes completely. For example, at amplitudes of about 1.7V, the highest peak is at 20kHz, not 10kHz (and it is more then 2 div higher than the 10kHz)
  • [*]with an amplitude of 2.1V, the picture changes: now the highest peak is at 10kHz and 2 div higher than the 20kHz
    • [/list]

    Maybe this has to do with the fact that only 256 samples are used, i.e. only about 10 periods and this is not enough - does the FFT only give good results when there is an integer number of periods in the 256 samples? Could this explain why the sines have so many harmonics?



    I had another problem, which looks like a bug: when moving the “navigator B”, changing between the “trigger” and the “auto/norm…” menus - but not changing anything at the settings - sometimes the FFT result change completely!!!



    Two more questions:
    [list]

  • [*]if the number of samples is the problem, why not make an FFT with 4096 sample? Why is it more complicated than with 256?
  • [*]what is the vertical scale?
    • [/list]

    Patrick