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What's going on with this FFT? Keysight 1102g
Bud:
100MHz is good enough.
Anthocyanina:
with a 100MHz signal, centering the FFT at 100MHz, i can go up to 27µs/ before it starts doing something weird, and 32µs/ before it disappears. When at 50µs/ it shows up at 50MHz. This is all so strange :o
Someone:
--- Quote from: Anthocyanina on October 25, 2022, 12:38:17 am ---This is all so strange
--- End quote ---
Increasing the timebase (horizontal) at some point reduces the sampling rate as the scope only has so much memory. More memory is good, high sample rate is still available at slower timebases. But the missing bit....
The FFT is only calculated from 64K points, spread across the screen and decimated from the acquisition data. 64k/20us = 320MHz sampling, 160MHz nyquist. That signal reappearing at longer captures is from aliasing.
Leave the span equal to twice the centre frequency and it should all be pretty self explanatory as you change the timebase.
Anthocyanina:
oooh, thank you, this seems to be what makes the most sense, but I still wonder, since the sampling rate of the scope remains 2GSa/s at up to 20µs/, wouldn't the FFT still use the same points as it did at faster timebase? or is the FFT sample rate something that doesn't depend directly on the time domain sample rate? (if that question even makes sense :-//)
Thank you!
Someone:
As above, the sampling rate for the FFT is 64k points across the current timebase for most (all?) conditions.
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