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In these power spectral density plots shown above (taken from the web 1,[2]), the spectra show a roll-off at the highest frequencies, as shown in the figure. For my own example, when I calculate PSD from time domain data taken with a sampling rate of 10 MSa/s using Welche's method, this roll-off appears at half the sampling rate in the spectra, which is after 5 MHz. Is this an artifact of the FFT or Welche's method, or could you please explain the reason behind this roll-off?

I have attached two figures where I see this roll-off. Thank you for your time.

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    $\begingroup$ It's just an indication that the dataset has a lowpass characteristic; that is, as frequency increases, the spectral content decays toward zero. This is a common attribute of signals, especially signals that have had anti-aliasing filtering applied before digitization. It doesn't have anything to do with Welch's method; for example, try generating white Gaussian noise and see that the PSD estimate is nearly flat. $\endgroup$
    – Jason R
    Commented Oct 18, 2022 at 15:02

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It many cases this is caused by the data acquisition system. Most A/D converters have an anti-aliasing low pass filter. The choice of filter depends on the specific application but typically the cutoff frequency is at 80%-90% of the Nyquist frequency.

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    $\begingroup$ That is interesting! $\endgroup$
    – TM90
    Commented Oct 18, 2022 at 18:11
  • $\begingroup$ Thank you so much for the informed answer @Hilmar. Let me understand it more clearly. I understand that the DAQ may have anti-aliasing filters ranging close to the Nyquist frequency. Are these used in addition to the downsampling filters, (FIR filter)? $\endgroup$
    – Greta
    Commented Oct 19, 2022 at 5:55
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    $\begingroup$ It's the Analog to Digital converter that needs and anti-aliasing filter. Whether you need one for a D/A converter depends on the application. Down sampling also requires and anti-aliasing filter $\endgroup$
    – Hilmar
    Commented Oct 19, 2022 at 16:12
  • $\begingroup$ Thanks a lot, I got it. @Hilmar $\endgroup$
    – Greta
    Commented Oct 20, 2022 at 4:45

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