Simulating of Sample-and-Hold fails: "Buffer" block generates the error "All sample times must be discrete. No continuous or constant sample times are allowed"

Any suggestions ? Thanks in advance.

Well, when I remove "Buffer" and set the parameter "Samples per Frame" in "Sine Wave", the FFT is calculated. But such setup isn't reliable ... I mean it doesn't represent the behavior of "Sample-and-Hold". The block "Sample-and-Hold", once applied to sampled signal works as sinc(x) function (sinc(x) = sin(x)/x). enter image description here

So, when the frequency of input signal approaches Nyquist frequency, after "Sample-and-Hold" it is filtered out. It is not the case in my setup: none of 3 frequencies is attenuated. enter image description here

  • $\begingroup$ The image urls are missing. $\endgroup$
    – Gilles
    Nov 13 '16 at 15:21
  • $\begingroup$ I've just edited the original post. The last picture shows unreliable simulation: if, as proposed Marcus (at least if I understood correctly his suggestion), I "emulate" "Sample-and-Hold" by specifying the sampling rate in 3 sinusoidal waveforms, and then do FFT on the sum of 3 waveforms, the result is erroneous: "Sample-and-Hold" acts as sinc(x) filter (please see the fragment from textbook) and should attenuate frequencies close to Nyquist frequency ... but it doesn't: FFT shows that all 3 constituents have the same level. $\endgroup$
    – Pavel_47
    Nov 13 '16 at 18:30

I'm no Simulink expert, but I'm pretty sure you don't want sample and hold, but just sample as functionality: As far as I can guess, sample and hold doesn't convert from simulatedly continuous signal to simulatedly discrete signal, but just actually does what its name says, and probably generates a continuous signal, not a discrete one.

In fact, seeing that you're already using a DSP sine, I'd say: instead of using a separate block with a trigger, just change that DSP Sine's sampling rate.


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