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- 13th November 2016, 19:09 #1

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## How to simulate in Simulink the FFT of "Sample-and-Hold"

Hello,

I've met difficulties while trying to simulate sinusoidal waveform passed through "Sample-and-Hold" module.

On the snapshot below my model. When I run simulation, the error occurs:

*Error in 'Sample_and_Hold_test/Buffer': All sample times must be discrete. No continuous or constant sample times are allowed*

"Sample-and-Hold" block outputs continuous signal, but "Buffer" requires discrete one.

How to proceed ?

Thanks in advance.

P.S. Model is in attachment.

- 13th November 2016, 19:09

- 13th November 2016, 20:06 #2

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## Re: How to simulate in Simulink the FFT of "Sample-and-Hold"

I had a similar issue some time ago, and I solved by replacing all the blocks before S/H ( including itself ) by a discrete input, more precisely an array of discrete values. The reason behind this error seems like the DSP blocks are generating discrete outputs, but in the continuous domain.

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- 13th November 2016, 20:41 #3

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## Re: How to simulate in Simulink the FFT of "Sample-and-Hold"

Can you, please, attach some screenshot ... frankly speaking I didn't properly understood what you mean.

Thanks.

- 13th November 2016, 20:41

- 13th November 2016, 21:42 #4

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## Re: How to simulate in Simulink the FFT of "Sample-and-Hold"

There should be better alternatives to fix that issue, but the only one I see based on my previous experiments, could be by replacing all the stuffs before S/H blocks by an 1-D vector, in you case with the length of 4096 values. Don't know exactly what function could do that, but

*discretizer*is the first one which come to mind.--------------------------------------------------------------------------------------------------

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- 14th November 2016, 12:53 #5

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## Re: How to simulate in Simulink the FFT of "Sample-and-Hold"

Yes, it would be a solution ... i.e. transform the train of continuous values into 1-D vector in order to apply it to FFT, but unfortunately I didn't find any Simulink block capable to do it.

I turned my attention to**"Zero-Order Hold"**from Simulink**"Discrete"**library and also modified sinusoidal waveform output type from**Discrete**to**Continuous**.

This way**"Zero-Order Hold"**can be considered as "sampler".

Indeed it is the case - after**"Zero-Order Hold"**I can place**"Buffer"**and simulation runs, i.e.__no error__is generated.

Then I returned to my original task - simulate**"Sample-and-Hold"**behavior and its impact on incoming signal.

According to textbook (please see screenshot below), the**"Sample-and-Hold"**acts as filter having a characteristic__sinc(x)__.

So if I apply, let say, 3 sinusoidal waveforms to such**"Sample-and-Hold"**, it should filter out frequencies that close to__Nyquist__frequency.

But in my setup it doesn't ... all 3 sinusoidal waveforms manifest the same levels at the output of**"Sample-and-Hold"**.

In correct simulation output of*Sin_f3 < output of Sin_f2 < output of Sin_f1*.

So, the conclusion is: "Zero-Order Hold" cant transform**continuous**signal into**discrete**, but can't "emulate" real "Sample-and-Hold"

So, the problem apparently quite simple ... persists.

- 14th November 2016, 13:20 #6

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## Re: How to simulate in Simulink the FFT of "Sample-and-Hold"

Perhaps the Lookup Tables Block Library could make the job. There is already a 1-D Sin generator function there. According to the initial picture, this is exactly what you want to sample.

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- 14th November 2016, 13:20

- 14th November 2016, 14:37 #7

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## Re: How to simulate in Simulink the FFT of "Sample-and-Hold"

I've just tried your suggestion (at least as I understood it).

Unfortunately doesn't work ... well it works, but as in previous case "Sample-and-Hold" block doesn't manifest any filtering action ... i.e. the power level upon FFT doesn't change when input frequency changes.

Here below the results of 2 simulations:

- f1 = 233Hz
- f1 = 27133Hz

As you can see both have the same level, whereas the FFT output of 27133Hz must be much lower than 233Hz.

Fig.1 Model

Fig.2 Simulation with f1 = 233Hz

Fig.3 Simulation with f1 = 27133Hz

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