to plot frequency response you have to use spectrum scope from simulink library .
but you should take care of some things:
first if you are using analog signal you cant see it directly on spectrum scope as you should use azero order hold before it with agood sampling time.
also you should make the buffer size in spectrum scope bigger than the fft lenght
also both of them is power of 2 as 4, 8,16,64,128 ...etc.
i hope this help you
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