And do the same thing on FFTMON (U_3) to monitor the FFT component.

Please understand the spectrum of unit impulse is flat and any frequency
has the same amount. ("1", i.e. 0dB).

Next, left click on BTWLPF (Butterworth IIR type Low Pass Filter) to select
it and right click to choose "Open a Monitor".

Then you will see the frequency response of the IIR Filter. Its cutoff
frequency is 1kHz.

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