Filtered signal's frequency response is not correct
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I am trying to pass a pure noise signal through a filter, but the frequency response is not what I expected. I have attached the sample script I am using to filter the signal.
I created a digital FIR filter as shown:
![filter_response.jpg](https://www.mathworks.com/matlabcentral/answers/uploaded_files/196964/filter_response.jpeg)
I applied this to flat noise and expected to see abount -120 dB of attenuation at higher frequencies; however, I get the following frequency response:
![freq_response.jpg](https://www.mathworks.com/matlabcentral/answers/uploaded_files/196965/freq_response.jpeg)
A few things stand out as incorrect:
- Why isn't the amplitude flat in the stopband of the filter (~1000 Hz to ~64000 Hz)? I would have expected the orange filtered signal response (the 2nd figure) to match the filter response in the first figure.
- The attentuation in the orange filtered signal only reaches about -60 dB, when it should reach -120 dB. Why is this?
- I ran a similar test using Simulink and the spectrum analyzer produced the correct frequency response for the signal. Why is this different that using a MATLAB script?
![simulinkTest.JPG](https://www.mathworks.com/matlabcentral/answers/uploaded_files/196966/simulinkTest.jpeg)
![simulink_spectrum.JPG](https://www.mathworks.com/matlabcentral/answers/uploaded_files/196968/simulink_spectrum.jpeg)
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