Filter function doesn't do filtering

I have a notch filter designed for 50Hz. Then I generated the filter design code with Filter Design app:

function Hd = filter_notch

Fs=500; Fnotch=50; BW=1; Apass=1; 

[b, a] = iirnotch(Fnotch/(Fs/2), BW/(Fs/2), Apass);
Hd     = dfilt.df2(b, a);

This is it's frequency response:

Now, I filter input signal with this filter:

filter_block=filter_notch;
out_signal=filter(filter_block,in_signal);

but to my surprise at notch frequency, it amplifies the signal for about 10dB!! Here is the zoomed version. Blue is output, Red is Input. What is going on here?!

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Answer 1

[Star Strider]

I don’t have the DSP System Toolbox, so I cannot run your code.

The power spectral density is probably not the best way to evaluate your filter performance. I would use the fft (link) function instead. I would also use the filtfilt function to do the actual filtering.