pulse shaping by raised cosine filter

Hi
I have a question about pulse shaping by the raised cosine filter.
I select roll-off factor = 0.2.Fs/Fd=4 , delay= 9 , 10 , 11 , 12 , ...
I increase the length of the filter .
Frequency response is better in results.
I write a vhdl code for it. I synthesis it in FPGA.
I expect that frequency response is better. S/N is better ,
But is not. why?

thanks
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