Variation in original spectrum from Simulink spectrum scope block output

Hi,

I am working on a simulink model that generate HDL code using HDL coder.When I observe the output spectrum loading the VHDL code,I get a different output from that given by Spectrum scope block in Matlab environment.My spectrum do not give sufficient stop band attenuation.It was designed to give a stop band attenuation of 70dB in Matlab but gives a spectrum at -20dB with significant pass band ripple in the analyzer device.Please give some suggestions to sort out this issue.

Krishnakumar