fir deci simulation question

when trying to do a simulation of FIR decimation simulink block using a chirp block, it doesn't seem the FIR filter out the excessive Hz (ie i still see the full bandwidth). anyone know the proper way to setup a chirp and verify the output of FIR decimation block on spectrum analyzer.

below is my setup.

chirp from 0 to 8Mhz, 100ms sweep & target time, sample time 1/20e6.

spectrum setting as 20Mhz Fs wtih 1024 NFFT

filter dec by 4, direct form, elements as channels