Did you mean 20 dB per decade? One R-C filter rolls off at 20 dB per decade or 6 dB per octave. I am not familiar with Matlab, but it should automaticly display in semi-log form.
A simple filter with a series resistor then a capacitor to ground makes a 20dB/decade lowpass filter.
Why sim such a simple filter. You should see its gradual rolloff with your eyes closed.
I just wanted to know if anybody here knew how to put this into matlab. I am very new to matlab (just got it last week) and have no idea how to do this.