The ‘fre’ vector needs to be created in logspace and it then needs to be divided by 
because the frequency axis is in rad/sec. Then, it matches reasonably closely to the bode result.
because the frequency axis is in rad/sec. Then, it matches reasonably closely to the bode result. fre=logspace(-1,2,100)/(2*pi);
omg=2*1i*pi.*fre;
H=tf([1,0],[1,2,3]);
bode(H)
figure
Hm=omg./((omg).^2 + 2.*omg + 3);
semilogx(fre*2*pi,20.*log10(abs(Hm)))
.
