s域：
无滞后：
num=[122.0501]
den=[0.8190 1]
g=tf(num,den);
margin(g);
grid on;
有滞后：
>> num=[106.9389];
>> den=[0.176 1];
>> g=tf(num,den,'iodelay',0.088)
106.9
exp(-0.088*s) * -----------
0.176 s + 1
Continuous-time transfer function.
>> margin(g);
>> grid on;
z域：
Ts=0.1
dnum=[1]
dden=[1,-1]
Zk=tf(dnum,dden,Ts)
margin(Zk)