%phase_lag.m
%Problem 6.55

clc
clear all

num=10
den=poly([0 -1.4 -3])/1.4/3

sys=tf(num,den)

figure(1)
margin(sys)

wc=0.8

T=20/wc

alpha=10

zero=1/T
pole=1/(alpha*T)

numc=[T 1]
denc=[alpha*T 1]

sysc=tf(numc,denc)

sysL=series(sys,sysc)

figure(2)
margin(sysc)

figure(3)
margin(sysL)