f1=20;
f2=100;
df=f2-f1; % frequency
dt=0.002; % sample rate
T=4; % time
handles.t=0:dt:T-dt;
handles.s=sin(2.*pi.*handles.t.*(f1+(df/((T-dt)*2).*handles.t)));

handles.zero=ifft(handles.s);
 handles.zero=angle(handles.zero);

figure
plot(handles.t,handles.zero)
title('Zero phase signal');
xlabel('Time')
ylabel('Amplitude')