Thread Subject: Calculation of phase spectrum for one-sided exponential signal...

Hallo!

I try to calculate an amplitude and phase Fourier spectrum
of a one-sided exponential signal in the following way:

---CODE---
Fs = 10000; % Sampling frequency, Hz
dt = 1/Fs; % Sample time
L = 10001; % Length of signal

t = (0:L-1)*dt; % Time vector

A=1; % Exponent multiplier
k=30; % Exponent k

% One-sided exponential definition
s=A*exp(-1*k*t);

% Signal graphical output
plot(t, s, 'DisplayName', 's', 'YDataSource', 's'); figure(gcf)
%%

%dw=2*pi/((length(s)-1)*dt);
%w=(0:1:(length(s)-1))*dw;

% Frequency vector definition
df=1/((length(s)-1)*dt);
f=(0:1:(length(s)-1))*df;

% Complex Fourier spectrum calculaiton
z=fft(s)*dt;

% Amplitude Fourier spectrum calculaiton
p=abs(z);

% Amplitude Fourier spectrum graphical output
plot(f,p);

title('Amplitude Spectrum of Signal')
xlabel('Frequency (Hz)')
ylabel('|S(jf)|')

%%
% Phase Fourier spectrum calculaiton
ph=angle(z);

% Phase Fourier spectrum graphical output
plot(f,ph);

title('Phase Spectrum of Signal')
xlabel('Frequency (Hz)')
ylabel('Angle(S(jf))')
---CODE---

Amplitude Fourier spectrum values (vector p) are correct
with precision of 0.0001 (I suppose it's related to finite
length of the exponential signal). However, phase Fourier
spectrum values (vector ph) have very unclear behaviour - on
the low frequencies they decrease and on high frequencies
they increase, instead of steadily decrease from 0 to -Pi/2...

Most likely I'm doing a wrong thing.

Could you please help me to figure out my mistake?

Thank you in advance for you answer!

P.S. Happy New 2008 Year to Everyone!