% Dr. Durant, EE3221, W1D2, 12/2/2021, updated 11/30/2022
% Explore the Fourier Series of the sampling impulse train
% pulseTrain_FS

% Equation 6.155 (page 323) shows that the pulse train (used to sample an
% analog signal) has a FS with all terms equal to f_s.
% δ_{T_s}(t)
%  =     ∑_{n=-∞}^∞ δ(t-nT_s)        [definition]
%  = f_s ∑_{n=-∞}^∞ e^{j 2π n f_s t} [Fourier Series]
% (Note: I'm using LaTeX notation for equations, _ is subscript/text below,
% ^ superscript/text above.)

% The terms of the FS have conjugate symmetry (X_k = X*_{-k}) since the pulse train is real.
% Also, since the terms are all real, only the even (cosine) portions
% remain in the trigonometric form of the FS, which is expected since the
% pulse train itself is even. (See Table 5-3, page 206 to review forms of the FS.)
% a_0 = fs
% a_k = 2 fs, k ≥ 1

% Let's show the first several terms of the the FS...
N = 20; % number of FS terms to show after n=0
fs = 1000; % sampling frequency (= fundamental frequency), hertz
Ts = 1/fs;
Omega0 = 2*pi*fs; % sampling frequency, radians/sec

t = -2*Ts : Ts/1000 : 2*Ts; % 4 periods of pulse train, 1000 samples per period

x = [  fs*ones(size(t)); ... % n = 0 case
     2*fs*cos( (1:N)' * Omega0 * t)]; % arg is column × scalar × row = rectangular matrix

h = plot(t,x, t,sum(x,1)); % plot can return "handles" to each of the plotted lines
set(h(end),'LineWidth',3) % ...which can be used to set their properties
% Here, we're making the final, sum line (end) thicker (width=3).
title(sprintf("δ_{T_s}(t) ≅ f_s ∑_{n=-%g}^{%g} e^{j 2π n f_s t}; fs = %g",N,N,fs))
% Hint: For convenience, you can also write \delta in a string instead of δ
xlabel('Time (s)')