Error during sine-wave pwm reconstruction

Hey guys, I'm trying to convert a sine/cosine signal into a pwm one by comparing the current sine function with a triangular/sawtooth function. Afterwards, the created pwm signal is given to a low pass filter, which is to reconstruct the original sine wave. However, the resulting function differs from the original in sense of the amplitude, and the phase isn't shifted according to the filter order (5*90°).

By increasing the frequency of the triangular function, it also decreases in amplitude. Testing those functions via wolframalpha for instance gives the desired trajectory though.

Have you any clue what might went wrong?

clear;  

%---------------------------------------------------
% generation pwm signal
%---------------------------------------------------
f_sig=2.5e3;%10;           % signal frequency cosine
f_saw = 20*f_sig;          % signal frequency triangular
dt=1/50000;                % step width
tmax=pi/4;                 % simulation time
%---------------------------------------------------

 %---------------------------------------------------
 % filter design
 %---------------------------------------------------
 fa = 20*f_sig;           % sampling frequency
 fn = 0.5*fa;             % Nyquist frequency
 fg = 2.6e3;%20           % cut-off frequency
 Wn = fg/fn               % scaled frequency
 ord = 5;                 % filter order
 %---------------------------------------------------

 %---------------------------------------------------
 % signal calculations
 %---------------------------------------------------
 Np=floor(tmax/dt);
 tp=zeros(1,Np); uSp=tp;
 uPWMp=tp;uCosp=tp; 

 k=0; t=0;  uCosTP=0;
 while t< tmax
    uCos=cos(f_sig*2*pi*t); % cosine

    uS=2/pi*atan(tan(pi*t*f_saw));%2*(f_saw*t-floor(f_saw*t))-1; % triangular  function between +-1
    uPWM=sign(uCos-uS); % pwm modulated cosine signal

    % store plot values
    k=k+1;tp(k)=t;

    uSp(k)=uS;            % triangular
    uCosp(k)=uCos;        % cosine
    uPWMp(k)=uPWM;        % pwm cosine
    t=t+dt;
 end;
 %---------------------------------------------------

 %---------------------------------------------------
 % filter design
 [b,a] = butter(ord,Wn,'low'); % Butterworth low pass
 %---------------------------------------------------

 %---------------------------------------------------
 % plot results
 figure(1);
 plot(tp,uSp,':', tp,uCosp, tp,uPWMp); grid on;
 figure(2)
 plot(tp,uCosp,tp,filter(b,a,uPWMp));
 %---------------------------------------------------