MATLAB: Variable Amplitude sine wave

amplitudeMATLABsine wave

Hello, I am trying to learn some more about the function fft, for that I have a code like this
%Time specifications:
   Fs = 1000;                   % samples per second
   dt = 1/Fs;                   % seconds per sample
   StopTime = 0.6;             % seconds

   t = (0:dt:StopTime-dt)';     % seconds
    % Sine wave:
   Fc = 500;                     % hertz
   x = sin(2*pi*Fc*t);
   % Plot the signal versus time:
   figure;
   plot(t,x);
   xlabel('time (in seconds)');
   title('Signal versus Time');
   zoom xon;
Now I dont understand why, but the output of the plot is a sinusoidal wave that increases in amplitud. why is that? As far as my understanding goes, that should not happen. I know that it has something to do with the Fs value. but what is the relationship exactly. How should I pick a good Fs value to avoid this?
Thank you very much

Best Answer

Margarita, that's because the dt value is too large. Try
 dt = 1e-6;
instead and you should receive a proper sinusoidal signal with a constant amplitude of 1. As far as next steps are concerned, the optimal value of dt depends on your particular application, whatever you need to do.
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