Variable Amplitude sine 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

[Shravankumar P]

you need to change the frequncy Fc ; try Fc=10 to 50 ;

or you need to change the Fs, eg: Fs=100000;

Answer 1

[Mischa Kim]

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.

[Margarita]

Thank. But why is the dt generating that behaviour?

[Mischa Kim]

The frequency of the signal is 500 Hz, the period is therefore 1/500 s which is exactly 2*dt. So you evaluate the sine pretty much dead on at its zero crossings. That's the reason why the signal amplitudes in your plot are tiny (1e-13).

[Margarita]

Thank you

Answer 2

[Shravankumar P]

See this Attached file with Fc=10Hz