Variable Amplitude sine wave

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Margarita 2014-7-2

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编辑： Shravankumar P 2014-7-2

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

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Shravankumar P 2014-7-2

编辑：Shravankumar P 2014-7-2

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

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

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Answer 1

Mischa Kim 2014-7-2

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编辑：Mischa Kim 2014-7-2

在 MATLAB Online 中打开

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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Mischa Kim 2014-7-2

编辑：Mischa Kim 2014-7-2

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 2014-7-2