why sin(x) function doesnt no return periodic sin waves?

Well, it does, but only in certain circumstances. I've got to add a line-power- like signal to a simulated ECG in order to show that interference. I've defined, as is requested by standard calculations, a time vector as 1:length(signal)/ sampling frequency and then i computed the line signal as A*sin(2*pi*w*t). Obviously i previously define A e w, and everything is good at w=50, which is standard frequency in most countries for line interferences, but, as soon as i switch to different frequencies, such as 50.5, the sin wave is completely wrong since it has some sort of modulation in the amplitude which cannote be true for a sin wave. Help?

[Adam]

Posting some plots or code might help as I don't really understand what you mean. A*sin(...) should always return a true sine wave of some frequency or other and with some magnitude or other, subject to discrete sampling constraints, of course.