Any repetitive signal can be constructed by adding suitable sinusoids : its Fourier Series.
This web page allows you to select a signal and see how, as more sinusoids are added, the true function is approached.
Choose a function and it is plotted with its Fourier Series for up to n. The an and bn coefficients for the function are shown.
Press Sweep n and n will be increased gradually from 0..25.
Square Wave start at 0
SqWave centred at 0
Uneven Square Wave
SawTooth
Triangle Wave
n (0..25)