Forming equation of a sine wave with varying amplitude

i need to have the equation of a sine wave that i have. is there any suggestion or any tool that can ease my work? any suggestions, much thanks.

Answer 1

[Image Analyst]

Maybe try this: http://www.mathworks.com/matlabcentral/fileexchange/36904-matlab-script-for-digitizing-a-published-graph

Answer 2

[Star Strider]

If you have the data for the signal you posted in the plot, start by calculating the Fourier transform uding the fft function. That will give you some idea of the frequency components of your signal.