I have created a fairly simple Excel Spreadsheet to allow you to magnify a very small distortion in a captured sine wave. This can help you find the cause of the distortion. Start by downloading the spreadsheet, then follow the instructions below.

In Excel under Data, go to the far right and select Data Analysis. If you don't see the option "Data Analysis" on the far right of the screen, follow these instructions on this site.

This tool is pretty easy to use, but you have to have a coherently captured wave that has an integer number of cycles of the input wave. You can't use a non-coherently captured wave. A windowed wave may or may not work, try it at your own risk. This file is setup for a wave of 2048 samples. If you have some other size wave, you will need to make significant modifications to the spreadsheet.

It's pretty simple, every important column contains a comment explaining what it's there for. Magnifying the distoriton is a two step process. After pasting the wave in question into column B and entering the number of cycles of the wave, do a Fourier Transform by choosing the Fourier Analysis option in the Data menu. Use Column B as the input and Column C as the output. This should take a few minutes.

Then when that is done, select your magnification factor in cell D2. The default is 1000, but that is kind of high, so be careful. After that, perform an Inverse Fourier Transform by choosing the Fourier Analysis option in the Data menu again, but this time set the input to Column E and the output to Column F. Also make absolutely sure to click the radio button, Inverse on the Fourier Transform dialog box.

Once you do the second step, you can see the wave with the distortion magnified, like this:

Have fun!