How to interpolate the peak location of a DFT or FFT if the frequency of interest is between bins
by Matt Donadio
If the actual frequency of a signal does not fall on the center frequency of a DFT (FFT) bin, several bins near the actual frequency will appear to have a signal component. In that case, we can use the magnitudes of the nearby bins to determine the actual signal frequency.
3.1.1 What are "upsampling" and "interpolation"?
"Upsampling" is the process of inserting zero-valued samples between original samples to increase the sampling rate. (This is called "zero-stuffing".) Upsampling adds to the original signal undesired spectral images which are centered on multiples of the original sampling rate.