The structure for a CIC interpolator filter is shown in the following figure. This structure is similar to that of a CIC decimator with the integrator and comb sections in reverse order. In this case, there is an up-sampling of data by a factor R between the comb and integrator sections. This rate increase is done by inserting R-1 zero-valued samples between consecutive samples of the comb section output. The up-sampled and filtered data stream is output at the sample rate fs.
For interpolation, the response of the CIC filter is applied to the up-sampled (zero-valued samples inserted) input signal. The effect of this processing is shown in the following figure in a filter with rate change R = 7, number of stages N = 4, and differential delay M = 1. The peaks in the output interpolated signal show the effect of the magnitude response of the CIC filter applied to the spectrum images of the up-sampled input signal.