When the distance between positive and inverse Fourier transform system changes, the CCD is adjusted to the focal plane by variable value of interval between two peaks on CCD with computer.

This algorithm has utilized the signal to carry on the signal characteristic that Fourier includes in the density of the power spectrum after Fourier transform.

The method measuring electrical parameters is based on ac sample technology, the FFT way is used in software design, so the design of circuit, the function and the accuracy is fully improved.