A modified cascade fast Fourier transform in a spectrum analysing system

The principle of a modified cascade fast Fourier transform (CFFT) spectral analyser is described, and its qualities are pointed out. Optimum methods to keep aliasing phenomena under control are treated where the importance of the overlap factor is stressed. The CFFT is implemented on a 16 bit microcomputer. The basic cycle time is 210 nsec and an external simultaneous fast multiplier is used. Only one analog low pass filter is needed. The frequency resolution will depend on the size of the CFFT and can be varied by simple software instructions. A computational, speed saving of more than 50 % compared to the traditional use of a pure CFFT is accomplished.