User`s guide

FFT

5-173

5FFT

Purpose Compute the FFT of the input.

Library Transforms

Description The FFT block computes the fast Fourier transform (FFT) of each channel in

the M-by-N or length-M input,

u, where M must be a power of two. To work

with other input sizes, use the Zero Pad block to pad or truncate the length-M

dimension to a power-of-two length. The output is always complex-valued and

sample-based (it is unoriented 1-D vectors for unoriented inputs).

y = fft(u,M) % Equivalent MATLAB code

The kth entry of the lth output channel, y(k, l), is the kth point of the M-point

discrete Fourier transform (DFT) of the lth input channel.

(5-2)

For information on block output characteristics and how to configure the block

computation methods, see the following sections:

•“Input and Output Characteristics” on page 5-173

•“Ordering Output Column Entries (Output in bit-reversed order

Parameter)” on page 5-176

•“Selecting the Twiddle Factor Computation Method” on page 5-177

•“Optimizing the Table of Trigonometric Values” on page 5-178

•“Algorithms Used for FFT Computation” on page 5-179

•“Example” on page 5-179

Input and Output Characteristics

The following table describes all valid block input types, their corresponding

outputs, and the dimension along which the block computes the DFT.

Note For M-by-N and length-M inputs, M must be a power of two. To work

with other input sizes, use the Zero Pad block to pad or truncate the length-M

ykl,() uml,()e

j– 2π m 1–()k 1–()M⁄

m 1=

∑

= k 1 … M,,=