Embedded Target for Texas Instruments C6000 DSPs

Radix-2 FFT

Compute the radix-2 decimation-in-frequency forward FFT of a complex input vector

Library

C62x DSP Library--Transforms

Description

The Radix-2 FFT block computes the radix-2 decimation-in-frequency forward FFT of each channel of a complex input signal. The input length of each channel must be both a power of two and in the range 16 to 32,768, inclusive. The input must also be in natural (linear) order. The output of this block is a complex signal in bit-reversed order. Inputs and outputs are signed 16-bit fixed-point data types, and the output data type matches the input data type.

You can use the Bit Reverse block to reorder the output of the Radix-2 FFT block to natural order.

The Radix-2 FFT block supports both continuous and discrete sample times. This block supports little-endian code generation.

Dialog Box

Algorithm

In simulation, the Radix-2 FFT block is equivalent to the TMS320C62x DSP Library assembly code function DSP_radix2. During code generation, this block calls the DSP_radix2 routine to produce optimized code.

Examples

The output of the Radix-2 FFT block is bit-reversed. This example shows you how to use the Bit Reverse block to reorder the output of the Radix-2 FFT block to natural order.

The following code calculates the same FFT as the above model in the workspace. The output from this calculation, y2, is then displayed side-by-side with the output from the model, c. The outputs match, showing that the Bit Reverse block does reorder the Radix-2 FFT block output to natural order:

• k = 4;
  n = 2^k;
  xr = zeros(n, 1);
  xr(2) = 0.5;
  xi = zeros(n, 1);
  x2 = complex(xr, xi);
  y2 = fft(x2);
  
  [y2, c]
     0.5000             0.5000          
     0.4619 - 0.1913i   0.4619 - 0.1913i
     0.3536 - 0.3536i   0.3535 - 0.3535i
     0.1913 - 0.4619i   0.1913 - 0.4619i
          0 - 0.5000i        0 - 0.5000i
    -0.1913 - 0.4619i  -0.1913 - 0.4619i
    -0.3536 - 0.3536i  -0.3535 - 0.3535i
    -0.4619 - 0.1913i  -0.4619 - 0.1913i
    -0.5000            -0.5000          
    -0.4619 + 0.1913i  -0.4619 + 0.1913i
    -0.3536 + 0.3536i  -0.3535 + 0.3535i
    -0.1913 + 0.4619i  -0.1913 + 0.4619i
          0 + 0.5000i        0 + 0.5000i
     0.1913 + 0.4619i   0.1913 + 0.4619i
     0.3536 + 0.3536i   0.3535 + 0.3535i
  

0.4619 + 0.1913i 0.4619 + 0.1913i

See Also

Bit Reverse, FFT, Radix-2 IFFT

Matrix Transpose

Radix-2 IFFT