Designing a Fixed-Point Filter Part 1: Selecting a Filter Structure (Digital Filter Design Toolkit)

In fixed-point implementations, different filter structures have different memory and multiplier requirements. These requirements can cause different finite word length effects. In this situation, converting the default filter structure of a filter design to a different structure might provide better finite word length performance.

In Part 1 of this tutorial, you select a filter structure for the reference floating-point filter. Later in the design process, you can change the implementation structure if the filter design does not meet the filtering requirements.

Complete the following steps to select a filter structure for the reference floating-point filter by using the DFD Convert Structure VI.

1. Design a reference floating-point filter, if you have not already done so. This tutorial uses the lowpass Butterworth filter from Part 1 of the Designing a Floating-Point Filter tutorial.

   Note  Refer to labview\examples\Digital Filter Design\Getting Started\Tutorials\Designing a Floating-Point Filter\Designing a Floating-Point Filter Part 1.vi for the reference floating-point filter you use in this tutorial.

2. Add the DFD Convert Structure VI to the block diagram. 
3. Wire the filter out and error out outputs of the Classical Filter Design Express VI to the filter in and error in inputs of the DFD Convert Structure VI.
4. Right-click the target structure input of the DFD Convert Structure VI and select Create»Constant from the shortcut menu.
   
   A block diagram constant appears to the left of the DFD Convert Structure VI. This constant represents the target structure that this VI uses to convert the filter structure.
5. Click the target structure constant to select the target structure this VI uses to convert the filter structure. In this tutorial, select the IIR Cascaded Second-Order Sections Form II Transposed structure.
   
   The block diagram now resembles the following figure.

   

6. Select File»Save As and save this VI as Designing a Fixed-Point Filter.vi in an easily accessible location.

After you select a filter structure for the reference floating-point filter, you can scale the filter coefficients in Part 2 of this tutorial.

Note  Refer to labview\examples\Digital Filter Design\Getting Started\Tutorials\Designing a Fixed-Point Filter\Designing a Fixed-Point Filter Part 1.vi for a completed version of the digital filter from this tutorial.