Creating Fixed-Point Filter Models (Digital Filter Design Toolkit)
In a fixed-point implementation, after quantizing the coefficients of a reference floating-point filter, you also must configure all other quantizers, such as the input, output, and multiplicand quantizers. This process creates a fixed-point model for the filter. You must create a fixed-point filter model before you simulate the filtering process or generate code from the filter. You can use either the DFD FXP Modeling VI or the DFD FXP Set Quantizer VI to create fixed-point filter models. Refer to the Details section of the DFD FXP Set Quantizer VI for information about guidelines on the quantizer settings.
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Note After quantizing the coefficients of a reference floating-point filter, you automatically obtain a fixed-point filter model with the following default values: input word length = output word length = 16. The Specifying the Word Length and Integer Word Length topic contains more information about the effects of different word length values. |
If you use the DFD FXP Modeling VI to create the fixed-point filter model, complete the following steps:
- Specify the word lengths for the input and output signals.
- Set the rounding mode for the output signal.
The DFD FXP Modeling VI automatically calculates all quantizer settings of the fixed-point filter model. You can use the DFD FXP Get Quantizer VI to retrieve the quantizer settings or use the DFD FXP Set Quantizer VI to modify the quantizer settings.
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Note Output quantizers generally have different integer word lengths from the input quantizers. Before generating code, check the quantizer settings to confirm or modify the settings you want to use for the fixed-point filter. If you reduce the default output integer word length, National Instruments recommends that you change the overflow mode to Saturate. |
If you use the DFD FXP Set Quantizer VI to create the fixed-point filter model, you must configure the quantizers.
Refer to the Lowpass_Step 4_Model and Simulate FXP Lowpass VI in the labview\examples\Digital Filter Design\Case Studies\Single-Rate Filter directory for an example that demonstrates how to create a fixed-point model of a lowpass filter and simulate the filtering result.
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