# Designing a Fixed-Point Filter Part 4: Analyzing a Fixed-Point Filter (Digital Filter Design Toolkit)

In Part 3 of this tutorial, you quantized the coefficients of the reference floating-point filter. In Part 4 of this tutorial, you analyze the fixed-point filter to determine how the characteristics of the quantized fixed-point filter deviate from the characteristics of the reference floating-point filter.

Note If you did not complete Part 3 of this tutorial, refer to labview\examples\Digital Filter Design\Getting Started\Tutorials\Designing a Fixed-Point Filter\Designing a Fixed-Point Filter Part 3.vi for a completed version of the digital filter from that part. |

Complete the following steps to analyze a fixed-point filter by using the Filter Analysis Express VI.

- Place the Filter Analysis Express VI on the block diagram.

Place Find

The**Configure Filter Analysis**dialog box appears with**Sample Data**labels on the graphs. These labels remain on the graph until you run the VI. - Leave the default settings and click the
**OK**button to close the dialog box and return to the block diagram. - Wire the
**filter out**and**error out**outputs of the DFD FXP Quantize Coef VI to the**filter in**and**error in**inputs of the Filter Analysis Express VI.**Note**If you do not see the**error in**input to the Filter Analysis Express VI, move the cursor over the down arrows at the bottom of the Filter Analysis Express VI. When a double-headed arrow appears, click and drag the border of the Express VI to add two terminals. When you release the border, the**error in**input appears. - Right-click the
**magnitude**output of the Filter Analysis Express VI and select**Create»Graph Indicator**from the shortcut menu.

A new indicator appears to the right of the Filter Analysis Express VI. This indicator represents a waveform graph in the front panel window. This graph displays data from the**magnitude**output of the Filter Analysis Express VI.

The block diagram now resembles the following figure. - Double-click the
**magnitude**indicator to display the front panel window of the VI and see the graph. - Click the
**Run**button to run the VI. The following figure displays the magnitude graph of the filter design.**Note**You can customize the graph by clicking the plot legend and configuring the plot labels, display, and colors.

In the previous figure, notice the magnitude response of the quantized fixed-point filter matches the magnitude response of the reference floating-point filter. - (Optional) Create graph indicators for other outputs of the Filter Analysis Express VI, such as the
**phase delay**,**group delay**,**Z Plane**,**impulse**, and**step**outputs. Then, display the front panel window and run the VI again. Make sure the filter analysis graphs of the fixed-point filter match the filter analysis graphs of the reference floating-point filter. - Select
**File»Save**to save the VI.

After you analyze and confirm that the fixed-point filter meets the filtering requirements, you can create a fixed-point filter model in Part 5 of this tutorial. If the filter does not meet the filtering requirements, try one or more of the following steps:

- Redefine the filter specifications of the reference floating-point filter.
- Change the implementation structure of the reference floating-point filter.
- Modify the quantization settings of the filter coefficients.

Note Refer to labview\examples\Digital Filter Design\Getting Started\Tutorials\Designing a Fixed-Point Filter\Designing a Fixed-Point Filter Part 4.vi for a completed version of the digital filter from this part of the tutorial. |

Previous: Quantizing a Floating-Point Filter | Next: Creating a Fixed-Point Filter Model |