Learn more about Filter Design Toolbox

Designing Advanced Filters in FDATool

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Overview of FDATool Features Using FDATool with Filter Design Toolbox Software Example — Design a Notch Filter

Overview of FDATool Features

Filter Design Toolbox software adds new dialog boxes and operating modes, and new menu selections, to the Filter Design and Analysis Tool (FDATool) provided by Signal Processing Toolbox software. From the new dialog boxes, one titled Set Quantization Parameters and one titled Frequency Transformations, you can:

• Design advanced filters that Signal Processing Toolbox software does not provide the design tools to develop.

• View Simulink models of the filter structures available in the toolbox.

• Quantize double-precision filters you design in this GUI using the design mode.

• Quantize double-precision filters you import into this GUI using the import mode.

• Analyze quantized filters.

• Scale second-order section filters.

• Select the quantization settings for the properties of the quantized filter displayed by the tool:

  • Coefficients — select the quantization options applied to the filter coefficients

  • Input/output — control how the filter processes input and output data

  • Filter Internals — specify how the arithmetic for the filter behaves

• Design multirate filters.

• Transform both FIR and IIR filters from one response to another.

After you import a filter in to FDATool, the options on the quantization dialog box let you quantize the filter and investigate the effects of various quantization settings.

Options in the frequency transformations dialog box let you change the frequency response of your filter, keeping various important features while changing the response shape.

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Using FDATool with Filter Design Toolbox Software

Adding Filter Design Toolbox software to your tool suite adds a number of filter design techniques to FDATool. Use the new filter responses to develop filters that meet more complex requirements than those you can design in Signal Processing Toolbox software. While the designs in FDATool are available as command line functions, the graphical user interface of FDATool makes the design process more clear and easier to accomplish.

As you select a response type, the options in the right panes in FDATool change to let you set the values that define your filter. You also see that the analysis area includes a diagram (called a design mask) that describes the options for the filter response you choose.

By reviewing the mask you can see how the options are defined and how to use them. While this is usually straightforward for lowpass or highpass filter responses, setting the options for the arbitrary response types or the peaking/notching filters is more complicated. Having the masks leads you to your result more easily.

Changing the filter design method changes the available response type options. Similarly, the response type you select may change the filter design methods you can choose.

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Example — Design a Notch Filter

Notch filters aim to remove one or a few frequencies from a broader spectrum. You must specify the frequencies to remove by setting the filter design options in FDATool appropriately:

• Response Type

• Design Method

• Frequency Specifications

• Magnitude Specifications

Here is how you design a notch filter that removes concert A (440 Hz) from an input musical signal spectrum.

1. Select Notching from the Differentiator list in Response Type.

2. Select IIR in Filter Design Method and choose Single Notch from the list.

3. For the Frequency Specifications, set Units to Hz and Fs, the full scale frequency, to 10000.

4. Set the location of the center of the notch, in either normalized frequency or Hz. For the notch center at 440 Hz, enter 440.

5. To shape the notch, enter the bandwidth, bw, to be 40.

6. Leave the Magnitude Specification in dB (the default) and leave Apass as 1.

7. Click Design Filter.

   FDATool computes the filter coefficients and plots the filter magnitude response in the analysis area for you to review.

When you design a single notch filter, you do not have the option of setting the filter order — the Filter Order options are disabled.

Your filter should look about like this:

For more information about a design method, refer to the online Help system. For instance, to get further information about the Q setting for the notch filter in FDATool, enter

doc iirnotch

at the prompt. This opens the Help browser and displays the reference page for function iirnotch.

Designing other filters follows a similar procedure, adjusting for different design specification options as each design requires.

Any one of the designs may be quantized in FDATool and analyzed with the available analyses on the Analysis menu. For more general information about FDATool, such as the user interface and areas, refer to the FDATool documentation in the Signal Processing Toolbox documentation. One way to do this is to enter

doc signal/fdatool

at the prompt. The signal qualifier is necessary to open the reference page in Signal Processing Toolbox documentation, rather than the page in Filter Design Toolbox documentation. You might also look at the general section on FDATool in the Signal Processing Toolbox User's Guide.

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Using FDATool with Filter Design Toolbox Software

Switching FDATool to Quantization Mode

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