Learn more about Filter Design Toolbox

Viewing SOS Filter Sections

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Using the SOS View Dialog Box Example — View the Sections of SOS Filters

Using the SOS View Dialog Box

Since you can design and reorder the sections of SOS filters, FDATool provides the ability to view the filter sections in the analysis area — SOS View. Once you have a second-order section filter as your current filter in FDATool, you turn on the SOS View option to see the filter sections individually, or cumulatively, or even only some of the sections. Enabling SOS View puts FDATool in a mode where all second-order section filters display sections until you disable the SOS View option. SOS View mode applies to any analysis you display in the analysis area. For example, if you configure FDATool to show the phase responses for filters, enabling SOS View means FDATool displays the phase response for each section of SOS filters.

Controls on the SOS View Dialog Box

SOS View uses a few options to control how FDATool displays the sections, or which sections to display. When you select View > SOS View from the FDATool menu bar, you see this dialog box containing options to configure SOS View operation.

By default, SOS View shows the overall response of SOS filters. Options in the SOS View dialog box let you change the display. This table lists all the options and describes the effects of each.

Option	Description
Overall Filter	This is the familiar display in FDATool. For a second-order section filter you see only the overall response rather than the responses for the individual sections. This is the default configuration.
Individual sections	When you select this option, FDATool displays the response for each section as a curve. If your filter has five sections you see five response curves, one for each section, and they are independent. Compare to Cumulative sections.
Cumulative sections	When you select this option, FDATool displays the response for each section as the accumulated response of all prior sections in the filter. If your filter has five sections you see five response curves: The first curve plots the response for the first filter section. The second curve plots the response for the combined first and second sections. The third curve plots the response for the first, second, and third sections combined. And so on until all filter sections appear in the display. The final curve represents the overall filter response. Compare to Cumulative sections and Overall Filter.
User defined	Here you define which sections to display, and in which order. Selecting this option enables the text box where you enter a cell array of the indices of the filter sections. Each index represents one section. Entering one index plots one response. Entering something like {1:2} plots the combined response of sections 1 and 2. If you have a filter with four sections, the entry {1:4} plots the combined response for all four sections, whereas {1,2,3,4} plots the response for each section. Note that after you enter the cell array, you need to click OK or Apply to update the FDATool analysis area to the new SOS View configuration.
Use secondary-scaling points	This directs FDATool to use the secondary scaling points in the sections to determine where to split the sections. This option applies only when the filter is a df2sos or df1tsos filter. For these structures, the secondary scaling points refer to the scaling locations between the recursive and the nonrecursive parts of the section (the "middle" of the section). By default, secondary-scaling points is not enabled. You use this with the Cumulative sections option only.

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Example — View the Sections of SOS Filters

After you design or import an SOS filter in to FDATool, the SOS view option lets you see the per section performance of your filter. Enabling SOS View from the View menu in FDATool configures the tool to display the sections of SOS filters whenever the current filter is an SOS filter.

These next steps demonstrate using SOS View to see your filter sections displayed in FDATool.

1. Launch FDATool.

2. Create a lowpass SOS filter using the Butterworth design method. Specify the filter order to be 6. Using a low order filter makes seeing the sections more clear.

3. Design your new filter by clicking Design Filter.

   FDATool design your filter and show you the magnitude response in the analysis area. In Current Filter Information you see the specifications for your filter. You should have a sixth-order Direct-Form II, Second-Order Sections filter with three sections.

4. To enable SOS View, select View > SOS View from the menu bar.

   Now you see the SOS View dialog box in FDATool. Options here let you specify how to display the filter sections.

   

   By default the analysis area in FDATool shows the overall filter response, not the individual filter section responses. This dialog box lets you change the display configuration to see the sections.

5. To see the magnitude responses for each filter section, select Individual sections.

6. Click Apply to update FDATool to display the responses for each filter section. The analysis area changes to show you something like the following figure.

   

   If you switch FDATool to display filter phase responses, you see the phase response for each filter section in the analysis area.

   

7. To define your own display of the sections, you use the User defined option and enter a vector of section indices to display. Now you see a display of the first section response, and the cumulative first, second, and third sections response:

   • Select User defined to enable the text entry box in the dialog box.

   • Enter the cell array {1,1:3} to specify that FDATool should display the response of the first section and the cumulative response of the first three sections of the filter.

8. To apply your new SOS View selection, click Apply or OK (which closes the SOS View dialog box).

   In the FDATool analysis area you see two curves — one for the response of the first filter section and one for the combined response of sections 1, 2, and 3.

   

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Reordering the Sections of Second-Order Section Filters

Importing and Exporting Quantized Filters

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