Signal Processing Toolbox™

Analyzing the Filter

Displaying Filter Responses

You can view the following filter response characteristics in the display region or in a separate window (see Displaying the Response in FVTool):

• Magnitude response

• Phase response

• Magnitude and Phase responses

• Group delay response

• Phase delay response

• Impulse response

• Step response

• Pole-zero plot

• Zero-phase response — available from the y-axis context menu in a Magnitude or Magnitude and Phase response plot.

If you have Filter Design Toolbox product installed, two other analyses are available: magnitude response estimate and round-off noise power. These two analyses are the only ones that use filter internals.

For descriptions of the above responses and their associated toolbar buttons and other FDATool toolbar buttons, see fvtool.

You can display two responses in the same plot by selecting Analysis > Overlay Analysis and selecting an available response. A second y-axis is added to the right side of the response plot. (Note that not all responses can be overlaid on each other.)

You can also display the filter coefficients and detailed filter information in this region.

For all the analysis methods, except zero-phase response, you can access them from the Analysis menu, the Analysis Parameters dialog box from the context menu, or by using the toolbar buttons. For zero-phase, right-click the y-axis of the plot and select Zero-phase from the context menu.

For example, to look at the filter's magnitude response, select the Magnitude Response button on the toolbar.

You can also overlay the filter specifications on the Magnitude plot by selecting View > Specification Mask.

Note   You can use specification masks in FVTool only if FVTool was launched from FDATool.

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Using Data Markers

You can click the response to add plot data markers that display information about particular points on the response.

To move a data marker, grab its black square at the corner of the marker. Dragging the marker with your mouse changes the Frequency and Magnitude values.

To change the properties of a data marker, right-click the marker to display the properties menu:

• Alignment — Change the position of the marker. Available options are top-right, top-left, bottom-right, and bottom-left.

• Font Size — Change the font size.

• Movable — Allow the marker to be moved on the response.

• Interpolation — Select Nearest to force the marker to snap to nearest point along the plotted curve. Select Linear to interpolate between points along the plotted curve.

• Track Mode — Restrict the marker to be movable in the x, y, or xy direction.

• Delete — Delete the selected marker.

• Delete all — Delete all markers.

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Drawing Spectral Masks

To add spectral masks or rejection area lines to your magnitude plot, click View > User-defined Spectral Mask.

The mask is defined by a frequency vector and a magnitude vector. These vectors must be the same length.

• Enable Mask — Select to turn on the mask display.

• Normalized Frequency — Select to normalize the frequency between 0 and 1 across the displayed frequency range.

• Frequency Vector — Enter a vector of x-axis frequency values.

• Magnitude Units — Select the desired magnitude units. These units should match the units used in the magnitude plot.

• Magnitude Vector — Enter a vector of y-axis magnitude values.

The magnitude response below shows a spectral mask.

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Changing the Sampling Frequency

To change the sampling frequency of your filter, right-click any filter response plot and select Sampling Frequency from the context menu.

To change the filter name, type the new name in Filter name. (In fvtool, if you have multiple filters, select the desired filter and then enter the new name.)

To change the sampling frequency, select the desired unit from Units and enter the sampling frequency in Fs. (For each filter in fvtool, you can specify a different sampling frequency or you can apply the sampling frequency to all filters.)

To save the displayed parameters as the default values to use when FDATool or FVTool is opened, click Save as Default.

To restore the default values, click Restore Original Defaults.

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Displaying the Response in FVTool

To display the filter response characteristics in a separate window, select View > Filter Visualization Tool (available if any analysis, except the filter specifications, is in the display region) or click the Full View Analysis button:

This launches the Filter Visualization Tool (fvtool).

Note   If Filter Specifications are shown in the display region, clicking the Full View Analysis toolbar button launches a MATLAB figure window instead of FVTool. The associated menu item is Print to figure, which is enabled only if the filter specifications are displayed.

You can use this tool to annotate your design, view other filter characteristics, and print your filter response. You can link FDATool and FVTool so that changes made in FDATool are immediately reflected in FVTool. See fvtool for more information.

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Computing the Filter Coefficients

Editing the Filter Using the Pole/Zero Editor