Analysis and Output

This chapter describes the steps involved in analyzing and generating the output of a digital filter design. It also includes background information on the available options. Digital Filter Designer provides you with a number of analysis and output options from printing specifications to analyzing filter characteristics and generating implementation schematics.

Once a filter is designed, its characteristics are displayed automatically. By default Digital Filter Designer generates both ideal and quantized coefficients for a filter that you design. These coefficients can be displayed in a table then imported into Agilent Ptolemy filter components. You can also generate schematics for further simulation using Advanced Design System.

The analysis and output options for a digital filter design using Digital Filter Designer are:

Printing the Filter Specifications

Digital Filter Designer enables you to print the filter design specifications for future reference and use. Printing the filter specifications can be accomplished using the standard functionality of the Print and Print Setup functions.

The printed output lists the filter, parameter, and implementation specifications.

Filter type: IIR
Response type: Lowpass
Method type: Chebyshev I
Order: Auto order
Frequency unit: MHz

Defining the Analysis Options

Use the setup analysis options to define the analyses that are plotted, to define the detail used in the display, and to retain previous data for comparison.
To customize the analysis that is plotted:

  1. Choose Design > Setup Analysis from the Digital Filter Designer menubar to display the Setup Analysis dialog box.
  2. Define the level of detail to be used.
    The detail used in the display is determined by the number of frequency points and by the number of unit pulse response points.
    Frequency Points
    Increasing the number of points leads to a corresponding increase in the display detail. The number of frequency points is always rounded and expressed as a power of two.
    Number of Unit Pulse Response Points
    Increasing the number of points leads to a corresponding increase in the pulse duration for which the pulse response is computed. This setting is only useful for an IIR filter because the unit pulse response for an FIR filter is always a finite duration.
  3. Specify the data to be displayed.
    Select one or more of the Compute options as desired. Data for the options you select here is displayed in the graphs that are generated for analysis.
    Frequency Response
    Select this option to plot the frequency response for the filter design.

    Unit Pulse Response
    Select this option to plot the unit pulse response for the filter design.

    Unit Step Response
    Select this option to plot the unit step response for the filter design.

    Group Delay
    Select this option to plot the group delay response for the filter design. This option is only available for an IIR filter.

    Poles/Zeros
    Select this option to plot the poles and zeros for the filter design. This option is only available for an IIR filter.

    Eye Diagram
    Select this option to plot the eye diagram and eye closure for a FIR filter design. Keep in mind that a lower eye closure can be achieved with an even order filter.
  4. Plot the specifications.
    Select the option to include a plot of the magnitude in the frequency response plot. The magnitude plotted is based on the filter specifications and can be used to visually verify the shape of the filter based upon the specifications you have entered.
  5. Keep data for comparison.
    Select the option to keep previous data if you wish to retain it for a comparative analysis of different iterations of a design.

Displaying the Analysis Plots

The analysis plots for a filter are displayed automatically in a Data Display window after the filter design process is completed. In addition, analysis plots for a filter that has been designed previously can also be displayed.

Editing an Analysis Plot

Click a plot to select it and choose Edit > Item Options to display the Plot Traces & Attributes dialog box. Use the Datasets and Equations drop-down list in the dialog box to choose the dataset . Click data from the list on the left and click Add to move it to the list on the right. Digital Filter Designer consistently uses the label "data" for any data that is generated while designing a filter.

Repeat this process to add all the desired data before you click OK to close the dialog box and display the plot.
Refer to the online help or printed documentation for details on using the features and functionality available within the Data Display environment.

Displaying Coefficients

Coefficients for a filter can be displayed after the design process is completed. In addition, coefficients for a filter that has been designed previously can also be displayed.
Digital Filter Designer generates two sets of coefficients, ideal and scaled, for a filter design.

Choose View > Coefficients from the Digital Filter Designer menubar to display the coefficients generated for the designed filter. A table of the filter coefficients is displayed in the Data Display window.


In the case of an FIR filter, the coefficients are listed in a single column. To scroll through the table of coefficients, click the table to select it and choose the desired option from the View > Scroll Data submenu in the Data Display window menubar or click the appropriate button in the Data Display window toolbar.
Refer to the online help or printed documentation for details on using the features and functionality available within the Data Display environment.
The coefficients for an IIR filter in each row correspond to a second-order section and the coefficients are listed as
corresponding to the transfer function of the section as follows:

For the scaled coefficients, the pairing logic used follows the rule that pairs the poles closest to the unit circle with the zeros nearest to them. In addition, the sections are ordered using the logic that it is best to keep the sections in a sequence from the least peaked to the most peaked (typically those with poles closest to the unit circle). For more information on the pairing and ordering logic used for IIR filter coefficients, refer to "Digital Filters and Signal Processing" by Leland B. Jackson, pp. 321-323.


Generating a Schematic

When generating a schematic, keep in mind that the topology of the schematic you generate is affected by the implementation options you have selected. For information on the available implementation options, refer to Choosing an Implementation Structure in Implementing a Digital Filter.

To generate a schematic from your digital filter design:

  1. Choose Design > Generate Schematic from the Digital Filter Designer menubar to display the Generate Schematic dialog box.

    If you disabled conversions by choosing Options > Disable/Enable Conversion , or if you selected Floating Point as the numeric format while designing the filter, you will be able to generate a schematic for only an Ideal (infinite precision) filter.
    Specify the filename to be used for saving the schematic. A scaled floating-point or single precision schematic can't be generated if the simulator does not provide the library support for the required components.
  2. Enter the desired specifications in the dialog box and click OK or Apply to generate the schematic.
    Note Keep in mind that various implementation structures are used to minimize quantization effects that occur as a result of coefficient scaling with the use of a fixed-point numeric format and finite-precision arithmetic. As a result, when you use the ideal floating-point numeric format, the schematic generated remains constant for all implementation structures.
  3. The generated schematic is displayed in a new Schematic window within Advanced Design System.
    If Advanced Design System is not running, it is launched and then the schematic is displayed in a new Schematic window.

The generated schematic can now be edited or printed on its own. It can also be placed within another schematic, if desired.

Note In a schematic generated for a scaled filter design, a component is added to provide global parameters for the entire filter structure. Use this component to change design parameters such as bit width to propagate the change across the entire design rather than manually editing the parameters for each component.

Retrieving and Placing Coefficients

Given an existing filter design, you can generate a schematic from its coefficients directly from within an Advanced Design System Schematic window. To do so, choose Tools > Digital Filter > Retrieve And Place Current Ideal Coefficient Filter or Tools > Digital Filter > Retrieve And Place Current Scaled Coefficient Filter and use the dialog box that is displayed to specify a filename for the schematic. Using the coefficients you choose to retrieve (ideal or scaled), a new schematic is generated and saved using the filename you specify.

Saving a Design File

Choose File > Save Filter Design to save changes to an existing filter design file. If you wish to create a new filter design file or save changes to a new file, choose File > Save As and use the File Save As dialog box to save the file using the desired filename and location.

If you have computed the ideal and the implementation coefficients for the filter design, separate files containing each set of coefficients are typically saved in the synthesis subdirectory as ASCII files using the MDIF format. These files are used by Digital Filter Designer to store information on the filter you design. The naming convention used for the .mdb files is <design_name>ideal.mdb and <design_name>scaled.mdb .

In the MDIF file, the coefficients for an FIR filter are listed in a single column whereas those for an IIR filter are listed in six (6) columns where each row corresponds to a second-order section and the coefficients are listed as
corresponding to the transfer function of the section as follows:

Coefficient data without the header and footer information is also typically saved in the synthesis subdirectory as ASCII files using the .txt extension. Use these text files to add the filter coefficients to a component within Advanced Design System or your application of choice. The naming convention used for the .txt files is <design_name>ideal.txt and <design_name>scaled.txt .

In the text files, the coefficients for an FIR filter are listed in a single column just as in the MDIF file. Coefficients for an IIR filter are listed in six (6) columns where each row corresponds to a second-order section and the coefficients are listed as
corresponding to the transfer function of the section as follows:

This format facilitates the use of this data within Advanced Design System.

Using Coefficients in Agilent Ptolemy Components

Ideal filter coefficients that you generate using Digital Filter Designer can be read into Agilent Ptolemy components you place within a schematic.
To use the filter coefficients for a component:

  1. Place the component.
    From the Numeric Signal Processing palette, choose the FIR or the BqdCscd (IIR) component and place it on the schematic.
  2. Edit the component parameters.
    Select the component and then choose Edit > Component > Edit Component Parameters to display the parameters for the component.

Click the Taps parameter and enter the relative or absolute path and filename for the text file containing the coefficients you wish to use. Be sure to enter a left angle bracket (<) before the path and filename.

Click Apply or OK to close the dialog box and use the filter coefficients from the specified file.

 

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