Digital Filter Designer Basics

The Digital Filter Designer offers you an integrated environment for creating and simulating a filter design based upon your specifications. Once a filter is designed, you can view an analysis of its simulation in a Data Display window that is launched automatically. The data displayed in this window can also be configured to use a variety of plot and tabular options.

Digital Filter Designer uses the Advanced Design System project paradigm. A project acts as a container for the data generated in designing, simulating, and analyzing one or more filter designs. It is also used to store the analysis displays, filter coefficients, and implementation schematics.

You will need to create or open a design project before you begin using Digital Filter Designer for your filter design tasks. Once you open or create a project, you can begin the process of either modifying an existing filter design or creating a new filter design using one of several available methods to define the filter specifications.

In addition to being able to print the filter specifications and the contents of any given display, coefficients or filter taps generated using Digital Filter Designer can be exported in an ASCII format. Also, you can generate an implementation schematic of the digital filter.

Launching Digital Filter Designer

The Digital Filter Designer can be launched independently or from within the Advanced Design System (ADS). Depending upon your computer platform and configuration, you can launch the Digital Filter Designer using any one of the following methods:

From an ADS Schematic window:

Choose Tools > Digital Filter > Start Digital Filter from the Schematic window menubar for a signal processing design.

When you launch Digital Filter Designer this way, it uses the current design project directory and you can continue on to creating or modifying a digital filter design.

On a UNIX workstation:

To launch Digital Filter Designer without launching the ADS, type dfilter in a terminal window.

When you launch Digital Filter Designer in this manner, you need to open or create a project before you can continue on to creating or modifying a digital filter design.

Tip Navigate to within a project directory before you type dfilter in a terminal window to facilitate access to designs within the project directory.

On a PC:

To launch Digital Filter Designer without launching the ADS, choose Start > Programs > Advanced Design System 2008 > ADS Tools > Digital Filter Designer .

When you launch Digital Filter Designer this way, you need to open or create a project before you can continue on to creating or modifying a digital filter design.

Once Digital Filter Designer is launched and a project has been opened, the specification options in the Main window become active. Choose Help > What's This? from the Digital Filter Designer menubar and click within the window to display context-sensitive, online help for using the item you clicked.

Using Projects

Digital Filter Designer uses the Advanced Design System paradigm of projects. A project acts as an organizer of one or more designs that may be created to accomplish a larger, overall design goal. A project is also used to keep together all the information generated in the process of designing, simulating, analyzing, and implementing a digital filter design.

Creating a New Project

You will need to create or open a design project before you can begin using Digital Filter Designer for your design tasks. To create a new design project, choose File > New Project from the Digital Filter Designer menubar and display the New Project dialog box.

Enter a name and path for the new project or click Browse and use the New Project File Browser dialog box to define the path.

Once you have defined a name and path, click OK in the New Project dialog box to create the project. A feedback message is displayed to confirm that the current working directory has been changed to the project you specified.

The Digital Filter Designer window is then updated to display the default filter specifications which you can modify as desired to complete your digital filter design tasks.

Opening an Existing Project

To open and work within an existing project, choose File > Open Project from the Digital Filter Designer menubar and use the Open Project dialog box that is displayed to locate the existing project you wish to use.

Once you have selected the project you wish to use, click OK in the Open Project dialog box to open the project.

Managing Designs

Digital Filter Designer offers the standard file handling and management capabilities. Even though more than one file or component is used to make up a design file, the interface is designed to present a simple and familiar environment.

Creating a Design

Choose File > New Filter Design from the Digital Filter Designer menubar to create a new design file. An alert message is displayed to confirm whether you wish to save any changes made to the currently open file.

After you choose to save or discard any changes, a new, untitled design file is created and the Digital Filter Designer window is updated to display the default filter specifications which you can modify as desired to complete your digital filter design tasks.

Using Command Lines

Digital Filter Designer includes a command line interface that you can use to automate some of your filter design tasks. Choose Options > Command Line to display the Command Line dialog box.

By keeping this dialog box open while you complete your filter design tasks, you can generate a list of commands that you can reuse either directly or with any enhancements or parameters you wish to modify.

Opening a Design

Choose File > Open Filter Design from the Digital Filter Designer menubar to open an existing filter design file. An alert message is displayed to confirm whether you wish to save any changes made to the currently open file.
After you choose to save or discard any changes, the File Open dialog box is displayed to enable you to identify the existing file you wish to open.

Once you have selected the design file you wish to use, click OK in the File Open dialog box to open the file and proceed with your digital filter design tasks.

Retrieving Filter Designs

Choose Tools > Digital Filter > Retrieve and Place Current Ideal ( Scaled ) Coefficient Filter from an Advanced Design System Schematic Window to retrieve a filter design and generate a schematic using the specified filter coefficients. You must be running Digital Filter Designer to perform this operation. A schematic of the designed filter will be created and saved using the filename you specify.

Using the Design List

If the design file you wish to open is located within the current project directory, choose File > Design List from the Digital Filter Designer menubar to display the Design List dialog box that lists all the existing design files in the current project.

Click a design file to select it and click OK to open it. An alert message is displayed to confirm whether you wish to save any changes made to the currently open file. After you choose to save or discard any changes, the design file you selected is opened.

Using File History

If you wish to reopen one of the last four designs you created or modified using Digital Filter Designer, choose the design from the list at the bottom of the Digital Filter Designer File menu. An alert message is displayed to confirm whether you wish to save any changes made to the currently open file. After you choose to save or discard any changes, the design file you selected is opened.

Importing Ideal Coefficients

To design a digital filter using an existing set of ideal coefficients, choose File > Import Ideal Coefficients File from the Digital Filter Designer menubar and display the Import Ideal Coefficients File dialog box.

Enter a name and path for the file containing the ideal coefficients you wish to use or click Browse and use the File Import Ideal Coefficients dialog box to define the filename and path.

Once you have defined a name and path, click OK in the Import Ideal Coefficients File dialog box to import the file and proceed with your digital filter design tasks.

Importing an AEL Ideal Coefficients Function

Digital Filter Designer also enables you to test, view, edit, and import an AEL function you've created to generate the ideal coefficients for a custom, Gaussian, or Edge standard digital filter. Using this import function you can create your own special FIR filter that can then be scaled, quantized, and implemented for simulation and synthesis.

To design a digital filter using an AEL function to generate its coefficients, choose File > Import Ideal AEL Coefficients Function from the Digital Filter Designer menubar and display the Import Ideal AEL Coefficients Function dialog box.

Enter a name and path for the file containing the AEL function you wish to use or click Browse and use the File Import Ideal AEL Coefficients Function dialog box to define the filename and path.

You can use the Import Ideal Coefficients AEL Function dialog box to display and edit the function, and to test it. For details on using AEL to create a function, refer to Manipulating Arrays".

Click OK in the Import Ideal AEL Coefficients Function dialog box to import the function and proceed with your digital filter design tasks.

Note An AEL template that can be customized is included with the current installation of Digital Filter Designer. You can modify this template to design symmetric FIR filters according to GSM specifications using the desired rolloff, symbol period, and tolerable order values. The AEL template is located in the $HPEESOF_DIR/dfilter/function directory.

Ideal Coefficients File Format

The ideal coefficients file you import needs to be created using a specific format. An example template is shown here. Note that for an FIR filter, coefficients must be entered in a decimal format, while those for an IIR filter are entered in a matrix format.

FIR Filter	IIR Filter
BEGIN FIR# FIR % X 0.0012 0.234 ... 0.0023 END	BEGIN IIR# IIR (time_dim parm_format T t QuantizeCoeff) % X b(0,1)...b(0,2) a(0,1)...a(0,2) b(1,1)...b(1,2) a(1,1)...a(1,2) ... END IIR

Where:
time_dim specifies the units used for the sampling time step t. The available formats are SEC, MSEC, USEC, NSEC, and PSEC.
parm_format specifies the format used for the filter coefficients. The available formats are:
RI for real and imaginary
MA for magnitude and angle (degree)
DB for 20log (magnitude) and angle (degree)
T is placed to make the file more readable and to indicate the following number is the sampling time and t is the actual sampling time
QuantizeCoeff is used to indicate to the application the coefficients may be quantized, if needed.

Instrument Link

If your computer has a GPIB card, and its supporting software installed, you can use the Instrument Link feature. This allows FIR filter coefficients to be exported to instruments such as the Agilent ESG-D signal generator and eliminates the need to manually enter each coefficient with the FIR table editor.

To export a digital filter design, choose File > Export Ideal Coefficients from the Digital Filter Designer menubar. The Export Ideal Coefficients to Instrument dialog appears.

Enter the address of the instrument that you want to connect to. The GPIB address for the Agilent ESG-D can be found by pressing the Utility key on the instrument.

Enter the GPIB name. The GPIB, or symbolic interface name, is the name assigned to the GPIB interface card when the card is installed. The name is required by the Digital Filter tool to transfer data to an instrument. For the E2050A LAN/HP-IB gateway box, the symbolic name is in the form:
lan[IP_address]:hp-ib_name
For example, if an E2050 LAN/HP-IB gateway box with an IP address of 18.9.134.213 is to be addressed, the symbolic interface name is:
lan[18.9.134.213]:hp-ib
The symbolic interface name can be redefined at any time. For details on changing the hp-ib_name, refer to the documentation for the E2050A.
When you have entered the information, click OK to export the design data to the instrument.

Copying and Saving a Design

Choose File > Save As from the Digital Filter Designer menubar to save a copy of the currently open filter design file. Use the File Save As dialog box that is displayed to provide a name and location for the file you wish to create.

If you have computed the ideal and the implementation coefficients for the filter design, separate files containing each set of coefficients are saved as ASCII files using the MDIF format. The naming convention used for these files is <design_name>ideal.mdb and <design_name>scaled.mdb respectively and the files are saved in the synthesis subdirectory within the project directory. Files without the header information are also saved in the same location using the same naming convention and the .txt extension. Ideal coefficients from these files can be used for components within Agilent Ptolemy or any other application.
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
where
and
denote the numerator and denominator coefficients, respectively. For details on the coefficient formats, or for information on how the coefficient values can be displayed, refer to Displaying Coefficients in Analysis and Output. For details on using these coefficients, refer to Using Coefficients in Agilent Ptolemy Components in Analysis and Output.

Generating Outputs

Digital Filter Designer can be used to print filter specifications, display plots for analyzing filter designs, view filter coefficients, and generate schematic outputs.

Printing Specifications

Choose File > Print Filter Design from the Digital Filter Designer menubar to generate a printout of the filter specifications. Choose File > Print Setup from the Digital Filter Designer menubar to modify the printer and printing specifications, as desired.
The printed output lists the current filter, parameter, and implementation specifications.

Plotting an Analysis

Choose Design > Design Filter from the Digital Filter Designer menubar to design a filter based upon the entered specifications. After the filter design process is completed, various plots are displayed automatically for your analysis.

The data computed and plotted for your analysis depends upon the kind of filter being designed, FIR or IIR, and the analysis options you specify. To define the options and detail used in the display of plots for analysis, choose Design > Setup Analysis from the Digital Filter Designer menubar to display the Setup Analysis dialog box. Click Help within the dialog box to display online help instructions on the available options.

Use this dialog box to select the analysis options. Some of the options, such as the display of Poles/Zeros is only available for IIR filters and the Eye diagram plot option is available for all FIR filters. Group Delay is available for both IIR and FIR filters. Options that you select are used to display plots for analyzing your filter design.
Choose Design > Analyze from the Digital Filter Designer menubar to display the plots for designed filter.

Generating a Schematic

Choose Design > Generate Schematic from the Digital Filter Designer menubar to generate a schematic based upon a filter design. Use the Generate Schematic dialog box that is displayed to specify a name and type for the schematic to be generated.

The schematic is generated and displayed in a new schematic window that is launched within Advanced Design System.

Viewing Coefficients

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