DspEdu 2.1 - IIR Filter Design Module

1. What does it do ?

This module implements the design of IIR Filters by Analog Filter Transformation .

Three approximation functions - Butterworth , Chebyshev I and Chebyshev II - are supported for Low Pass , High Pass , Band Pass and Band Elimination Type Filters. Filter Impulse Response Sequence ,Filter Impulse Response Sequence and Frequency Response of the Filter are plotted with Slider Scale value read-out feature.The pole-zero in z-plane will also be displayed.Only Bilinear Transformation is currently supported.

The filter design data can be saved into *.iir file in the default project directory. The frequency response data output also can be stored in *.txt file and the plots can be saved in *.ps (Postscript Format) files in the default project directory.

2. Tab Windows

2.1 'Design Data' Window

Contains pulldown boxes for selecting or entering 8 design parameters. The filter specifications are in discrete domain and the passband and stopband edges are in radians.The allowed maximum gain deviation in passband and stopband are to be entered in their real value and not in dB. The Bandwidth Parameter is used only in Bandpass and Band Elimination Designs. In that case the product of passband edges and stop band edges are taken to be equal and the values entered in the relevant boxes are taken to be the values pertaining to the left side of frequency response with respect to center of the band.

2.2 'Design Output' Window

Shows the design specifications , the calculated prewarped frequencies , order of the filter and discrete-domain poles and zeros with their multiplicity. The window content can be copied to clipboard by right clicking and invoking 'select all' under 'Edit' menu followed by 'Copy' under the same menu.

2.3'Pole-Zero Plot' Window

Shows the Pole-Zero plot in the z-domain with respect to unit circle.The multiplicity of poles and zeros will also be displayed.

2.4 'Frequency Response' Window

Shows the one-sided frequency response plots (when the window is raised frequency response at 1 degree interval over 0 to 180 degrees on the unit circle is calculated and the plots are updated if necessary) of the design currently on display in the 'Design Outputs' window. Shows Magnitude and Angle.

Two slider scales appearing at the bottom of the plots allow readout of index and spectral data at that index. Magnitude spectra will have even symmetry and Angle spectra will have odd symmetry.

The plot windows have cursor lines with value display. Cursor lines can be moved by one division by Left/Right arrow keys.Pressing Ctrl-Left or Ctrl-Right moves the cursor line by larger divisions for faster traversal.

2.5 'Time Response' Window

Shows the Impulse and Step Responses of the designed filter. The plot window comes with a slider scale which can be used to recalculate and display the waveforms with more number of points in them. The default display length is 64 points.

The plot windows have cursor lines with value display. Cursor lines can be moved by one division by Left/Right arrow keys.Pressing Ctrl-Left or Ctrl-Right moves the cursor line by larger divisions for faster traversal.

2.6 'Menu Help' Window

Raises the window and puts a brief help on the menu items in it.

3. Menubar Items

3.1 File Menu

3.1.1 Load Saved Project

Load a Sampled & Windowed Sequence and the Window Sequence used by opening a *.iir file saved in an earlier session.The default directory will be '/Projects/iirfilter'

3.1.2 Save Project
Brings up a standard Windows Save Dialog box which opens into '/Projects/iirfilter' by default and shows *.iir as the default extension.The number of points in the sequences and sequence data will be stored as an ascii file with selected name and 'iir' extension.The program assumes write permission in the chosen directory , overwrites with prompt and does not provide confirmation dialog.

3.1.3 Save Transfer Function
Brings up a standard Windows Save Dialog box which opens into '/Projects/tfa' by default and shows *.tfa as the default extension.The discrete-time poles and zeros and their multiplicity will be stored as an ascii file with selected name and 'tfa' extension so that it can be loaded in the DTFA (Discrete-Time Transfer Function Analysis Module) Module later for more detailed study of the frequency response. However the gain scale factor used in these two modules are different. In this module the designs assume that all the designed filters need a gain of unity in the passband and scaling is done suitably in frequency response and time response. No such scaling is done in the DTFA module and DTFA module takes the transfer function as the quotient of product of zero factors and product of pole factors. The program assumes write permission in the chosen directory , overwrites with prompt and does not provide confirmation dialog.

3.1.3 Frequency Response Outputs
Brings up a standard Windows Save Dialog box which opens into '/Projects/iirfilter' by default and shows *.txt as the default extension. The design data alongwith the 180 point frequency response data will be stored as an ascii file with selected name and 'txt' extension. The program assumes write permission in the chosen directory , overwrites with prompt and does not provide confirmation dialog.

3.1.5 Save Plots
Brings up a standard Windows Save Dialog box which opens into '/Projects/iirfilter' by default and shows *.ps as the default extension.The four plots which currently appear in the four plot areas will be saved as four postscript files with selected names and 'ps' extension. The program assumes write permission in the chosen directory , overwrites with prompt and does not provide confirmation dialog.

3.2 Options

3.2.1 Display Color Options

3.2.2 Display Mode

3.2.3 Set Frequency Points

3.2.4 Set Time Response Points

3.3 Magnify

3.4 Redraw

3.5 Help

3.6 Exit

4. Restrictions in the Evaluation Version

5. Author