Filter Design For DsPIC DSC Digital Filter Design And Analysis System
Filter Design for dsPIC DSCDigital Filter Design and Analysis SystemMomentum Data Systems, Inc.
Copyright/Trademark InformationCopyright 2008Momentum Data Systems17330 Brookhurst Street, Suite 230Fountain Valley, CA 92708World rights reserved. No part of this publication may be stored in a retrieval system, transmitted,or reproduced in any way, including but not limited to photocopy, photograph, magnetic or otherrecord, without the prior agreement and written permission of Momentum Data Systems.Information in this manual is subject to change without notice and does not represent a commitment on the part of Momentum Data systems. The software described in this reference guide isfurnished under a license agreement and may be used or copied only in accordance with the termsof the agreement.This manual was produced and printed using FrameMaker.FrameMaker is a registered trademark of Adobe.All trademarks are copyrighted to their respective owners.Microchip, dsPIC, and MPLAB are trademarks of Microchip Technology Inc. and are used underlicense.
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Filter Design for dsPIC DSCTable of contentsChapter 1 - Introduction1.1 Design Capabilities1-21.2 General System Features.1.3 Installation1.3.11-41-5Application Software Installation 1-51.4 Input Specifications1.5 Design Methods1-61-71.6 System Operation1-8Chapter 2 - Examples2.1 IIR Lowpass2-22.2 IIR Bandpass2-52.3 FIR Lowpass2-72.4 Equiripple FIR Lowpass2-102.5 Using a dsPICfd Filter in dsPICworksChapter 3 - System Operation3.1 Toolbar3.1.13-3Status Bar3.2 File Menu3-53-63.2.1 Load Specification File 3-73.2.2 Print 3-83.2.3 Printer Setup 3-83.2.4 System Settings 3-93.2.5 User Information 3-103.2.6 Exit 3-103.2.7 About dsPIC‘ FD Filter Design3.3 View Menu3-113-123.3.1 Toolbar 13-23.3.2 Status Bar 3-123.3.3 Text Filter File View 3-133.4 Design Menu3.4.13.4.23.4.33.5 Filter Menu3.5.13.5.23.5.33.5.43-22Lowpass 3-23Highpass 3-24Bandpass 3-25Bandstop 3-263.6 Output Menu3.6.13.6.23.6.33.6.43.6.53.6.63-14IIR Design 3-15FIR Design with Windows 3-17Equiripple FIR Design 3-203-27Plot Control 3-27Snap to Grid 3-30Quantization 3-30Create Specification File 3-31Create Filter Coefficient File 3-32Create Plot Data Files 3-333.7 Code Generation Menu3-342-12
3.7.13.7.23.7.3Create C Code 3-35C Code File Generation 3-35Microchip dsPIC30F/33F Code Generation3.8 Window Menu3.8.13.8.23-353-40Select Plots 3-41Display Control 3-41Chapter 4 - Filter Coefficient Files4.1 IIR FILTER COEFFICIENT FILES4.1.14.1.24-2General Discussion on Cascaded Biquad Sections 4-3Fractional Fixed Point Cascaded Biquad Sections 4-54.2 FIR FILTER COEFFICIENT FILEChapter 5 - Addendum5.1 References5-25.2 IIR Design Output Example5-34-9
CHAPTE R 1Introduction to FilterDesign for MicrochipTechnology dsPIC30F/dsPIC33FThis manual is a reference guide to the Filter Design System dsPICFd specificallydeveloped for use with Microchip Technology’s dsPIC30F/dsPIC33F family. It isintended to explain the usage of the system and what the various screen displays andcomputer printouts mean. It is not intended to be a tutorial on digital signal processingsince several excellent texts on the subject exist and it is assumed that the user has had acertain amount of academic or professional exposure to the subject. Several referencesare given in Chapter 4, should more in-depth information be required.This software tool dsPICFd will allow the user to design coefficients for digital filtersand provides a seamless manner of getting a filter routine and its associated coefficientsinto an MPLAB project.More advanced versions of QEDesign are also available. Please call Momentum DataSystems for further information. These systems support significantly more powerful filterdesign techniques.dsPIC Filter Design Reference GuidePage 1 - 1
1.1 Design CapabilitiesInfinite Impulse Response Filter Design: Lowpass, Highpass, Bandpass, and Bandstop Filters Filter orders up to 10 for Lowpass and Highpass Filters (4 for dsPICFdlite) Filter orders up to 20 for Bandpass and Bandstop Filters (8 for dsPICFdlite) Five Analog Prototype Filters are available:–Butterworth–Tschebyscheff–Inverse Tschebyscheff–Elliptic–Bessel Digital Transformations are performed by the following method:–Bilinear Transformation Method Reports show design details such as all transformations from normalized lowpassfilter to desired filterFinite Impulse Response Filter Design: Design Method Selection–FIR Windows Design–FIR Equiripple Design (Parks-McClellan) Lowpass, Highpass, Bandpass, and Bandstop filters An FIR filter can have up to 513 taps (64 for dsPICFdlite) Following window functions are supported:–Rectangular–Hanning (Hann)–Hamming–Triangular–Blackman–Exact Blackman–3 Term Cosine–3 Term Cosine with continuous 3rd Derivative–Minimum 3 Term Cosine–4 Term Cosine–4 Term Cosine with continuous 5th Derivative–Minimum 4 Term Cosine–Good 4 Term Blackman HarrisPage 1 - 2dsPIC Filter Design Reference Guide
–Harris Flat sian Reports show design details such as window coefficients and Impulse Responseprior to multiplying by the window function Filters are designed for a maximum gain of 1Code Generation Features The filter design program supports C callable assembly language reoutines thatimplement the designed filter. The user creates the desired filter within dsPICfd. When a satisfactory filter hasbeen created, the code generator can be invoked to create the files associated withthe filter. An option to create a C main program to execute the filter using the simulator isallowed. There is support for 32-bit coefficients in the dsPICFd system. This capability is notavailable for dsPICFdlite.Other General Features MPLAB C30 computer syntax is used for all calls to assembly language routines. Graphical Output includes:–Magnitude Response vs. Frequency–Log Magnitude vs. Frequency–Phase Response vs. Frequency–Group Delay vs. Frequency–Impulse Response vs. Time (per sample)–Step Response vs. Time (per sample)–Pole and Zero Locations (IIR only)dsPIC Filter Design Reference GuidePage 1 - 3
1.2 General System Features.Page 1 - 41.Recycling of input for comparative analysis - input for a filter is retained until a newspecification is called for. This allows various IIR designs to be compared with FIRwindow designs as well as FIR equiripple designs.2.Use of 64 bit floating point for all design calculations ensures that maximumaccuracy is maintained for the design calculations. Certain critical calculations arecarried out in 128 bit precision.3.Specification file which allows the retention and retrieval of filter specifications.4.Help screens for all data entry fields.5.Cursor tracking on all frequency domain plots. X and Y coordinates read outautomatically as the cursor is tracked on the function by holding the mouse buttondown and dragging across the window as desired.6.Graphical Zooming is available.dsPIC Filter Design Reference Guide
1.3 Installation1.3.1 Application Software InstallationThe system is automatically installed by executing the following command at the DOSprompt or the run dialog.For dsPICFD: drive :\dsPICFD.msiFor dsPICFDLite: drive :\dsPICFDLite.msiNote: this executes SETUP.EXE on the cd which decompresses the files and installs thesoftware group.dsPIC Filter Design Reference GuidePage 1 - 5
1.4 Input SpecificationsThe magnitude specification of the passband of a filter is given in the form-20log10(1.0-δ1)This assumes the magnitude range of the passband is from [1-δ1,1], δ1 0 and δ1 1Stopband specifications are given in the form-20log10(δ2)Thus the magnitude of the stopband is from [0, δ2] and δ2 1Note that: -20log10(1.0-δ1) and -20log10(δ2) are positive numbers. These values are oftenreferred to as passband ripple and stopband ripple, but a more accurate description wouldbe maximum passband attenuation and minimum stopband attenuation.These forms are used throughout all IIR and FIR designs for Lowpass, Highpass,Bandpass and Bandstop Filters.1.01-δ1δ2FrequencyAll IIR and FIR filters have their stopband and passband filters specified in this manner.There are no unique specifications for each filter type. Such transformations if requiredfor design calculators are all handled automatically by the design program.Note that the definitions of the passband and stopband values are positive numbers.Furthermore -20log10(1.0-δ1) -20log10(δ2) .Page 1 - 6dsPIC Filter Design Reference Guide
1.5 Design MethodsFor the IIR design, a normalized lowpass analog transfer function is generated based onthe given filter specifications. This normalized lowpass analog transfer function is thentransformed to an unnormalized analog transfer function (lowpass,highpass,bandpass orbandstop) via the analog frequency transformation formulae. Values for the frequencytransformation are suitably chosen to compensate for the prewarping caused by thesubsequent bilinear transformation.The unnormalized transfer functions for Lowpass, Highpass, Bandpass and BandstopFilters are then transformed to the digital domain via the bilinear transformation. Detailsof the transformations are printed in the output file SFIL.OUT. Output for each designwill be accumulated in this file until it is printed. The filter characteristics including theimpulse response are simulated via cascaded second order sections with the poles andzeros grouped using the L. B. Jackson algorithm1 to minimize stability problems.For the FIR design with windows, the number of taps in the filter are determined usingthe window calculation, again based on input specifications. The coefficients for the tapsare then determined by using the Fourier series design techniques for computing theimpulse response and the window coefficients.The Equiripple FIR design uses the Remez2 exchange algorithm to determine an optimalsolution. The solution is optimal in the minimax sense - that is the maximum error overthe entire frequency band is minimized. This produces equiripple results in both thepassband and stopband.1. Jackson, L. B. Digital Filters and Signal Processing - see Addendum (8)2. Jackson, L. B. Digital Filters and Signal Processing - see Addendum (8)dsPIC Filter Design Reference GuidePage 1 - 7
1.6 System OperationThe dsPICFD system was designed to be intuitively easy to use without sacrificing anyof the powerful design capabilities required for a sophisticated engineering application.The top menu bar provides all the necessary options to design filters and will savepreviously inputted data thus allowing specifications to be changed with the minimum ofeffort. To alter a field, simply move the cursor into the relevant field using the mouse orEnter keys and re-enter the desired data.The menus and dialogs are largely self-explanatory, thus allowing the system to be usedwith the minimum of difficulty. The user merely selects the desired option/s by followingthe menu and dialog prompts. All dialog boxes have a cancel box which will cause thesystem to revert to the previous dialog box.It is not necessary to input data in floating point format as the system will automaticallyconvert integers to the correct format. For example, 5000, 5000.0, and 5E03 are all equallyacceptable forms of input.To start the system: Double-click on the dsPICFD icon to launch the system or enter thefollowing command: drive: \path\ dsPICFD.exeor for dsPICFDlite use: drive: \path\ dsPICFDlite.exePage 1 - 8dsPIC Filter Design Reference Guide
CHAPTE R 2dsPIC Filter DesignExamplesThe authors of dsPICFD believe that the most efficient way to become familiar with theimplementation and potential of the system is to step through the examples provided. Thefollowing pages contain examples of the various design methods and are organized in thefollowing manner:DESIGN METHOD and PROBLEM STATEMENT TOGETHER WITH INPUT,OUTPUTS are SFIL.OUT (Analysis provided with each design) and PLOTS.The examples implemented are:TABLE 2-1Example ListingExampleFilter Type1IIR Lowpass (Tschebyscheff)2IIR Bandpass (Elliptic)3FIR Lowpass (Kaiser)4Equiripple FIR (Lowpass)dsPIC Filter Design Reference GuidePage 2 - 1
2.1 IIR LowpassThe first example is an IIR design with an inverse Tschebyscheff analog prototype.FIGURE 2-1Lowpass Filter SpecificationsThe frequencies are specified in Hertz. The sampling frequency is set at 5000 Hz, thepassband frequency is set at 800Hz and the stopband frequency is set to 1600 Hz. Notethat for a lowpass filter the stopband frequency must be greater than the passbandfrequency and both frequencies must be less than half the sampling frequency.Both the passband and the stopband ripples are specified in dB (see Section 1.4 ). Theripple values are all specified as positive values. The passband value must be thestopband value. Typical passband values are 0.1 to 3 dB. Typical stopband values are40dB or greater.Page 2 - 2dsPIC Filter Design Reference Guide
FIGURE 2-2Select Analog Prototype and Filter OrderInverse Tschebyscheff is selected for the analog prototype. The filter order will defaultto 5 unless overridden in this dialog box.dsPIC Filter Design Reference GuidePage 2 - 3
FIGURE 2-3System Responses for Example 1Some interesting things to note about this filter design - the zeros on the unit circlecorrespond to the frquencies in the log frequency plot where the response has a null .xxxxPage 2 - 4dsPIC Filter Design Reference Guide
2.2 IIR BandpassThis example is an IIR bandpass filter with an Elliptic Analog prototype. Since this is abandpass filter, there are two passband frequencies. The passband frequencies must becontained within the two stopband frequencies. This is explained in detail in the HELPmenu.FIGURE 2-4Bandpass Filter SpecificationsThis filter has relatively narrow transition bands. The Elliptic filter design method willbe significantly more efficient than other analog prototypes. In general, unless one hasa specific reason for picking a different analog prototype, the Ellliptic filer prototype should be used for IIR filter design.FIGURE 2-5Select Analog Prototype and Filter OrderdsPIC Filter Design Reference GuidePage 2 - 5
The elliptic filter design is selected and the default filter order of 8 is also selected.FIGURE 2-6System Responses for Example 2Note that there are ripples in both the passband and the stopband. This is characteristicof all IIR filters that are generated using the elliptical designPage 2 - 6dsPIC Filter Design Reference Guide
2.3 FIR LowpassThe same lowpass filter specifications as in Example 1 are used for this FIR lowpass filter.FIGURE 2-7Lowpass Filter SpecificationsSeveral window functions are available for FIR design. However, the Kaiser windowfunction is usually selected for this type of design.The frequencies are specified in Hertz. The sampling frequency is set at 5000 Hz, thepassband frequency is set at 800Hz and the stopband frequency is set to 1600 Hz. Notethat for a lowpass filter the stopband frequency must be greater than the passbandfrequency and both frequencies must be less than half the sampling frequency.Both the passband and the stopband ripples are specified in dB (see Section 1.4 ). Theripple values are all specified as positive values. The passband value must be thestopband value. Typical passband values are 0.1 to 3 dB. Typical stopband values are40dB or greater.dsPIC Filter Design Reference GuidePage 2 - 7
FIGURE 2-8Select Window Function and Filter OrderThe Kaiser window design is selected with a default order of 19. It appears from the filterorders that a rectangular window would be better. However, a rectangular window willnot provide a design where the first sidelobe of the stopband will be less than 45 dB.Page 2 - 8dsPIC Filter Design Reference Guide
FIGURE 2-9System Responses for Example 3An FIR filter has ripples in both the passband and stopband. The ripples in the stopbandare easily seen in the magnitude plot. The ripples in the passband can be seen by zoomingin on the passband portion of the log magnitude or magnitude plot.Also note that the phase plot is essentially a straight line and the group delay is a constant.Both of these phenomena are characteristic of an FIR design with symmetric or antisymmetric coeeficients. If an FIR filter does not have symmetric or anti-symmetriccoefficients, then the group delay is not a constant.dsPIC Filter Design Reference GuidePage 2 - 9
2.4 Equiripple FIR LowpassThe same lowpass filter specifications as in Examples 1 and 3 are used for this FIR lowpassfilterFIGURE 2-10Lowpass Filter SpecificationsThe Equiripple design can make use of both ripple values in the design of the filter whereasthe FIR design with windows cannot. The FIR design with windows uses the only oneripple value and will over design the other ripple value. The FIR design uses the smallestripple value. This is almost always the stopband ripple. Hence, the FIR design withwindows cannot be as efficient as the equiripple deisgn.FIGURE 2-11Select Filter OrderNote that the filter order is 11 - much less than the Kaiser window design.Page 2 - 10dsPIC Filter Design Reference Guide
FIGURE 2-12System Responses for Example 4The magnitude plot for this design is considerably different than the magnitude plot forthe Kaiser window design. The equiripple design makes use of the two ripple values andthus requires fewer taps or coefficients to achieve the desired design specifications.An FIR window design has only one ripple value for its design calculations. The smallestripple value will be used for the design calculation - usually this is the stopband ripplevalue - hence the filter is over-designed in the passband. What that means is that thepassband ripple is smaller than requested by the user.dsPIC Filter Design Reference GuidePage 2 - 11
2.5 Using a dsPICfd Filter in dsPICworksA filter can be exported for use in dsPICworks.Just create an FLT file. Then select said file when applying a filter in dsPICworks.Page 2 - 12dsPIC Filter Design Reference Guide
CHAPTE R 3System OperationSystem operation is controlled via the standard graphical user interface of a main menubar with pull-down menus and dialog boxes. The main menu bar consists of the followingentries:TABLE 3-1Main Menu BarSelectionDescriptionFileAllows loading of previously stored filter and analysis inputspecificationsViewView the design data and filter filesDesignSelection of Filter Design OptionsFilterSelection of Filter TypesOutputSelects plotting options and output filesCodegenGenerates dsPIC assembly codeWindowStandard Windows Operations to select output and graphicaloptionsdsPIC Filter Design Reference GuidePage 3 - 1
After the system has been started, the following screen will be displayed:FIGURE 3-1Start-Up ScreenThe FILE, VIEW, DESIGN, FILTER, OUTPUT, CODEGEN and WINDOW menu itemsare covered in this chapter.Page 3 - 2dsPIC Filter Design Reference Guide
3.1 ToolbarThe toolbar allows the user to select the most commonly used features by button.The toolbar appears as follows:FIGURE 3-2Toolbar buttonsThe buttons and equivalent Menu options are detailed below in the order in which theyappear on the toolbar.Load FilesFILE/Load Filter Specification FileAbout/Printer OptionsFILE/About,FILE/PrinterPlot Display OptionsWINDOW/Cascade,WINDOW/TileDesign Type C Filter Design Reference GuidePage 3 - 3
Filter s,FILTER/BandstopQuantization OptionOUTPUT/QuantizationStart DesignFILTER/Start DesignOutput OptionsOUTPUT/Create Filter Specification File,OUTPUT/Create Filter Coefficient FileOUTPUT/Create C Code File.Page 3 - 4dsPIC Filter Design Reference Guide
3.1.1 Status BarFIGURE 3-3Status Bar DisplayThe Status Bar which appears at the bottom of the screen displays the followinginformation:StatusSnap to Grid On/OffQ16 - ResolutiondsPIC Filter Design Reference GuidePage 3 - 5
3.2 File MenuSelect the file menu by placing the cursor over the word FILE, click and hold the selectionby moving the cursor to the appropriate item in the drop-down menu, prior to releasingthe mouse button. Note that the enabled options are listed in black with disabled optionsappearing in gray. The FILE menu appears as follows:FIGURE 3-4File MenuCertainly the most powerful feature of this menu is the ability to automatically loadpreviously stored specifications via FILE/Load Specifications.Page 3 - 6dsPIC Filter Design Reference Guide
3.2.1 Load Specification FileIf the FILE/Load Specifications option was selected, the following dialog box will bedisplayed.FIGURE 3-5Filter Specification File SelectionAll filter specification files suffixed by .SPC will be displayed. A filter specification filecan be created by selecting OUTPUT/Create Specification File after a filter design iscomplete. The dialog box corresponding to the current design method will be startedautomatically after a specification file is selected. If necessary, the design method will bechanged to a method that can design the filter.dsPIC Filter Design Reference GuidePage 3 - 7
3.2.2 PrintOn completion of a filter design, filter plots can be printed. The print menu does not applyto text windows. Text files must be printed outside of QED Lite.FIGURE 3-6Print SelectionNote: hardcopy plots are always printed in black and white.3.2.3 Printer SetupSelect the desired printer setup. Note these parameters are not saved after the system exits.Page 3 - 8dsPIC Filter Design Reference Guide
FIGURE 3-7Printer SelectionThe printer setup box will vary according to the type of printer.3.2.4 System SettingsThe system settings dialog box controls global aspects of dsPIC FD operations.FIGURE 3-8System Setting OptionsdsPIC Filter Design Reference GuidePage 3 - 9
Exit Mode: These options determine what information is retained on exiting theapplication. Open a view window when a text file is created: If this option is selected, a Viewwindow is opened with the contents of the text file. This applies to all output files.3.2.5 User InformationThe text in the User Information will be displayed on the graphical displays and printouts.FIGURE 3-9User Information Screen3.2.6 ExitExits the application.Page 3 - 10dsPIC Filter Design Reference Guide
3.2.7 About dsPIC FD Filter DesignDisplays the current version number and technical support information. Technical supportis available for customers subscribing to maintenance contracts - please contact our salesdepartment for further information.FIGURE 3-10About ScreendsPIC Filter Design Reference GuidePage 3 - 11
3.3 View MenuThe items for this menu are shown as follows:FIGURE 3-11View Menu Items3.3.1 ToolbarThis option toggles the Toolbar display on and off.3.3.2 Status BarThis option toggles the Status Bar display on and off.Page 3 - 12dsPIC Filter Design Reference Guide
3.3.3 Text Filter File ViewAll files can be read in text format. Simply select the file type by scrolling through theavailable file types. Text files cannot be printed within dsPIC FD.FIGURE 3-12Text DisplaydsPIC Filter Design Reference GuidePage 3 - 13
3.4 Design MenuThis section describes the filter design capabilities of the system. Infinite Impulse Response (IIR) Design Finite Impulse Response (FIR) Design Equiripple FIR DesignThe following menu is the primary means of starting the design of digital filters.FIGURE 3-13Design MenuThe various design methods and filter types will be discussed in some detail although, itshould be noted, that some filter types are specific to a particular design method.An output file (SFIL.OUT) is automatically created showing detailed calculations for thedesigns.Page 3 - 14dsPIC Filter Design Reference Guide
3.4.1 IIR DesignSelects the system design method to Infinite Impulse Response (IIR) design. IIR designsall have feedback loops in the difference equations that implement the filters - that is theoutput value in a linear function of previous output values as well as current and previousinput values. IIR designs are classical filter designs using analog design methods whichtransform the resulting S domain filter to a Z domain filter.FIGURE 3-14IIR Design OptionsFilter TypeIIR designs for Lowpass, Highpass, Bandpass and Bandstop filters all use analogprototypes. After selecting “Next” on the IIR Design Menu the following dialog box willbe displayed.FIGURE 3-15Estimated Filter Order/Analog TypedsPIC Filter Design Reference GuidePage 3 - 15
The estimated orders for the five types of analog filters are displayed. In selecting a filtertype, one should bear in mind certain salient characteristics of these analog filters. TheButterworth filter is maximally flat in the passband and exhibits no ripples in either thepassband or the stopband. The Tschebyscheff and Inverse Tschebyscheff exhibit a morerapid cutoff than the Butterworth filter, but at the expense of ripples in either the passbandor the stopband. The Tschebyscheff filter has its ripples in the passband whereas theInverse Tschebyscheff filter has its ripples in the stopband. The Elliptic filter has thesharpest cutoff from the passband to the stopband, but has ripples in both the stopbandand the passband and has a higher nonlinearity in its phase response. These characteristicscarry over to the digital form of the transfer function. The Bessel filter order is estimatedusing the Butterworth order calculation. This order is probably optimistic and some finetuning is required. The Bessel filter does not
Page 1 - 2 dsPIC Filter Design Reference Guide 1.1 Design Capabilities Infinite Impulse Response Filter Design: Lowpass, Highpass, Bandpass, and Bandstop Filters Filter orders up to 10 for Lowpass and Highpass Filters (4 for dsPICFdlite) Filter orders up to 20 for Bandpass and Bandstop Filters (8 for dsPICFdlite) Five Analog Prototype Filters are available:
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When you first begin using dsPIC IAR C/EC Compiler, you should read Part 1: Using the compiler in this reference guide. When you are thoroughly familiar with the dsPIC IAR C/EC Compiler and have already configured your project, you can f
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