Michael J Wooding G6IQM

Filtech Professional - A Review

VHF Communications 1/1996

Dabbling a little in electronics as I do I have in the past ‘designed’ filters the hard way - you know: Now I know that there are far cleverer engineers than me, but I equally know that there are many of you out there who use the above method, going round and round until the desired result, or a close approximation of it, is produced. However, help is at hand. Reach for your trusty PC and run Filtech or Filtech Professional from Number One Systems, or if you have not yet purchased your copy, read on and see what you are missing!

FILTECH

Filtech and Filtech Professional are advanced circuit synthesis programs that produce both active and passive filter circuits from your specifications. With no mathematics or tedious manipulation of tables required, these programs make the design of complex filter circuits fast, simple, accurate and in terms of development time, very cost effectively.

All that you need to specify for your filter are the frequency limits for the type of filter (i.e. lowpass, highpass, bandpass or bandstop), the stopband attenuation required and the maximum acceptable passband ripple. Then simply run the program, fine tune the design if required and the filter is simulated and designed for you, including all the component values.

Both versions of the program include a built-in circuit simulator allowing you to test the circuit immediately, comparing the simulated result with the original specification. As with all computer design programs the values calculated for the components are real and not necessarily preferred values. However, you can force the program to select components to the nearest preferred value from a list of different ranges and check the effect on the filter performance at a glance.

Filtech can design Bessel, Butterworth and Chebyshev filters up to 6th order types, whereas the enhanced version Filtech Professional can design the above plus Caur (elliptic) up to 12th order filters.

The essential features of Filtech are: The computer requirements for running either version of the program are an IBM PC or clone running under MS-DOS version 3 or later, an EGA or VGA screen and a minimum of 640k of RAM. To use all of the features of the program a mouse is also required. Supported printers are 9 or 24-pin Epson or IBM Graphics printers, Laserjet II Laser printers or equivalent. The programs will also run as DOS applications under Windows 3 and 3.1, Windows for Workgroups 3.11 and Windows 95 (and probably Windows NT as well, but as I don’t run that version I cannot say for sure).

DOCUMENTATION

As with all Number One Systems software packages these programs come with an extensive instruction manual in A5 binder format. This manual follows their well tried and tested format of a program Overview, followed by installation Instructions, a chapter on First Impressions, the Grand Tour, a Reference section, a chapter detailing Customisation, Netlist formats, linking to Analyser III (AC Circuit Simulation software package from Number One Systems), a chapter on Filter Design and numerous appendixes of filter circuits relating to Filtech designs.

The program Overview and Installation chapters are concise and detail the essential properties of the software and all you need to know to load it onto your hard drive.

The First Impressions chapter is in reality not just a quick tour of the major components of the programs, but actually a tutorial on the basic aspects of running the package and producing filter designs to example specifications, and reviewing how to fine tune a specification for the real world.

The chapter Grand Tour now takes you step-by-step through all the functions of the software at least once, guiding you through detailed filter design simulations producing working filter specifications for practical filters.

The Reference chapter describes in greater detail the options of the Specify, Parameters and View menus and how and when to use the powerful features available.

The Customising chapter allows you to configure the package specifically for your system and operation. The appendixes give information on various aspects of running the software in association with other packages and DOS, detailed information on filter design aspects and all the circuits that Filtech uses in its simulations.

RUNNING FILTECH

Filtech is essentially a DOS package, but as stated earlier runs perfectly well as a DOS application within Windows 3, etc. To run from windows of any ilk nothing more than creating an Icon (in Windows 95 a ‘Shortcut’ to the *.exe program) is necessary. For those of you like me who are graphically oriented, there is no icon provided for Windows so you just have to be inventive. For Windows 95 users nothing need be changed in the properties of the shortcut, unless of course you want to, but Filtech runs perfectly well in a standard DOS window without modification.

Upon execution of the program you are presented with the design screen as shown in Fig.1. The screen is essentially a piece of graph paper on which the spectrum of the filter performance will be plotted. At the top of the screen is the familiar tool bar of drop-down menus and at the bottom of the screen is an array of user-variable filter parameters.

The drop-down menus are: Configuration; Specify; Parameters; View; !Tools; Help; Quit; File; Model; Type and X-scale.

Configuration: this menu deals with the path settings, file parameters and general hardware settings for the program, printing, etc. Specify: this menu is a sort of non-menu! What selecting this feature does in fact is to return you to the original design screen, which then allows you to change any of the settings and parameters prior to running the simulation again. Parameters: this menu allows you to select the style of filter to be designed (i.e. Chebychev, Butterworth, etc.), the allowable ripple and the order of filter. View: this is the ‘all-action’ selection box. Clicking on this item runs the simulation software and plots on the screen the actual spectrum of the filter as determined by the parameters previously entered.

!Tools: this menu gives access to basic DOS tools and also give the facility to print a Netlist for the designed filter (a Netlist is a list of components and their values and circuit specifications, etc.).

Help: not surprisingly this menu gives access to the in-built help dialogues.

Quit: guess!

File: the menu for loading and saving filter specifications and for printing screen dumps.

Model: the menu for selecting the type of filter (i.e. lowpass, highpass, etc.).

Type: is a click-selectable box for selecting Passive or Active filter types.

X-Scale: this box selects either linear or logarithmic scaling of the spectrum display. The design screen, as described earlier, is essentially a graph on which the spectrum of the filter is plotted. The design screen has on it graticule lines representing the frequency limits, attenuation and ripple characteristics as determined by the user-selectable parameters. As these selectable parameters are changed so the screen display graticule alters in accordance with the values entered.

Once the filter specifications have been entered then a simple click on the View menu sets the simulation package into motion. Upon completion of the simulation a plot of the filter is printed onto the screen (Fig.2). If the design is not quite as it should be then any of the parameters can be altered and the simulation rerun, which can be repeated until the spectrum meets the specification.

However, there is a problem with systems that use mathematical analysis to solve a problem rather than the empirical method as I have used in the past described at the beginning of this review. The problem is that the science can be too exact, per se: components end up having values such as 2.36947W or 3.68956229pF - not exactly what one would call helpful in the world of the Exx ranges of components generally available! Fear not, the answer is at hand. A feature within the File menu called Overlay Preferred is the solution to this problem. By selecting this item a sub menu appears giving the choice of selecting preferred value series for the resistors, capacitor and inductors, or the choice of leaving the values as True Value, that is the values as calculated by Filtech.

Having selected the required preferred ranges for the components then a press of the Escape key closes the menu and automatically reruns the simulation. The resulting spectrum is plotted on the screen in red leaving the original plot viewable (Fig.3), so that not only can the plot be compared with the selected parameters but it can also be compared with the original mathematical plot.

Now that we have fine tuned our filter design it is time to commit this theoretical device to a practical buildable unit. To do this you once again select the File menu and from it select Print the Netlist, which results in a printout similar to Fig.4. Now what do you do? Refer to Appendix D is the answer. Filtech simulates filters on the basis of pre-defined circuit modules, the circuits of which are shown in Appendix D. To build a filter you refer to the filter tables in the Appendix which show which groups of modules are connected together for a particular design. From Fig.4 it can be seen that the filter designed is a 4th order passive bandpass Chebychev type. By reference to the tables in the handbook the necessary circuit modules are selected. The component references and values shown on the Netlist (Fig.4) are then simply transferred to the circuit elements and there is your filter.

CONCLUSIONS

Although I am not a design engineer but just a plain ordinary RF Comms engineer, I am an active radio amateur, and as such I do need to design the odd filter or two for specific applications. Thus I am conversant with the basic of filter design, however, faced with the power and versatility of Filtech I was able to design complex filters with but a few key presses. The ability to simulate the filter and see the results of any changes to the parameters makes the entire task a pleasure.

This software system lives up to the pedigree of the previous Number One Systems packages that I have reviewed and used in the past. Simplicity of operation with a well-defined graphical user interface makes the software easy to use with a fast learning curve. The handbook is concise, but detailed enough to explain all of the facilities of the package.

Within a short while I was running the software with ease and felt able to design very complex filters without the need to know more than the basic specifications. As with the rest of the range of Number One System’s simulation and design software, the time saving aspects of such a package must be extremely desirable to the design engineer as well as perhaps even the casual user.

I am not able to think of anything that I did not like about the software. There are one or two idiosyncrasies which a dedicated Windows user (me!) found a little odd. Namely, each of the drop-down menus has a ‘Leave the Menu’ option, somewhat unnecessary as the usual press of the escape key results in the same action! It appears not to be possible to expand the Help screen to full size, but it remains a small dialogue box - again odd for a Windows user. But these are nit-picking things, as far as the software itself is concerned I can certainly recommend it as a development aid. Not only will it save lots of design time in mathematical analysis, it will possibly save much prototype manufacturing time as well. A necessary addition to the development tools of any electronic design laboratory.

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