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QCkt The Quick Circuit Analyzer Written by Kevin McClaning Reference Manual Version 1.00 January 1988 (c) Copyright 1988 by RadioWare All Rights Reserved RadioWare Inc. P.O.Box 2016 Columbia, Md 21045 This document describes the QCkt circuit analysis package. This version of QCkt is being distributed under the ShareWare concept. The ShareWare concept is based on three principles: 1. People need to try programs before they buy them to see if they are useful. You have to use a program for a while before you can determine whether it's suitable for what you want to do. 2. Software authors can be supported directly by users, 3. Copying and networking, one of the major strengths of electronic data, can be encouraged. If user-supported software works, then everyone benefits. The user will benefit by receiving quality products at low cost along with being able to try out a package before purchase. The author benefits by being able to enter the commercial software market without large amounts of capital. User-supported software is generally not public domain material: most programs carry a copyright notice, including this one. The author has licensed you to use and copy the program under certain conditions. Likewise, ShareWare is not intended to be free software. It is intended to provide quality software at a low price, while directly supporting the author. The bottom line is this - If you're still using a ShareWare product after a few weeks, then the program is worth something to you and you should make a contribution. If you like this program and find it of use, then your registration of $25 will be greatly appreciated. It will be used to support future versions of the program as well as providing encouragement to produce other programs. What will you get for your registration? First, I will send you the next major revision at no extra charge. After that, you can always get the current version for a distribution and handling charge of $10. Secondly, your registration will encourage me to revise and improve QCkt. Thirdly, I'll put you on my mailing list for revisions of QCkt and other programs we write. Lastly, you'll get support. I will always respond to the queries of registered users but I may not have time to answer questions from non-registered users. All users of QCkt may : 1. Copy and freely distribute QCkt as long as: a. No fee is charged, except for copying and distribution charges not to exceed $12. b. QCkt is distributed only in it's original, unmodified form with all of it's documentation. c. It is not distributed with or as any part of any other package (software or hardware), unless prior written permission has been granted by RadioWare. 2. If you are using this product in a commercial environment, then your company MUST register or see about getting a site license. That way, I'll stay in business and you'll get support. Send all inquires to : RadioWare P.O. Box 2016 Columbia, Md 21045 Make all checks payable to RadioWare, Inc. Please use the enclosed order form and please PRINT your full name and address. Site licenses and commercial distribution licenses are also available. I will do customized versions at very reasonable prices. I would really like to hear your comments about QCkt. Even if you're not a registered user, please drop me a line if you have any comments, criticisms or complaints. With such help, I will modify QCkt to produce a better, easier-to-use program. Occasionally, I do a little teaching and I've found QCkt to be very helpful in this respect. Concepts such as matching, transmission lines and filtering become very clear when the student can twiddle with component values in an interactive environment (that's where many of the examples included on this disk have come from). The students get a good feeling if they see the filter they designed out of a book perform like it should. Also, they can see the effects of finite component Q and parasitic components on circuit performance. Anyway, if anyone out there uses QCkt in a teaching environment, I would really like to hear about their experiences. Personally, I think this will be the wave of the future. The Fine Print ... Disclaimer RadioWare makes no representation or warranties with respect to the content hereof and specifically disclaims any implied warranties to the suitability of this program for any particular purpose. You, the user, must determine that yourself. In addition, you should understand that using a program of this type on an IBM PC or similar compatible machine has inherent risks and that you may inadvertently damage or destroy valuable programs or data. RadioWare expressly declines to assume liability for any use of this program by you and your use of this programs constitutes your agreement to hold us blameless. RadioWare reserves the right to make changes from time to time in the context hereof without obligation to notify any person or persons of such changes. Trademarks MS-DOS is a registered trademark of Microsoft Corp. PC-DOS and IBM PC are registered trademarks of IBM Corp. Turbo Pascal is a registered trademark of Borland International Inc. The Smith Chart is registered by P.H.Smith of Analog Instruments, Inc. Table of Contents 1 .............................. Introduction 2 .............................. An Example 3 .............................. Entering and Saving A Circuit 4 .............................. Entering Frequency Sweeps 5 .............................. Variable Components and the Tune Mode 6 .............................. Plotting the Frequency Response 7 .............................. The Smith Chart 8 .............................. Some Examples 9 .............................. How QCkt Works 10 .............................. QCkt Limitations Appendices A .............................. Technical Specifications B .............................. Acknowledgements C .............................. References - Credit Where It's Due D .............................. Component Summary E .............................. Running Aspect.Com F .............................. Order Form QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Chapter One : Introduction This manual describes the operation and use of QCkt, an interactive circuit analysis program for the IBM-PC and close compatibles. This manual contains a description of the files found on the disk, an overview of program features and a reference guide for all of QCkt's commands. QCkt's personality is geared toward the practicing RF engineer. I do a lot of RF circuit design and analysis in the 20 - 1000 MHz range and none of the reasonably priced programs currently on the market did what I wanted to do. I wrote QCkt over many months with the following goals in mind: 1. Fast data entry - allow the user to enter the circuit of interest in a short time. 2. Fast sweep time - to allow the user to see the effects of component changes almost immediately. 3. Quick feedback when changing component values - one of the problems I have with programs currently on the market is that they don't let you twiddle with circuit components and immediately see the result. 4. Quick access to the circuit from almost any point in the program - you can change almost any plotting parameter or circuit value while in one of the plot modes and immediately see the results on circuit performance. QCkt will run on an IBM-PC or close compatible. Minimum system requirements are 256-K of memory, PC-DOS or MS-DOS 2.0 or greater and at least one floppy disk drive. QCkt needs a color/graphics card with either a color or monochrome monitor to function properly. You will need an 8087 numeric co-processor chip installed in 1 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. your machine to run QCkt-87, the 8087 version of QCkt. QCkt-87 runs about 4 times faster than QCkt -- a big difference when evaluating large circuits. Due to the Turbo Pascal compiler used to generate QCkt and QCkt-87, their file structures are incompatible. That is, QCkt cannot read or write QCkt-87 files. Likewise, QCkt-87 cannot read or write QCkt files. The files included with this package are : QCkt.Com - This contains the main driving routines used by QCkt. FarCode.Com - This module contains the code for the rectangular and Smith Chart plotting routines. QCkt will terminate with an error message if this file is not on the default drive at start up. Aspect.Com - This program generates the aspect ratio of your monitor. If the Smith Chart isn't round on your particular monitor and you can't correct it by fiddling with the controls, run Aspect.Com to correct the problem. Note: Aspect.Com creates a short file called Aspect.Dta on the default drive. This file contains the aspect ratio of your monitor which QCkt reads on start up. Aspect.Dta - This file contains the aspect ratio of your monitor, if needed. See the entry under Aspect.Com for more information. Ex*.Ckt - These are example circuit files discussed in the manual (Read on). QCkt.Doc - This manual in ASCII format. In order to run QCkt, you must have QCkt.Com and FarCode.Com on the default drive and directory. If you need Aspect.Dta (see Appendix E for more information), it must also be on the default drive and directory. One final note before we get into it. If you're a new user, go through the example in Chapter 2 before you do anything else. This will help acquaint you with some of the features and power of QCkt. Then, skim the rest of the manual while sitting in front of your computer. Try things out. Finally, go through the examples given in Chapter 8. This procedure will get you up and running quickly and painlessly. Use the other parts of the manual for reference or when you're having trouble. 2 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. 3 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Chapter Two - An Example If you're like me, then you don't want to read too much before you fire the computer up to play. This chapter will run through an example which will demonstrate most of QCkt's capabilities. The first example circuit we'll go through is on the disk under the name Exbpf214.Ckt. As the name implies, this is a 21.4 MHz bandpass filter. It is a Chebychev filter with a 1 Mhz 5-dB pass band, capacitively matched to 50 ohms. Before we get to the example, let's clear up a few terms. When I ask you to hit the Alt-S key, for example, this means hold down the Alt key (lower left hand corner of your keyboard) and, while still holding the Alt key down, strike the S key (either a capital-S or small-S will do - it makes no difference). Another item to clear up is the subject of default answers. Typically, when the program asks the user for input, it will show a default value in parenthesis. The value of the default varies depending upon how you answered the question last time, the present state of the circuit currently in memory and what you're doing at the time. Anyway, to accept the default, all you have to do is hit the return key. For example, suppose the program asks: "Which Component (A, B, C, D or E) : (A) ...". The choices you are allowed to enter are A, B, C, D or E (either upper or lower case) and answer A is the default. Hitting the return key is the same as typing an "A", then hitting the return key. Finally, when the circuit lists possible responses to a question, you can respond with a number. Consider our example from before - The program asks: "Which Component (A, B, C, D or E) : (A) ... " The choices you can enter are A, B, C, D or E. You can also 4 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. enter 1 for A, 2 for B, 3 for C, etc. up to 5 for E. In general, you can enter a 1 for the first answer listed, a 2 for the second and so on. This feature speeds things up slightly by allowing you to keep your right hand over the numeric keypad at all times. Consider another example - suppose the program asks: "Continue (Y or N) : (Y) ... " Hit return for Yes. You can hit either "n", "N" or "2" for No. From the DOS prompt and with the QCkt disk in the default drive, type "QCkt" and wait for the main menu. Then, get the Exbpf214.Ckt file from the disk by selecting item 11. This puts up the QCkt retrieve file menu. Select option 2 and type "Exbpf214" (the program automatically appends the ".ckt" if you don't enter an extension). This will load the 21.4 MHz filter file from the disk and return you to the main menu. Loading a file this way loads a description of the actual circuit to be analyzed as well as several other system parameters such as component Q, 2 different frequency sweep ranges, plotting parameters and a host of other items. At the QCkt main menu, select item 3 to plot the gain and return loss of the filter we just retrieved from the disk. This brings up two more menus - don't bother with them right now. Just hit the return key twice to accept the default options and get a gain plot of the filter. The gain plot is really two different plots displayed on the same scale. The solid line you see is the transfer function of circuit in memory. This is the power dissipated in the load resistor divided by the maximum power available from the source. The second plot made up the little square boxes is the return loss of the circuit. This function reflects how well the circuit is matched to the system characteristic impedance - the lower this number is, the closer the circuit input impedance is to the system characteristic impedance. In the upper left corner is the current cursor frequency. This is the frequency currently being evaluated and it relates to the small, hollow box currently at the left hand side of the graph (it's about 35 dB down). If you press the '+' or '-' keys on the numeric keypad, this evaluation frequency will increment or decrement accordingly. Go ahead and try it now. You'll notice that as you change the evaluation frequency, the numbers at the bottom labelled Gain, Phase, R.L. (return 5 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. loss) and G.D. (group delay) also change. These number always reflect the characteristics of the circuit in memory at the evaluation frequency. Another way to change the evaluation frequency is to press the Alt-F key. A window opens up and asks you to enter a new frequency. Enter 21.4 MHz for the new frequency. When you hit return, the previous screen is restored and updated to reflect what the circuit is doing at 21.4 MHz. The frequency cursor (the small hollow box on the graph) is also moved to 21.4 MHz. Now, hit the Alt-I key. This will bring up a help menu which contains all of the commands you can use while in the plotting mode. It might be a good idea to do a Shift-PrtSc at this point to get a hard copy. Strike any key to get back to the plotting screen. Try the Alt-W key. This key lets you twiddle with the frequency sweep and plot parameters. The menu that you see displays the two sweeps currently in memory (labelled #1 and #2) along with their current step sizes. Take note that the current sweep is #1 by looking at menu option 4. Change the active sweep from sweep #1 to sweep #2 by selecting option 4. When the screen repaints, you'll see the active sweep has now become #2. Hit the return key once more to return to the plot. When we return to the plot, you'll notice that nothing has changed. QCkt simply saves the current graphics screen when it calls one of the Alt functions and restores it upon completion of the Alt task. To re-sweep the circuit with the current active sweep (now sweep #2), press Alt-S. The screen will be repainted with the new active sweep. This entire operation of striking Alt-W to edit the sweeps, changing the active sweep to #2 with one of the menu options, returning to the plotting screen and doing an Alt-S to re-sweep the circuit is done often. The whole process can be accomplished by hitting the Alt-A key to toggle the active sweep. Try it now. The active sweep will be changed back to sweep #1, the screen will be repainted and the circuit will be re-swept according to the first sweep's parameters. Let's try one more thing before we leave this plot. Press the Alt-G key. This key allows the user to enter global circuit and plotting parameters. These parameters are: the load and source resistance, the system characteristic impedance, the frequency where the electrical length of T-Lines is specified and, finally, capacitor and inductor quality factors or Q's. After hitting Alt-G, the program will ask you a question 6 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. about system characteristic impedance. Hit return to accept the default value of 50 ohms. Likewise, just hit return to accept the defaults for load and source resistance and for the T-Line question. When the program asks for inductor Q, take note of the default value. This number, along with capacitor Q, was loaded from disk when we loaded the circuit. The value displayed here (inductor and capacitor Q both = 10000) is artificially high and doesn't reflect the realizable, lossy components we have to use in the real world. Change the inductor Q value to a more reasonable value of 250 and the capacitor Q value to 1000 by entering these numbers in response to the appropriate questions and hitting return. Upon answering the capacitor Q question, you should find yourself looking at the gain plot of the circuit you saw previously. Make a mental note of the filter insertion loss (about 0) and the depth of the ripples (about 5-dB). Now, hit Alt-S to sweep the circuit with the new, more realistic Q values that we just entered. You'll note that the "real-world" circuit has an increased insertion loss (about 3 dB) and the ripple depth has decreased (also to about 3 dB). One of the great strengths of QCkt is that it allows you to twiddle with component values and quickly see the results on a plot. Finally, hit the Alt-X key to exit the gain plotting routine and return to the main menu. Main menu options 4 and 5 (plot phase and group delay, respectively) perform much the same as the gain plotting routine. Try them out for a spin! Remember that on-line help is available through the Alt-I key. When you're done playing with the phase and group delay options, return to the main menu (via Alt-X) and reload the original circuit by entering option 11 and retrieving the default file (which is now Exbpf214.Ckt). This is just in case you accidentally changed anything important while you were fiddling around. Now enter option 6, the Smith Chart plotting routine, from the main menu. Hit return twice to accept the default options and you'll be dropped into the Smith Chart plotting routine. This option lets you plot the input impedance of your circuit on the Smith Chart. Hit Alt-F and enter 21.4 to move the frequency cursor to 21.4 MHz. You'll see that at our center design frequency of 21.4 MHz, things don't look so good. Looking in the upper left-hand corner of the screen, we can see that we have a VSWR of about 10, our input impedance is nowhere near 50 Ohms and our return loss is about 0 dB (indicating a pretty poor match). 7 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Since the center of the Smith Chart represents a perfect match, we are interested in those frequencies where our input impedance is near the Smith Chart center. Increment and decrement the frequency cursor using the '+' and '-' keys by the numeric keypad to find the two frequencies closest to 50 Ohms. You should get about 21.05 and 21.80 MHz. If you don't, Alt-F your way to these frequencies to verify that they are indeed near the center of the chart. You'll notice that the impedance plot on the chart looks fairly rough. In the part of the spectrum where the input impedance of the circuit is changing rapidly, the plotting resolution is pretty coarse. You can see the points calculated by the program connected with straight lines. One of the options we didn't explore while we were doing the gain plot is the Alt-H key. This key will halve the current step size, giving finer plotting resolution but increasing the sweep time. Press Alt-H twice to plot 4 times as many points, then hit Alt-S to sweep the circuit with the new step size. You'll see that the plot is a lot smoother, but that the sweep time has also increased. You can double the current step size using the Alt-D key. If you're not all ready there, Alt-F your way back to 21.4 MHz and make a mental note of the VSWR at that point. Then, hit the Alt-C key to draw circles on the Smith Chart. The Smith Chart Circle menu will come up, asking you which type of circle you want to draw on the chart. Just hit the return key to accept the default of constant VSWR circles. Enter the VSWR at 21.4 MHz and hit return. When the screen repaints, you'll see the extra VSWR circle. It will be centered about the center of the chart and should pass through the frequency cursor at 21.4 MHz. And again, just for the heck of it, enter Alt-G to edit the global circuit variables again. As before, change the inductor Q to 250 and the capacitor Q to 1000. When the Smith Chart screen returns, hit Alt-S to sweep the circuit again and see the effect of realistic component Q. It does make a difference. 8 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Chapter Three - Entering and Saving A Circuit Entering a circuit into QCkt is really quite easy. Remember that, at present, QCkt can handle only ladder circuits with resistive terminations. For those of you who don't know, ladder circuits are circuits which contain only shunt and series element connected in a cascade fashion. This particular topology turns out to be quick and fairly easy to analyze. Before entering a circuit, it's a good idea to have a drawing of the circuit in front of you. Label the components starting with #1 at the load end and continuing on to the source resistor. Don't number the source or load resistors themselves. From the QCkt main menu, select option 1 to clear the circuit and enter a new one. The program will ask you if you're sure you want to do that - answer yes by typing a "y" or a "1". Any other response will send you back to the main menu. The program will then ask you how you want the global circuit variables set up. Enter values for the system impedance and the load and source resistance (all are typically 50 or 75 Ohms). The next question deals with transmission lines. When you enter a transmission line, you'll have to tell the program the desired characteristic impedance of the line you want to enter and it's electrical length in degrees at a specified frequency. This is where you enter that frequency. It will be the same for all of the transmission lines in your circuit. The last two questions ask for the quality factors or Q-values for inductors and capacitors. The defaults are 250 and 1000 for inductors and capacitors, respectively, which are pretty representative of realizable high-quality components. Hitting return after the capacitor-Q question leads us to the component selection menu. The components the user is able to include in his circuits are: 1. Capacitors - The value of all capacitors in this program are currently given in picofarads. All capacitors have the same global Q value which is held constant with frequency. The default value for capacitor Q is 1000. 9 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. 2. Inductors - The value of all inductors in this programs are currently given in microhenries. Like capacitors, all inductors have the same global Q value which can be (and usually is) different from the global capacitor Q. The default value for inductor Q is 250. 3. Resistors - The values of all resistors are entered in Ohms. 4. Parallel LC - An inductor and capacitor hooked together in parallel. As before, the capacitance is given in picofarads while the inductance is in microhenries. Global capacitor and inductor Q losses are applied to this combination. 5. Series LC - As in #4, but the two components are connected in series. 6. Parallel RC - A resistor and capacitor connected in parallel. 7. Series RC - A resistor and capacitor connected in series. 8. Parallel RL - A resistor and inductor connected in parallel 9. Series RL - A resistor and inductor in series. 10. T-Line - This is a lossless transmission line. The user enters a unique characteristic impedance (in Ohms) and electrical length (in degrees). If the line is in shunt with the circuit, the user is asked if the line is open- or short-circuited. The electrical length is the number of degrees the line appears to be at a given frequency. The frequency is specified as a global value and it is the same for all of the T-Lines entered in this program. The default is 100 MHz. 11. Parallel Resonator - This is simply a parallel LC circuit (see item #4 above) specified in a different way. Instead of asking for a value of L and C, the program asks for a value of L (in microhenries) and a center frequency (in MHz). The parallel LC will then be made to resonate at the given frequency. Global capacitor and inductor Q's are applied to this component. 12. Series Resonator - The same component as in item #11, except that the inductor and capacitor are 10 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. connected in series. 13. Defined Impedance Plane - This is an operation more that it is a component. If one of these is present in the circuit, the Smith Chart and return loss plots will reflect the impedance of the circuit looking from the impedance plane toward the load. The Smith Chart and return loss plots will not reflect the input impedance as the source sees the circuit. If there is more than one of these defined in a circuit, the last impedance plane (the one closest to the source) is used. 14. Duplicate an Existing Part - This is a very convenient operation which allows any previously defined part (ie. a part with lower number) to be duplicated. This is useful for some types of filters and matching circuits which exhibit a high degree of symmetry. A little sidenote - In all cases, you are allowed to enter negative values for components. Mathematically, this is not a problem for the evaluation software, but interpretation is up to you. Doing this, however, can mess up the graphics plotting routines. They won't crash, but will usually try to plot off of the outlined grid. I allow negative components for the simple reason that it can be interesting to plot such things. You can also settle a lot of silly arguments about circuit theory when they come up. From this point, enter the components in order; starting at the load end of the circuit and proceeding towards the source. The program will prompt you for the information it needs about any particular part. When you're finished, enter 99 when you're asked for a component (or just hit return). This will take you back to the main menu. At the main menu, enter option #2 to take a look at the circuit you just entered. The top three lines of the new screen give information on the global circuit parameters you entered when you started a new circuit. After that, the program displays the current circuit in memory starting at the load with block #1 and proceeding toward the source with increasing block numbers. After the block number is the component type (inductor, capacitor, etc.), followed by a description of the way the component is connected in the circuit (either shunt or series). Finally, the component value is displayed along with the proper units (uH, pF or Ohm). 11 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. If you want to change a particular circuit element, enter the block number of the component you want to change. You'll find yourself back at the QCkt component menu where you can enter a new component or change the value of the present one. When you're done, QCkt takes you back to the circuit description screen. A convenient feature of QCkt is that it allows you to enter new components from the circuit description screen. For example, if the maximum component number in your circuit is 9, you can enter 10 for the number of the block you'd like to change. You can then add a new component to your circuit. If you like, you can also change the global variables from this screen by entering 99 as the component you'd like to change. QCkt will then take you to the global editing screen. Main menu options 10 and 11 allow you to save your circuit file on disk. Select option 11 to retrieve a circuit from the disk. You'll come to the QCkt file menu which is reproduced here: Get Disk Menu - QCkt (Version X.XX) ----------------------------------------------------------------- Pick an Option: Default File is : Work.Ckt 1. Get Circuit From the Default File 2. Get Circuit From Another File 99. Return ----------------------------------------------------------------- What'll It Be : (1) ... You can get a file from the default circuit file (Work.Ckt, here) or from any other file. If you retrieve or save anything in any other file, that file then becomes the default file. For example, when we retrieved the example circuit Exbpf214.Ckt from the disk, that became the default file. This makes updating of your circuit files as you're working on them pretty easy. To save something in the default file amounts to selecting option #11 from the QCkt main menu and hitting return twice. If you need to save your circuit in a file other than the default file, select option #2. QCkt will ask for the name of the file you want to store data in. Only twelve characters (8-character name, decimal point and a 3-character extension) are allowed. The circuit will be saved on the default drive in the current directory. 12 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. If you enter a file name without an extension, QCkt will tack ".Ckt" onto the end of the name you enter. If you specify a file that doesn't exist, the program will beep, tell you about your mistake and take you back to the main menu. No data will be changed. QCkt main menu option 10 (the save to disk option) works exactly the same way as the retrieve function does. The following is a list of the information QCkt stores on disk: - Total number of blocks in your circuit, - The primary, secondary and tertiary items of each element (the Z0, length and Short/Open Circuit status of transmission lines, for example). Also, whether each element is in series or shunt, - Global Inductor and Capacitor Q, - The source and load resistances, - The frequency at which transmission line electrical are valid, - The system characteristic impedance, - The number of defined parallel branches, - The following data for each sweep: - Start frequency - Stop frequency - Increment frequency - The value of the bottom of the linear gain plot - The value of the top of the group delay plot - The tic mark interval for the horizontal and vertical axes for the gain, phase and group delay plots - The following data for the variable components: - The block number of the variable component - Whether the variable component is the primary, secondary or tertiary element of the block 13 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Chapter Four - Entering Frequency Sweeps Before you can plot your circuit, you have to set up several parameters to tell QCkt how to plot your data. These parameters include start and stop frequencies, vertical ranges, tic mark spacings and the like. You can do this from the QCkt main menu option #7 (among other places in the program). From the QCkt main menu, enter option #7 to get to the edit sweep menu. This menu displays the current parameters for two separate frequency sweeps, labelled sweep #1 and sweep #2. The menu displays the start, stop and step frequencies. The circuit is evaluated from the start frequency to the stop frequency, incrementing by the step frequency. For example, if the start frequency is 100 MHz, the stop frequency is 200 MHz and the step frequency is 1 MHz, the circuit will be evaluated and plotted at 100 MHz, 101 MHz, 102 MHz ... 199 MHz and 200 MHz for a total of 101 points. The more points you pick, the finer the graph will be but the longer it will take to complete a sweep. Some sidenotes - Anytime 0 MHz is encountered while analyzing a circuit, the circuit will be evaluated at 1 Hz. This allows you to enter 0 MHz as a start or stop frequency and prevents divide-by-zero run time errors. You can enter negative frequencies without any problem. Realistically, this is pretty useless although mathematically interesting. If you enter a frequency step size of 0 MHz, QCkt will default the step size to the following expression: Step Size = Abs(Stop Freq - Start Freq) / 100 which will produce 101 evaluation points. The edit sweep menu presents you with four options. The first three allow you edit either the first sweep, the second sweep or both sweeps. We'll get to that in a minute. 14 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. The fourth option allows you to toggle the active sweep between #1 and #2. The active sweep is the sweep used when you plot anything. The inactive sweep is always there, waiting to become the active sweep whenever you want. Two sweeps allow you to look at a circuit in two ways. First, you can look at the circuit up close, where the interesting things are happening (in the passband of a bandpass filter, for example). Secondly, you can get an overall picture of the circuit as it behaves over a large frequency range (to get an ideal of ultimate attenuation and that kind of thing). Anyway, select option #1 from the QCkt edit sweep menu to edit sweep #1. QCkt will remind you which sweep you're editing at the top of the page, then ask you for the new start, stop and step frequencies. Either enter new frequencies or hit return to accept the default values in parentheses. For example, if the screen looked like: Edit Sweep #1 ------------------------------------------------------------- Start Freq (MHz) : (19.4000) ... 15 Stop Freq (MHz) : (23.4000) ... 28 Step Freq (MHz) : (0.0500) ... ------------------------------------------------------------- It means that the previous start, stop and step frequencies were 19.4 MHz, 23.4 MHz and 0.05 MHz. The user entered new start and stop frequencies of 15 and 28 MHz, but accepted the default step frequency of 0.05 MHz. After you've entered the step frequency, QCkt will ask for a set of plotting guidelines. Since QCkt deals only with passive devices which have no gain, it reminds you that the top of the X-Y grid will be 0 dB, then asks for the attenuation value at the bottom of the screen. Again, just hit return to accept the default value in parenthesis or enter a new value. Now, QCkt will ask for the tic mark interval for the vertical grid. This tells the plotting routine how often you want to draw a horizontal line across the X-Y grid. Finally, QCkt will ask how often you want a vertical line drawn to mark off frequency. It then asks for the maximum expected group delay over the frequency range and finally, a group delay tic mark interval. 15 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. To recap, if the screen looked like this when you were finished: Edit Sweep #1 ------------------------------------------------------------- Start Freq (MHz) : (19.4000) ... 15 Stop Freq (MHz) : (23.4000) ... 28 Step Freq (MHz) : (0.0500) ... ------------------------------------------------------------- Vertical Parameters The Top of the Grid is 0 dB. How Far Down is the Bottom (dB) : (40.0) ... 50 Enter the Vert Grid Tic Mark Interval (MHz) : (5.0) ... ------------------------------------------------------------- Horizontal Parameters Enter the Horiz Grid Tic Mark Interval (MHz) : (2.0) ... 1 ------------------------------------------------------------- Group Delay Enter the Maximum Expected Group Delay (ns) : (1200) ... 1000 Enter the Group Delay Tic Mark Interval (ns) : (200) ... ------------------------------------------------------------- It means that the X-Y grid for this sweep will be plotted from 15 to 28 MHz in 0.05 MHz steps. The vertical part of the grid will go from 0 dB at the top of the screen to -50 dB at the bottom. QCkt will partition the vertical scale into 5 dB/div sections (ie. it will draw a horizontal line every 5 dB) and the horizontal scale into 1 MHz divisions (ie. a vertical line every 1 MHz). When plotting group delay, the vertical scale will cover 0 to 1000 ns and will be marked off every 200 ns. 16 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Chapter Five - Variable Components and the Tune Mode The concept of variable components is one of the most powerful features of QCkt. As I said in the beginning of this manual, I wanted to be able to change component values and immediately see the effect on both the transfer characteristic and input impedance of the circuit. This operation is realized through the tune mode using variable components. Basically, you can select up to five components (labelled A through E) that you want to vary. Then, when you are at any of the plotting screens (gain, phase, group delay or the Smith Chart), you can change the value of the selected component by entering the Tune Mode. Before we begin, load the circuit called Exbpf214.Ckt from disk. We will use this as the example here. To assign circuit element as variable components, first enter your circuit into the machine (either by hand or from the disk drive), then select the QCkt main menu option 8 labelled "Edit - Variable Components". This will take you to the following menu: 17 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Edit Variable Components - QCkt (Version X.XX) ----------------------------------------------------------------- Current Variable Components are : Component A : 3 Inductor Shunt L = 1.0000 uH Component B : 4 Capacitor Shunt C = 34.430 pF Component C : 5 Capacitor Series C = 2.0100 pF Component D : Undefined Component E : Undefined ---------------------------------------------------------------- What'll It Be ... 1. Edit One Component 2. Edit All Components 3. View the Parts List 99. Return ---------------------------------------------------------------- What'll It Be : (99) ... First of all, this screen shows you the current state of the variable components. In this case, component A is circuit element #3. That component is a 1.0 uH inductor connected in shunt. Similarly, component B is a shunt 34.43 pF capacitor. It is block #4. Variable component C is a series connected capacitor (C = 2.01 pF). Components D and E are not defined. The menu options allow you to either edit the variable component list (options 1 and 2) or view the circuit currently in memory (option 3). Select item #1 to edit a single component and you'll be asked which component you want to edit. Enter an "A" and you'll be presented with the following menu: Edit Component A ----------------------------------------------------------------- Present Part Is: 3 Inductor Shunt L = 1.0000 uH ----------------------------------------------------------------- Maximum Part Number is 9 Enter New Part Number for Component A : ( 3) ... 18 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Enter a new part number if you want to change which component in your circuit will be labelled as component A. Hit return if you want to accept the default. For this example, type an 8 to tell QCkt that you want component A to represent whatever circuit element is in block #8. When you hit the return key, you'll get the following menu: Edit Component A ----------------------------------------------------------------- Present Part Is: 3 Inductor Shunt L = 1.0000 uH ----------------------------------------------------------------- Maximum Part Number is 9 Enter New Part Number for Component A : ( 3) ... 8 ----------------------------------------------------------------- New Part is: 8 Capacitor Series C = 20.0000 pF ----------------------------------------------------------------- Present Value is: 20.000 Enter New Value ... If you want to change the present value of the component, you can do so at this point. It will be recorded as part of your circuit and the old value will be lost, so take care. For this example, change the value of the component to 10 by typing "10". Right before you hit return, your screen will look like: Edit Component A ----------------------------------------------------------------- Present Part Is: 3 Inductor Shunt L = 1.0000 uH ----------------------------------------------------------------- Maximum Part Number is 9 Enter New Part Number for Component A : ( 3) ... 8 ----------------------------------------------------------------- New Part is: 8 Capacitor Series C = 20.0000 pF ----------------------------------------------------------------- Present Value is: 20.0000 Enter New Value ... 10 QCkt will not allow you to assign certain components to be variable. The following components wouldn't make sense as variable components: - The impedance plane (component #13) - The duplicate component (component #14) although the component being duplicated can be made variable. 19 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Hit return to get back to the Edit Variable Component menu. Hit return a second time to get back to the QCkt main menu. Just in case you messed something up, retrieve Exbpf214.Ckt from the disk again via main menu option 11. If you don't have a print out of this circuit in front of you, this might be a good time to make one (use main menu option 2 and the Shift-PrtSc key). It would also help to draw this circuit out before you begin. Now, enter option 3 and hit return twice to get to the gain plotting screen and you'll see the now-familiar plots of this circuit's gain and return loss curves. Just to refresh your memory, this is a bandpass filter with a 1 MHz, 5 dB passband ripple. Using the variable components we've set up, we're going to try to narrow up the bandwidth of this filter and decrease the center frequency insertion loss. Alt-F your way to 21.4 MHz, the center frequency of this filter, and note the insertion loss of around 5 dB. Before going any further, we need to talk about the variable components of circuit we're about to adjust. The bandpass filter is made up to two identical, parallel LC circuits (components 3 and 4. Components 6 and 7 are duplicates of 3 and 4). The two tanks are coupled by a single capacitor (component 5). The resonant frequency of the tanks determines the center frequency of the filter while the coupling capacitor determines the shape and insertion loss of the passband. Now, enter Tune mode by hitting the Alt-T key. Tune mode allows you to change any of the variable components at will and immediately see the results of the change. When in tune mode, the last line of the screen prompts you to enter a new value for one of the variable components. If you enter a number and hit return, the circuit will be re-swept with this new component value in place. If you hit the Tab key, you will advance to the next variable component (ie. if you're currently editing component A and hit the Tab key, the last line of the screen will now ask you to enter a new value for component B). Hitting the Escape key will take you out of Tune Mode. Tab your way to component C and change it a few times by entering a new value and hitting return. Note that every time you change a component, the circuit will be re-swept without showing the return loss plot. The parameters of the filter at the cursor frequency are also updated and displayed on the bottom of the screen. Adjust component C to get the sharpest bandpass shape with 20 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. the least insertion loss. The center frequency will shift but we'll compensate for that in a minute. What you're doing is adjusting the coupling capacitor between two tanks. Decreasing this capacitor decreases the coupling between the two parallel LC elements and hence tightens up the bandwidth. You should be able to get the insertion loss to less than 0.2 dB when component C is 1.11 pF. You'll notice that the center frequency has shifted upward to 21.6 MHz, the ripple has decreased significantly and the 3-dB bandwidth is now about 500 kHz. With the cursor still at 21.4 MHz, start adjusting component B. Using the duplicate component function, you're actually adjusting the two capacitors in the tank circuits. We want to lower the center frequency back to 21.4 MHz. Play around with these components until the frequency is roughly centered at 21.4 MHz. If you're having trouble, the circuit I came up with had the following values: Component B = Component D = 35.43 pF and Component C = 0.710 pF. The final circuit had an insertion loss of about 0.16 dB with a 300 kHz bandwidth. Just for fun, Alt-X your way to the QCkt main menu, then enter option #6 to get to the Smith Chart. It's interesting to see what varying the coupling capacitor (component C) does to the input impedance on the Smith Chart. Enter the tune mode (Alt-T) and adjust Component C. As before, each time you adjust a variable component you'll get another sweep. 21 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Chapter Six - Plotting the Frequency Response Now on to the fun part - plotting things. QCkt will plot the following items on an X-Y grid: 1. Gain and return loss - These are measures of how well the circuit in memory accepts energy from a resistive source (ie. return loss) and how efficient it is at transferring that energy to a load resistor (gain). This is main menu option #3. 2. Phase - This is the phase of the voltage impressed upon the load resistor with respect to the source voltage. This is main menu option #4. 3. Group Delay - Different frequencies can take different amounts of time to get through a filter. Group Delay is a measure of how long it takes an signal placed upon the input to produce an output signal. Mathematically, group delay is calculated by taking the derivative of the phase plot with respect to frequency. For more information, see Chapter 9 on QCkt's Limitations. All of the plotting functions work pretty much the same except for what they plot. The information displayed and the place it's displayed on the screen doesn't vary nor do the Alt-Key commands. I tried to get things as uniform in command structure as possible. From the QCkt main menu, enter option #3 for the gain/return loss plot. Actually, you could have entered either #3, #4 or #5 for the gain\return loss, phase or group delay plots, but going through the gain\return loss plot will keep this narrative consistent with what you see on the screen. The first menu that pops up asks whether you want to sweep the circuit currently in memory, edit the variable components or edit the frequency sweeps currently stored in memory. We've already covered editing the frequency sweeps in chapter 4 and we'll get to the variable components in a later chapter. For 22 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. now, just hit return to accept the default task of sweeping the circuit in memory. Upon hitting return, QCkt then asks if you want to keep the current active sweep or switch to the other. Enter the appropriate answer and hit return to get to the plot. One thing to note: in the vast majority of cases, answering these questions usually boils down to hitting return twice before you get to the plotting screen. So even though these questions seem cumbersome now, they really aren't much of a bother at all once you begin using the program. You are now at the QCkt plotting screen and should see two distinct plots on the screen. The solid line is the circuit gain plot while the dotted line is the circuit return loss. The top two lines remind you of what you're doing. They tell you how to get to the help screen, reminding you of the file you're playing with and give you the current cursor frequency. The second line from the bottom of the screen gives the gain, phase, return loss (labelled "R.L.") and group delay (labelled "G.D.") at the cursor frequency. The vertical axis is labelled along the left side of the grid. This axis is labelled in dB, degrees or nanoseconds depending upon the plot you're doing. The horizontal grid is labelled at the bottom with the start and stop frequencies. The help screen is displayed by typing Alt-I (for information). This information menu is reproduced below: 23 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Plotting Help - QCkt (Version X.XX) ----------------------------------------------------------------- +/- : Increment/Decrement Freq Alt-A : Active Sweep Toggle Alt-D : Double Freq Step Size Alt-F : Enter New Cursor Frequency Alt-G : Edit Globals Alt-H : Halve the Freq Step Size Alt-I : This Help Screen Alt-N : Draw New Grid Alt-S : Sweep the Circuit Alt-T : Enter Tune Mode Alt-V : Variable Component Edit Alt-W : Edit Sweeps Alt-X : Exit Gain Plot : Solid Line is Gain, Dotted Line is Return Loss Tune Mode : Tab Key to Advance to Next Variable Component Esc Key to Leave Tune Mode Press Any Key to Continue ... The cursor frequency can be incremented or decrimented using the "+" or "-" keys, respectively. The graphics cursor will be updated as well as the gain, phase, return loss and group delay data at the bottom of the screen. The Alt-A key is the active sweep toggle. If the current sweep is #1, then Alt-A will change it to #2, clear the screen and re-sweep the circuit using sweep #2. The Alt-D and Alt-H keys perform opposite functions. The Alt-D key doubles the frequency step size while the Alt-H key halves it. QCkt doesn't respond to either of these keys in any way - it just quietly updates the step size. When the circuit is re-swept (using the Alt-S key, for example), QCkt will use the new step size. The plot sweep time and resolution will vary accordingly. The Alt-F key allows the user enter a new cursor frequency directly instead of stepping to it with the '+' or '-' keys. When hit Alt-F, a window opens up and you're asked to enter a new cursor frequency. When you hit return, the window will close and the screen will be updated to reflect the new cursor frequency. The Alt-G key allows the user to edit the global parameters. This allows the user to change component Q values, transmission 24 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. line parameters, system impedance, etc. quickly and see the results almost immediately. The Alt-N key repaints the screen with the current cursor frequency but does not re-sweep the circuit. This is useful when the screen has gotten messed up or too cluttered with data and markers. The Alt-S key repaints the screen and re-sweeps the circuit. Like Alt-N, it retains the current cursor frequency. The Alt-T key allows the user to enter Tune Mode. This causes several things to happen: First, the bottom line of the screen is replaced by a prompt asking you to enter a new value for one of the variable components. At this point, you can enter a new value for the specified component and hit return. The circuit will then be re-swept with the new component value. If you hit the tab key at this point, the next variable component will be displayed and you can change the value of that. If you hit the Esc key or Alt-X, you will exit tune mode. The second thing that happens when you enter tune mode is the program changes from the normal single-sweep mode to the re-sweep mode. Normally, QCkt does not re-sweep the circuit unless told to via the Alt-S (for Sweep) key. However, when changing the variable components, it's convenient to have QCkt re-sweep the circuit every time you change a component. When this alternate mode is active, QCkt re-sweeps the circuit every time one of the variable components is adjusted. To keep the screen from getting too cluttered, QCkt will repaint the screen after four sweeps. Also, if you're plotting gain and enter tune mode, the return loss is no longer plotted (this also helps keep the screen clear of clutter). The Alt-V key allows the user to view and edit the variable components. See chapter 7 on variable components for more information. The Alt-W key leads to the edit sweep menu. The user can then twiddle with the sweeps or change the active sweep. The Alt-X key is the exit key. Pressing this key will get you back to the QCkt main menu. 25 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Chapter Seven - The Smith Chart "Feel cheated ... " Professor Milton Kult, University of Akron, retired, commenting on the fact that he had only 3 weeks in which to explain the Smith Chart and it's uses. You can get to the Smith Chart plotting routine via the QCkt main menu option #6. As with the X-Y plots, the program asks if you want to sweep the current circuit (which is the default option), edit the variable components or change the current frequency sweeps. Enter the appropriate number or hit return alone to accept the default. QCkt then asks if you want to keep the currently active sweep. Answer appropriately and hit return. When you do, you'll be at the Smith Chart plotting screen. The Smith Chart takes up most of the screen. If it isn't round on the particular monitor you're using, quit now and run the Aspect.Com program provided (See Appendix E below for more information). The Smith Chart plotting routine behaves very much like the X-Y plotting routines described in Chapter 5. As before, information dealing with circuit behavior at the cursor frequency is presented but, this time, it's on the left side of the screen instead of the bottom. Starting at the top of the screen, we have the cursor frequency in MHz. Below that is the complex input impedance given in Ohms, followed by the VSWR and return loss. Then comes the reflection coefficient (labelled Gamma), the Smith Chart normalization impedance (usually 50 or 75 Ohms) and the state of the variable components at the present time. Finally, the current default file is given at the bottom of the screen. Also on the bottom of the screen is the label: "R = 0.5, 1, 2" This is a reminder that the three constant-resistance circles 26 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. displayed on the Smith chart are the R = 0.5, R = 1 and R = 2 Ohm circles. 27 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. As in the X-Y plots, you can get to a help screen by typing Alt-I (for Information). The help screen is reproduced here: Smith Chart Help - QCkt (Version X.XX) ----------------------------------------------------------------- +/- : Increment/Decrement Freq Alt-A : Active Sweep Toggle Alt-C : Draw Constant VSWR, Constant Gamma or Matching Circle Alt-D : Double Freq Step Size Alt-F : Enter New Cursor Frequency Alt-G : Edit Globals Alt-H : Halve the Freq Step Size Alt-I : This Help Screen Alt-N : Draw New Smith Chart Alt-S : Sweep the Circuit Alt-T : Enter Tune Mode Alt-V : Variable Component Edit Alt-W : Edit Sweeps Alt-X : Exit Tune Mode : Tab Key to Advance to Next Variable Component Esc Key to Leave Tune Mode Press Any Key to Continue ... These keys are exactly the same keys used by the X-Y plot to get things done. I refer you to Chapter 5 for information on these keys. However, there is one key that has been added - the Alt-C key. The Alt-C key allows you to draw either constant VSWR, constant Gamma (reflection coefficient) or matching circles. Pressing the Alt-C key will produce the following menu: Smith Chart Circles ----------------------------------------------------------------- 1. Constant VSWR Circle 2. Constant Gamma (Reflection Coefficient) Circle 3. Matching Circle 99. Return ----------------------------------------------------------------- What'll It Be : (1) ... The constant VSWR and constant Gamma circles are pretty much 28 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. the same thing, but the data is entered in different units. QCkt will prompt you for either the VSWR or Gamma value you're interested in, then return you to the Smith Chart plotting screen and draw the specified circle on the chart. In case you don't know, the circle's center is at the center of the Smith Chart and it's radius is determined by the value you entered for VSWR or Gamma. Anyway, these circles are useful if you're designing a circuit which must meet some VSWR specification. (They are also useful as a teaching aid. See the examples in Chapter 8 for more information). The matching circle is another useful tool. I refer you to the examples (ExLMat.Ckt and ExLMat2.Ckt) and to P.H. Smith's book listed in the reference section (Appendix C) for more information. 29 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Chapter Eight - Some Examples To help you understand how QCkt works and the best ways to use the program, I've included several example circuits on the disk with the program. The example circuit files I've included are named in the form EX*.CKT - that is, they all begin with the letters 'EX' and have the '.CKT' extension. The following four circuits are 100-MHz, 5-pole, low-pass filters. The first is a Butterworth, the second is a Chebychev, the third is an elliptic filter and the fourth is a Guassian filter. These are included here to allow you to compare and contrast the responses of these very different filters. The primary sweep goes from 0 to 300 MHz to give you an idea of the filter's ultimate rejection, stopband behavior, etc. The secondary sweep goes from 0 to 100 MHz and shows the basic passband characteristics of the filter. For comparison's sake, we will examine the attenuation of these filters at 250 MHz. ExBWLPF.Ckt - This filter was designed to have a 100 MHz, 3- dB cutoff frequency. In the gain plot using sweep #1, note the smooth, monotonic response and how the return loss gently increases. These characteristics are typical of a Butterworth filter. Alt-F your way to 250 MHz and read the attenuation. You should read about 38.3 dB of rejection. Examine the close in response by toggling to sweep #2 with the Alt-A command. Note the smooth roll off. The phase and group delay plots show relatively friendly performance. The peak group delay (using the Alt-F and +/- keys) is 8.2 ns at 93 MHz. The minimum group delay is found at 2 MHz and is 5.1 ns. This makes the group delay ripple 3.1 ns throughout the passband. Try increasing the Q-factors of the inductors and capacitors to 10000 and examine the effect this has on the filter's performance. The Smith Chart plot shows the input impedance of the filter. A Butterworth filter will smoothly rotate away from the center of the chart and approach it's final value smoothly. This particular filter appears as a low impedance out-of-band due to the shunt capacitors on the input. 30 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. ExCbLPF.Ckt - This is a 1-dB ripple Chebychev filter. It was designed to have a 3-dB cutoff frequency of 100 MHz. The most interesting thing about the gain plot is the return loss. Instead of smoothly rolling up like the Butterworth filter, the Chebychev shows ripple in the return loss as well as in the passband magnitude performance. Note the attenuation at 250 Mhz is 58 dB which is 19.7 dB better than the Butterworth filter. The price of the increased out-of-band rejection is ripple in the passband. Examining the close in response with the Alt-A command, we see that our filter does not really have 1-dB ripple as advertised. It droops as it approaches the cutoff frequency. This is due to the finite Q of our components. Use Alt-G to change both the inductor and capacitor Q to 10000 and resweep the circuit using Alt-S. The filter now behaves as the textbooks say it should. Change the capacitor Q back to 1000 and the inductor Q to 250 before continuing. Examining the group delay plot, we see the peak group delay is 20.1 ns at 96 MHz. The minimum delay is 6.1 ns and it appears at 29 MHz. This makes the in-band ripple 14 ns which 4 times as worse as the Butterworth filter. The Smith Chart is interesting. The input impedance sweeps around the chart's center several times before heading toward it's ultimate value. In this case, the ultimate impedance will be high due to the series inductor at the input of the filter. ExElLPF.Ckt - The magnitude, phase and group delay plots of an elliptic filter are all pretty interesting to look at. These filters are really a mess as far as their group delay and phase response are concerned. However, they do exhibit an excellent magnitude response. The example filter was designed for a shape factor of 2:1 with 40-dB minimum of attenuation in the stopband. The worst- case passband VSWR was around 1.04:1 with transmission nulls placed at 325 MHz and 209 MHz. The nulls are caused by components 2 and 4 - both are parallel-LC circuits placed in the series path. Sweep #1 goes to 500 MHz and allows you to see both nulls. Alt-F your way to the nulls (you may have to use the +/- keys also) -- I measured them at 210 and 324 MHz. Note that the stopband attenuation never rises above 40 dB which was one of the design goals. At our 250 MHz test frequency, I measured -40.3 dB of attenuation, a phase reading of 175.9 degrees and a group delay of 1.1 ns. Elliptic filters can have a steeper roll off than either Chebychev or Butterworth, but the price you pay is in the stopband performance. While the Butterworth and Chebychev roll off monotonically (ie. they never re-enter), the elliptic filter 31 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. will re-enter but is guaranteed by design never to exceed some minimum stopband attenuation value. Another price you pay for the improved amplitude response of the elliptic filter is in phase and group delay performance. Near the cutoff frequency, the phase response begins to show non- linear behavior. In the stopband, the phase and group delay responses are hideous; exhibiting huge jumps due to the transmission zeroes. The Smith chart shows the filters input impedance moves smoothly from the center of the chart to it's ultimate low- impedance value. ExGaLPF.Ckt - The Gaussian filter is a very low-Q filter. It has a poor magnitude rolloff characteristic but it is the least offensive as far as phase and group delay distortion go. Load ExGaLPF and plot the magnitude response out to 300 MHz (sweep #1). You can see the rolloff is pretty poor, as advertised. At 250 MHz, the attenuation is only -21.4 dB of attenuation compared with 38 dB for the Butterworth and 58 dB for the Chebychev. The phase at this frequency is 59.8 degrees and the group delay is 1.8 ns. Plotting the phase response shows the real beauty of the Gaussian filter. The response is remarkably linear throughout the passband and partially into the stopband (to around 150 MHz). Plot the group delay. You'll see that it's rock-solid until 125 MHz or so, then it gently rises outside of the passband. Because they are so gentle on the phase of a signal, Gaussian filters are used in places where phase must be preserved, such as television video. Due to their low, flat group delay, these filters are also used in fast sweeping receivers and spectrum analyzers. ExBPBRF.Ckt - This is a bandpass/bandreject filter I used in a transmitting/receiving system. The receiver operated at 650 MHz while the transmitter used the same antenna at 750 MHz. In order to protect the sensitive receiving circuitry from the transmitter, I designed a front-end filter to pass 650 MHz with presenting a high impedance to 750 MHz. Sweep #1 goes from 450 to 950 MHz and shows the passband centered around 650 MHz and the reject band centered around 750 MHz. The passband response is Chebychev with 0.5 dB of ripple. The insertion loss at 650 MHz is 1.54 dB and the filter provides 83 dB of rejection at 750 MHz. In practice, these numbers were- 3 dB at 650 and -70 dB at 750 due to component Q problems-- change the inductor Q to 120 and see for yourself. Sweep #2 goes from 600 - 700 MHz to show the passband response. Take sweep #1 out to 2000 MHz using the Alt-W key. Note the re-entrance at 1420 MHz. This is there because this filter uses 32 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. a capacitor as a coupling element (ie. element #4). If we change this capacitor to an inductor, the re-entrance problem will disappear. Try it -- change element #4 to a 70 nH inductor. This will cause the bandpass center frequency to shift slightly and the hump at 1420 MHz will disappear. Tune component #2 slightly to bring the filter back onto frequency (using the Alt-T key to enter tune mode). The Smith Chart shows the Chebychev response clearly. Note how the filter approaches a high impedance at the 750 MHz notch frequency. With in capacitor in place as the coupling element, you can also see the re-entrance occur at 1420 MHz. ExLMat.Ckt - This is an exercise in interactive matching. The circuit is a simple L-matching network between two resistive elements: R-Source = 50 Ohms and R-Load = 1000 Ohms. We want the match to be perfect at 50 MHz. Load ExLMat.Ckt from the disk and get a printout of the circuit using the Shift-PrtSc key. Draw a picture of the circuit and label the components. The circuit consists of a shunt capacitor across the 1000-Ohm resistor followed by a series inductor. Next, look at the variable components menu to confirm the shunt capacitor is variable component A and the series resistor is variable component B. Write this on your diagram. The purpose of this little exercise is to match the 1000 Ohm resistor to 50 Ohms at 50 MHz. We'll do this using the Smith chart and variable components. Enter the Smith Chart plotting routines and sweep the circuit. Since the component values are all 0, the plot isn't very interesting. Set the cursor frequency to 50 MHz using the Alt-F key. Now, enter the tune mode using the Alt-T key and start adjusting component A (the capacitor in parallel with the 1000 Ohm resistor). Note how adding capacitance moves the cursor point in a circle. Adjust the capacitor until the cursor falls onto the R=1 circle. Now, start adjusting component B, the series inductor, until the cursor falls onto the center of the Smith Chart. If you're having trouble, I made the inductor (component B) to be 0.68 uH and the capacitor (component A) to be 14 pF. Alt-X your way out of the Smith Chart plotting routines and plot the magnitude response of the matching network (main menu option #3). You'll see that the circuit is matched perfectly only at 50 MHz. Using the Alt-F and +/- keys, we find the 3-dB 33 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. points of the match are at 37 and 61 MHz. This is a bandwidth of 24 MHz or 48%. ExLMat2.Ckt - This is another matching example but with a twist. The bandwidth of the L-match circuit of ExLMat.Ckt was about 48% but we'd like to try to do better (ie. make it wider). This circuit consists of two L-matching networks in series to produce a wider bandwidth. As before, we're trying to match a 50-Ohm R-Source to a 1000-Ohm R-Load but this time we're going for an intermediate match to 224 Ohms (224 Ohms is the geometric mean of 50 and 1000). Load ExLMat2.Ckt and get a print out of it as you did before. Then, go to the Smith Chart plotting routine. Variable component A is a capacitor in shunt with the 1000-Ohm resistor. Component B is a series inductor. Components C and D are a shunt capacitor and series inductor, respectively. Finally, we have the 50-Ohm source resistor. First, we have to find out where 224 Ohms is on the Smith Chart. This is easy enough to do -- Use Alt-G to edit the global variables and change the load resistance to 224 Ohms. Then re- sweep the plot using Alt-S. Since all of our components started with zero value, we will see a mark on the Smith Chart which represents the load resistor (224 Ohms). Make a mental note of where 224 Ohms is on the Smith Chart. Now, Alt-F your way to 50 MHz and Alt-T into the tune mode and adjust components A (the capacitor in shunt with the 1000-Ohm load resistor) and B (the series inductor) until the impedance looking into the circuit is as close to 224 + j0 Ohms as possible. Then, start playing around with components C and D to move the 224 Ohms to 50 Ohms. The values I got for this circuit were 5.9 pF (component A), 1.33 uH (component B), 26.6 pF (component C) and 0.295 uH (for component D). Now, plot the magnitude response of the matching circuit. Note that the 3-dB bandwidth of the match goes from 31 to 74 MHz. This is 43 MHz or 86%. The bandwidth of the two element match of ExLMat.Ckt was 43%. 34 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Chapter Nine - How QCkt Works I've included this chapter for the more curious of you out there. Several people have asked how QCKt does what it does. QCkt makes use of a technique called the Modified Tack-Hammer method, fully described in Heyward's excellent book Introduction to Radio Frequency Design. This method is fairly crude, but it isn't very complicated, it's intellectually satisfying and it gets the job done. QCkt starts with the load resistor and assumes that it has one volt across it. The program then travels up the ladder toward the source resistor, evaluating the impedance of each element as it goes. QCkt calculates the impedance seen looking toward the load at every step and adjusts the voltage according to this impedance. The analysis program stops when it gets to the source resistor. This process generates the input impedance and the voltage present at the source. QCkt then adjusts the load/source voltage ratio according to the load and source resistors to get meaningful numbers to plot. This entire process is repeated for every evaluation frequency. 35 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Chapter Ten - QCkt Limitations When writing a program like QCkt, there are many trade offs and considerations to make. I originally wrote this program for my own, personal use and I feel the compromises I made were reasonable. They were made either because I don't like answering the same question over and over, to avoid run-time, divide-by-zero errors, for the sake of speed or because of personal taste. In any case, if the program starts behaving funny or gives you an answer you don't like, consult this chapter first. You may have run into one of my compromises. Frequency Sweep - Any time the program tries to evaluate the circuit at 0 Hz, it will be evaluated at 1 Hz. Step Size - QCkt takes the absolute value of any step size that you enter. If you enter a 0 for the step size, Qckt will default to the value Abs(Stop Freq - Start Freq)/100 which will give 101 points. Component Values - Generally, you may enter any component value that you want - positive, negative or zero. Positive values are obviously OK. Mathematically, negative components are no problem for the evaluation software but they may screw up the plotting routines. They won't crash, but they may try to plot to strange places on the screen. Any component specified to be zero is assumed to have a value of 1E-12. Other parameters that are not allowed to be zero (besides components) are: system impedance (Z0), load resistor, source resistor, inductor Q, capacitor Q and the frequency at which electrical lengths are specified. These also default to 1E-12 if you enter a 0. Number of Components - The maximum number of elements QCkt 36 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. allows is 98. This is pretty much a tradeoff between amount of memory used and realizable circuits. It's fairly difficult to design a passive circuit with 98 elements that will perform properly. Group Delay - Group delay is calculated by taking the derivative of the phase plot with respect to frequency and changing the sign. QCkt takes the derivative by evaluating the circuit at the cursor frequency and at the cursor frequency minus the step size. It then subtracts one phase from the other, adjusts the sign appropriately then divides by the step size to obtain the derivative. This means that the value of the group delay will change slightly depending upon the step size. For more accurate group delay data, decrease the frequency step size. Hardcopies - To make hardcopies of your circuit, list the circuit and Shift-PrtSc the data over to your printer. The circuit listing software prints out one screen at a time to allow you to get the entire circuit. You can also use the Control-PrtSc option on your computer to send data from your screen to your printer. To make hardcopies of the graphics screens, it's necessary to have loaded the Graphics.Com program that came with your DOS. Simply type "Graphics" from the DOS prompt prior to starting QCkt. You can then Shift-PrtSc your plots out to your printer. If there is a lot of interest in QCkt, I will upgrade the printing utilities to something a little more sexy. Variable Components - If a component is set up to be a variable component (Component A, for example) and it is changed from the Examine/Modify main menu option, QCkt will clear Component A and declare it undefined. Compatibility - Due to the Turbo Pascal compiler used to generate QCkt and QCkt-87, their file structures are incompatible. That is, QCkt cannot read or write QCkt-87 files. Likewise, QCkt-87 cannot read or write QCkt files. 37 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Appendix A - Technical Specifications QCkt was written using the Borland Turbo-Pascal compiler V3.0. It is primarily Pascal with a small amount of assembler thrown in. The QCkt program is actually made up of two modules: QCkt is the main module and handles user the input/output functions as well as formatting data for the FarCode module. FarCode contains the circuit analysis and plotting routines. QCkt.Com is the main module. However, since I needed more than the 64-k of code that Turbo Pascal allows, I load the FarCode.Com module on the heap of QCkt when QCkt is initializing. 38 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Appendix B - Acknowledgements This program and manual would not have been possible without the support, love and/or publications of the following people: My wife and newborn son, Christopher, who allow me to fiddle with my computer until all hours of the morning, John Cooper, Terry Fry, Tom Vito, France Bolei and the many others I've spent happy hours with discussing/arguing about some silly aspect of circuit theory or computer programming, All of the people on the computer bulletin boards who have donated their precious time and software to the public domain. 39 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Appendix C References - Credit Where It's Due The following are a list of a few of the books I've consulted in the course of writing this program and manual: Williams, Arthur B., Electronic Filter Design Handbook, McGraw-Hill Book Company, New York, NY, 1981, ISBN 0-07-070430-9. Hayward, W.H., Introduction to Radio Frequency Design, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1982, ISBN 0-13-490021-0. Jordan, Edward C., Ed., Reference Data for Engineers: Radio, Electronics, Computers, and Communications, Seventh Edition, Howard W. Sams & Co., Inc., Indianapolis, Indiana, 1985, ISBN 0-672-21563-2. Smith, Phillip H., Electronic Applications of the Smith Chart, McGraw-Hill Book Co., New York, New York, 1969, Library of Congress Catalog Card Number 69-12411. 40 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Appendix D - Component Summary This appendix is meant as a quick reference guide to the components. It is essentially a repeat of the data found in Chapter 3 on Entering and Saving a Circuit. 1. Capacitors - The value of all capacitors in this program are currently given in picofarads. All capacitors have the same global Q value which is held constant with frequency. The default value for capacitor Q is 1000. 2. Inductors - The value of all inductors in this programs are currently given in microhenries. Like capacitors, all inductors have the same global Q value which can be (and usually is) different from the global capacitor Q. The default value for inductor Q is 250. 3. Resistors - The values of all resistors are entered in Ohms. 4. Parallel LC - An inductor and capacitor hooked together in parallel. As before, the capacitance is given in picofarads while the inductance is in microhenries. Global capacitor and inductor Q losses are applied to this combination. 5. Series LC - As in #4, but the two components are connected in series. 6. Parallel RC - A resistor and capacitor connected in parallel. 7. Series RC - A resistor and capacitor connected in series. 8. Parallel RL - A resistor and inductor connected in parallel 9. Series RL - A resistor and inductor in series. 10. T-Line - This is a lossless transmission line. The user enters a unique characteristic impedance (in Ohms) 41 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. and electrical length (in degrees). If the line is in shunt with the circuit, the user is asked if the line is open- or short-circuited. The electrical length is the number of degrees the line appears to be at a given frequency. The frequency is specified as a global value and it is the same for all of the T-Lines entered in this program. The default is 100 MHz. 11. Parallel Resonator - This is simply a parallel LC circuit (see item #4 above) specified in a different way. Instead of asking for a value of L and C, the program asks for a value of L (in microhenries) and a center frequency (in MHz). The parallel LC will then be made to resonate at the given frequency. Global capacitor and inductor Q's are applied to this component. 12. Series Resonator - The same component as in item #11, except that the inductor and capacitor are connected in series. 13. Defined Impedance Plane - This is an operation more that it is a component. If one of these is present in the circuit, the Smith Chart and return loss plots will reflect the impedance of the circuit looking from the impedance plane toward the load. The Smith Chart and return loss plots will not reflect the input impedance as the source sees the circuit. 14. Duplicate an Existing Part - This is a very convenient operation which allows any previously defined part (ie. a part with lower number) to be duplicated. This is very convenient for some types of filters and matching circuits which exhibit a high degree of symmetry. 42 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Appendix E - Running Aspect.Com One of the problems I had when writing QCkt was that the Smith Chart would come out elliptical on some monitors and circular on others. For example, the Smith Chart on my Zenith monochrome monitor at home is fine, but it turns out to be a narrow ellipse on the IBM monitor at work. Usually you can solve this problem by fiddling with the controls on your monitor. Sometimes you can't. If you can't, I've included a program called Aspect.Com on the disk. Aspect.Com draws a horizontal line on the screen and asks you to get out a ruler, measure the line's length and enter the number. Aspect then draws a vertical line and repeats the request. The unit you use to measure the line is immaterial, but it's easiest to use a centimeter scale. That way, you don't have to convert fractional inches to decimal before entering the number into the computer. Since Aspect.Com knows how many pixels it lit up on the screen when it drew the two lines and you've told it how long the lines were physically, it can figure out a number of pixels per unit length for both the vertical and horizontal directions. It then divides the horizontal number by the vertical number to get a ratio. This ratio is stored in a file on the default drive under the name Aspect.Dta. When QCkt first comes up, it loads Aspect.Dta from the default drive if it's present (if it isn't present, QCkt just uses an internal default value for the aspect ratio). All of the data heading for the Smith Chart screen is then massaged by this aspect value before it hits the screen. So, the circles are drawn so they look round and the data plotted by the Smith Chart routines is plotted accurately. 43 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Appendix F ----------------------------------------------------------------- QCkt - Circuit Analysis Program - Order Form ----------------------------------------------------------------- RadioWare P.O.Box 2016 Columbia, Md 21045 PRICE PRICE DESCRIPTION QUANTITY EACH EXTENDED ----------------------------------------------------------------- QCkt program disk - Try Before ______ $10.00 $_______ Buying - Latest version, complete program with manual and examples on disk, NO technical support. QCkt - Registered Owner - ______ $25.00 $_______ Includes a free update to the NEXT version - manual and examples on disk - All the technical support you need. QCkt - Upgrade to Registered ______ $15.00 $_______ Owner - If you all ready sent in $10.00. QCkt - Upgrade to Newest ______ $10.00 $_______ Version - Registered Owners ONLY. Subtotal $_______ 5.0% Maryland State Sales Tax $_______ (Maryland Residents Only) Total $_______ Please make checks (U.S. funds only) payable to: RadioWare Name_______________________________________ Date ________________ Address__________________________________________________________ _________________________________________________________________ City________________________ State__________ Zip Code____________ Description of Computer System___________________________________ 44 QCkt - The Quick Circuit Analyzer - (c) 1988 RadioWare, Inc. Commercial users of QCkt MUST register their copies at a price of $25.00 each for the first 20 copies, $20.00 for every copy thereafter. In addition, a site license permitting unlimited copying within the licensed institution is available. Please write for details. 45