Microwave measurements for seniors: applications of computer-based instruction

IEEE TRANSACTIONS ON EDUCATION, VOL. 33, NO. I , FEBRUARY 1990 149

Microwave Measurements for Seniors: Applications of Computer-Based Instruction

Abstract-Computer-based instruction has widespread potential for use in undergraduate courses in microwave measurements. It provides a means for strengthening students’ proficiency in the laboratory in the face of possible equipment shortages and competition for faculty time. At the same time, computer-based instrumentation is becoming more and more widely used, particularly in the microwave part of the spectrum. We discuss here the integration of computer-based instruction with advanced, automated instrumentation, making use of the automated system’s capability for individualized, self-paced tutoring in its own operation. Details are presented on two such computer-based tutorials: one to provide an introduction to a microwave vector network analyzer, and the other to train students in the operation of a spectrum analyzer. This technique has broad potential for use in systems of PC instruments, and as an instructional tool for industry as well as academia.

I. INTRODUCTION ECENT studies of engineering education emphasize R the importance of a strong laboratory experience for

engineering graduates. Also emphasized is achieving the proper balance in the undergraduate curriculum between coverage of the broad fundamentals and exposure to the details in specialized areas or topics.

In this day of increasingly sophisticated instrumentation, faculty members working with undergraduate laboratories find themselves walking an interesting instructional tightrope between basic and advanced instrumentation. Surely we would agree that students should first become proficient with basic instrumentation and principles of experimentation, including such things as the effect of the measuring instruments on the quantity being observed. Experience with more advanced instrumentation would normally come later, and we would seek ways of presenting this instruction as efficiently as possible.

Modern, advanced instrumentation presents a dilemma that makes instructional efficiency a matter of importance. Two properties of these instruments are 1) they are rela- tively expensive, especially those designed for use in the microwave portion of the spectrum, so most schools cannot afford to equip their laboratories with large numbers of them; and 2) to achieve understanding of and proficiency with such equipment, much instruction is required and each student needs a lot of one-on-one time with the

Manuscript received December 3, 1988; revised April 21, 1989. The author is with the Department of Electrical Engineering and Com-

IEEE Log Number 8929910. puter Engineering, Iowa State University, Ames, IA 5001 1 .

instruments. Clearly these properties work against each other when we try to introduce large numbers of students to advanced instrumentation with limited faculty re- sources and equipment funds.

But advanced instrumentation has a third property that offers a solution to the dilemma posed by the first two: namely, the equipment has the capability of being inter- faced with a computer. If a computer is made a part of the instrumentation system, the system then contains the mechanism for computer-based instruction in its own operation.

This idea of integrating the realms of computer-based instrumentation and computer-based instruction, and its application to two tutorials used in the Department of Electrical Engineering and Computer Engineering at Iowa State University, are the basis for this paper. In Section I1 we look briefly at the configuration and some of the terminology of automated instrumentation systems, and note how the teaching role of the computer is imple- mented. Section I11 describes the background of the students entering our senior-level elective course in microwave measurements, and the course itself. In Sections IV and V we describe the two tutorials: one on the operation of a microwave vector network analyzer, and the other on a microwave spectrum analyzer. Experience in using these tutorials is presented as well. We conclude in Section VI with general comments on the applicability of this means of instruction.

11. AUTOMATED INSTRUMENTATION AND AUTOMATED INSTRUCTION

Fig. 1 shows a sort of generic automated instrumentation (AI) system based on the IEEE-488 Interface Stan- dard [ 13. At various times, each instrument in the system acts in one of three roles:

Listener-When addressed by the processor, it ac- cepts instructions. Examples in Fig. 1 are the programmable power supply and the frequency synthesizer. Al- though these units are obviously “talkers” to the UUT, they are “listeners” from the point of view of the digital interface.

Talker-When addressed by the processor, it sends data out on the interface bus. Examples in Fig. 1 are the multimeter and the frequency meter. They monitor the response of the UUT and place data on the interface to be read by the processor.

OO18-9359/90/0200-0149$01 .OO 0 1990 IEEE

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MGITAL INTERFACE SYSTEM

- - - _

ANALOG MEASUREMENT SYSTEM - 7 - t -

t- UNITUNDERTEST

Fig. 1. A typical system of automated instruments connected via an IEEE- 488 interface bus.

Controller-Usually a computer, although some instruments have a degree of built-in control capability. Called a “processor” in Fig. 1 , it is programmed to ad- dress the other instruments, to direct the flow of data, to accept, store, and process the data, and to print or plot the results (here via the printer, which is another “listener’ ’).

When we integrate AI with computer-based instruction (CBI), we are simply adding a fourth role to the cast of players :

Teacher-This role is played by the controlling computer also.

The major task, therefore, is to write a program for the controller that serves as a computer-based tutorial and also controls the instruments in the system-changing their settings, interrogating the settings the student has made to see if they are correct, obtaining and processing data and presenting them in various forms, and so forth.

There is quite a rich, diverse literature in the field of CBI (sometimes denoted CAI, for computer-aided instruction). References [2] through [5] are a sampling of this literature. An important point found in these references is that there are some kinds of instruction for which computers are not well suited. If a tutorial does nothing but present frame after frame of text material on the computer screen to be scrolled through and read by the student, that function is probably better served by a good textbook. The book may be read more conveniently and is available for later reference at the student’s conve- nience. Our approach, therefore, is to provide background information from textbooks and class notes, and then to use the computer to do what it can do best: facil- itate the student’s exposure to the instruments by means of an appropriate tutorial.

In a 1972 paper [6 ] , Neal and Meller commented on another potentially inefficient method of using CBI, this

time in a laboratory situation where students are being trained to operate instruments. They noted that in some applications, a CBI computer placed on the laboratory bench may be likened to a blindfolded instructor. The progress of the tutorial is quite limited if the computer has to depend solely on a dialogue with the student in order to guide the student through the sequence. A much more fruitful application is to provide an interface between the computer and the student’s instruments that can indepen- dently report reliable information to the computer about what the student has done. Their approach was to add po- tentiometers to the control shafts of several basic laboratory instruments. The computer could sense the control settings by reading the potentiometer outputs whenever it was programmed to do so, and compare them to whatever settings the student was supposed to have made at that point in the tutorial.

We contend here that the availability of digital instrumentation interfaces such as the IEEE-488 standard, which not only provide for the direct interrogation of instrument control settings but which let the computer set those controls as well, most fully realizes the goals of Neal and Meller and provides the instructor with CBI capability of almost unlimited potential.

111. SENIOR LABORATORY COURSES IN APPLIED ELECTROMAGNETICS

Electrical Engineering Majors in the Department of Electrical Engineering and Computer Engineering at Iowa State University take a two-semester required sequence of courses, EE 212 and 313, in electromagnetic fields, waves, and transmission lines. Normally these courses come in the second-semester sophomore and first-semester junior years. They include a thorough treatment of transmission lines, under both transient and sinusoidal steady-state conditions. Complex voltage and current, re-

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flection coefficient, standing waves, and input impedance are covered for sinusoidal steady-state signals, but there is no laboratory component in either of these courses.

At the senior level, students may elect any of three courses in applied electromagnetics. One of these, EE 412, Electromagnetic Waves, is a rather general “waves in all kinds of media” course, and does not have a laboratory. Two others do have a laboratory meeting each week as well as three recitation meetings. They are EE 413, Microwave Engineering, and EE 416, Antenna En- gineering and Radiowave Propagation. So far as text material is concerned, the core course EE 313 is satisfactory as a prerequisite for all three senior courses. But for EE 413 and 416, the students need, as an additional prerequisite (or co-requisite, at the instructor’s discretion), a course in microwave measurements.

EE 41 1, Introduction to Microwave Instrumentation, is a one-credit laboratory-oriented course that provides the background for the laboratory work to be done in the other senior electives. It is this course in which the CBI tutorials described in this paper are used.

Along with laboratory work, EE 41 1 includes some lec- ture-discussion coverage of the Smith chart, detection of microwave power, and the directional coupler, as well as those items of equipment that are unfamiliar to the students from their previous laboratory work in circuits and electronics-the slotted line, for example. One very important characteristic of microwave instruments is emphasized repeatedly: many of them have low impedance inputs and are designed to terminate a transmission line, as opposed to most equipment the students have used pre- viously which have high-impedance inputs and are designed to be connected in shunt across a circuit. Thus, these instruments must dissipate all the power incident in the line, and are susceptible to burnout if the incident power level is too high. Most students readily appreciate this point; a very few who did not have found it a very expensive lesson.

Most of the laboratory experiments in EE 41 1 in effect “point to” the microwave vector network analyzer. The first few lab tasks are as follows:

1) Pulsed transmission lines: the transmission, reflection, attenuation, and distortion of short pulses along a 200-meter length of RG-8 coaxial cable. The instructor can break into the line at various points and insert un- known series or shunt loads, and ask the students to determine the nature and location of the loads by observa- tions made with an oscilloscope at each end of the line. The principle of the time-domain reflectometer (TDR) is clearly illustrated here.

2) The TDR: two are available, an HP 140/1415 and a Tektronix 7704/7S12; the latter is emphasized due to its fast step-function risetime and high resolution.

3) Phase and amplitude on a transmission line: here a HP 8405 Vector Voltmeter is connected to the probe of a General Radio 874-LBA slotted line, and the complex voltage as a function of position is measured for different load impedances.

4) Microwave power detection: the responses of crystal and barretter detectors are measured as functions of applied power, using HP 415E SWR meters. Absolute power levels are measured with thermistor detectors and microwave power meters.

5) Now each lab team is assigned a “test load,” con- sisting of a length of RG-63 coaxial cable (nominal characteristic impedance = 125 Q ) terminated in a 50 Q load. The cable is one-half wavelength long at a frequency de- notedfo (about 850 MHz). They determine the theoretical complex input reflection coefficient over a frequency range from below 0.5fo to above 1 .5f0, and plot it in both Smith chart and rectangular formats. Then the laboratory exercise continues with the students measuring their test load over the same frequency range by several different means, as follows:

a) Slotted line with crystal detector and SWR meter. Although industry today rarely uses this classic but slow and cumbersome technique, the slotted line has an important advantage in an introductory course: by probing and observing the voltage distribution along a line, the student can actually “see” the standing waves and how they shift as the test load is replaced by a reference shorting termination. Students are shown how data on VSWR and the positions of voltage minimal may be processed on a Smith chart to yield the complex impedance and reflection coefficient of the load. After a sample data analysis on a Smith chart, the students use a program on a desktop computer to process the rest of their data.

b) Dual directional coupler with matched crystal detectors and SWR meters to read relative power at the two coupler outputs. This is the students’ first introduction to the use of a dual directional coupler to separate the incident and reflected waves so that their relative amplitudes may be measured separately. Although this method yields only the magnitude of the reflection coefficient, it does introduce the idea behind the reflection test unit in the network analyzer to be used later.

c) TDR-here the student must consider qualitatively the relationship between time-domain and frequency-domain measurements and analysis.

With this background, the student can now appreciate that if we had a dual directional coupler, two detectors capable of presenting both relative amplitude and relative phase, and a way of inserting additional line in the incident-wave channel so as to place the reference plane at any desired location, we would have the ingredients for an instrument with which to measure complex reflection coefficient over a wide range of frequencies. It is pre- cisely these devices that are present in the HP 8741A Re- flection Test Unit and the HP 8411A Harmonic Fre- quency Converter used with the HP 8410A Network Analyzer (see Fig. 2).

These points are covered in the course notes, and rein- forced by the instructor in a demonstration of this equipment. At this point, the stage is set for the computer-based tutorial on the network analyzer. This tutorial is the subject of the following section.

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152 IEEE TRANSACTIONS ON EDUCATION. VOL. 33, NO. I . FEBRUARY 1990

RF

Optional; Used above 500 MHz Only

e Primary R-F Slgnal Path

U

Fig. 2. Block diagram of the automated microwave network analyzer system.

I v . CBI TUTORIAL ON THE NETWORK ANALYZER The HP 8410A Network Analyzer is not a state-of-the-

art instrument. Ours was obtained in 1970. Newer analyzers feature integrated packaging, more versatile displays and graphical output capability, built-in computa- tional power for data analysis and error correction, built- in synthesized swept signal sources, and even some tutorial slant to their operating software.

But we assert that the 8410A-based system is preferred as the analyzer on which to give students their first introduction to this kind of equipment. They must use a separate sweep oscillator, and it together with all other required components must be connected together before the system can be used. When all this has been done, the students can clearly see the signal paths and the components necessary to obtain the data that their previous laboratory experience showed them they must have in order to measure complex reflection coefficient. Furthermore, the operation of the resulting system requires certain manual ad- justments and calibrations which reinforce the students’ understanding of how the system produces the desired data.

The Hewlett-Packard Company, through a university gift program, has provided the Electromagnetics Labora- tory with a microwave sweep oscillator, some desktop computers, an S-parameter test set, and an analog-to-dig- ita1 converter, all configured for interfacing via the IEEE- 488 instrumentation bus (designated “HP-IB” by Hew-

lett-Packard) so that the network analysis system may be placed under computer control. Thus, the system gives us the best of both worlds: its “transparent” building-block configuration is ideal for a student’s first introduction to network analysis, and the computer interface permits automated data collection and the implementation of a CBI tutorial.

The CBI tutorial is designed to lead the student through this complexity of calibrating and operating steps. It can do so with individual students or small groups of students, taking over the chore of repetitive instruction and freeing the instructor for more important tasks.

In Fig. 2 is shown a block diagram of the complete system. The 8741A Reflection Test Unit contains the dual directional coupler and the “line stretcher” or phase shif- ter, while the 841 1A Harmonic Frequency Converter contains the dual sampling-type detectors that preserve both relative amplitude and relative phase of the incident and reflected signals. At this point the student, given the background as described in item 5b) of Section 111 above, can easily appreciate why these components are necessary for the measurement of complex reflection coefficient and impedance.

Data are captured from the polar display by feeding to the A / D converter the horizontal and vertical deflection signals proportional to the real and imaginary parts of the reflection coefficient at any frequency. The controlling program includes the rectangular-to-polar conversion necessary to find the magnitude and phase of the reflection coefficient, as well as calculating VSWR and complex input impedance. Hewlett-Packard recommends [7] using both a polar display and a rectangular display, and that magnitude data on a decibel scale be obtained from the latter. The components needed to do this are available, but have not yet been incorporated into the system.

The program that presents the tutorial, operates the sweep generator, and gathers data is called “GYMNAS- TICS” (getting your microwave network analyzer settings together in correct sequence). The program loads some subroutines, after which the main menu appears as shown in Fig. 3. This presents several options, some of which are used for every set of measurements and some of which are calibration steps to be performed at the start of a session.

When the user selects “new frequencies” with softkey k2, the program performs the following sequence of steps:

1) It prompts the user to enter starting and ending frequencies and the frequency step size, up to a maximum of 100 frequency steps.

2 ) It prints out the selected frequency range and step size, for user verification. This, incidentally, is the point to which the program goes when softkey k3 is selected in the main menu (Fig. 3 ) . If the user wishes to make changes at this point, he or she may return to step 1.

3) Now the program addresses the sweep oscillator and causes it to sweep repeatedly over the frequency range selected above. In a series of frames of instructions ap- pearing on the screen, the user is asked to place a shorting

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I-1ENU

The program w i l l r e t u r n t o t h i s menu a f t e r e a c h s t e p i n t h e meas3urement p r o c e s s . S e l e c t t h e n e x t o p e r a t i o n by means of t h e a p p r o p r i a t e ’ s o f k k e y ’ , l a b e l l e d k C ) t h r o u g h k ? , on t h e t o p 0.f t h e keyboard cinder t h e l e f t . - h a n d d i s k d r i v e . C h o i c e s are asd f o l l o w s :

k i . > : A;D c o n v e r t e r zero- -and-ga in s e t t i n g r o u t i n e . T h i s s h o u l d b e d o n e a s t h e .first s t .ep e a c h t i m e t h e e q u i p m e n t is t u r n e d on.

k1: C a l i b r a t e s t h e s y s t e m hy l o c a t i n g t h e c e n t e r and t h e r a d i u s of t .he p o l a r d i s p l a y on t h e n e t w o r k a n a l y z e r , i n A / D c o n v e r t e r u n i t s . This s h o u l d b e d o n e o c c a s i o n a l l y d u r i n g a l o n g s e s s i o n .

4 o r measurements .

For a new test ... p e r h a p s w i t h a d i f f e r e n t t e s t - c h a n n e l g a i n s e t t i n g , f o r example.

k9: L-e.ts you exit t h e program.

k2: P r o m p t s you t o e n t e r a new r a n g e of f r e q u e n c i e s t o u s e

\.:3: Lets you ‘LISE t .he p r e v i o u s l y - e n t e r e d set of f r e q u e n c i e s

c:

Fig. 3. Main menu for the tutorial program GYMNASTICS.

termination on the output of the reflection test unit at the point where the reference plane is to be, and adjust the reference plane extension, the test channel gain, and the amplitude and phase verniers so as to produce the desired short-circuit locus on the network analyzer’s polar reflection coefficient display.

At this point the student needs to respond to a question that, in an earlier version of the program, proved to be confusing for some people. It has to do with whether or not the selected position for the reference plane is beyond the reference plane extension capability of the reflection test unit. If it is, the computer can obtain data at each frequency with the short in place, and then use those data to adjust the data obtained later with the test load connected, thus providing a sort of first-order “electrical” or “computed” reference plane extension. To give the user a clearer basis for deciding whether the step is necessary, we used the graphics capability of the computer to add a series of three frames of information. These frames are shown in Fig. 4.

If the student enters “A” in response to the prompt in the third frame, the program prompts for the value of the test channel gain in decibels. It then directs the student to remove the reference short and place the test load on the line. The oscillator is still sweeping continuously over the selected frequency range so the user can see on the polar display how the data will look. The user may change the test channel gain if desired-to increase accuracy and resolution with very small reflection coefficients, for example. The user enters the new test channel gain into the computer, whether or not it was changed

Then the computer steps the oscillator through the selected frequency range, which in turn involves the following sequence at each frequency:

1) the frequency is sent to the oscillator; 2) the frequency is sent to the source synchronizer;

3) the synchronizer adjusts the oscillator frequency until the counter displays the selected frequency (a 1.5 s delay in the program provides time for this to take place).

At each frequency the computer reads the reflection coefficient data from the A / D converter. When the entire frequency range is covered and the computations are performed, the computer lists on the screen the frequency, complex reflection coefficient, VSWR, and complex impedance for each step. The user may then select a hard- copy listing of the data from a printer, or a plot of the data in Smith-chart form, or both. Fig. 5 shows an example of a Smith-chart plot from the pen plotter.

After the computer delivers the hard-copy output, the program returns to the main menu.

When the student has gone through all this, he or she has data on the same test load from four weeks worth of experimentation: the TDR, the slotted line, the dual directional coupler with crystal power detectors, and the network analyzer. The student’s assignment is now to write a report, analyzing the four sets of data. Two spe- cific questions are to be answered in this report: a) In which set of data or in which experimental system do you have the greatest confidence, and why? b) What is your best estimate of the characteristic impedance of the section of RG-63 coaxial line in your test load, and what tolerance do you place on it?

Student interest in the use of this program has been very high. This is due in part to the novelty of running a lab this way-it is their first exposure to automated instrumentation-and in part to the fact that when the computer addresses the various instruments in turn, assigning frequencies and obtaining data, the visual effect is mildly spectacular.

During Spring semester 1988, this course was taught by someone other then the author for the first time. This provided a good test of the tutorial’s ability to stand on

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154 IEEE TRANSACTIONS ON EDUCATION, VOL. 33, NO. I, FEBRUARY 1990

I f your- des i red re fe rence p lane l o c a t i o n i s w i t h i n t h e range o f t h e mechanical fiEFERENCE PLANE EXTENSION (RFE) adjustment, you w i l l be a b l e t o make t h e r e f l e c t i o n c o e f f i c i e n t locus f i t w i t h i n a very smal l r e g i o n as shown i n t h e f i g u r e below. (Use s o f t k e y k4 t o cont inue.)

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I f you cannot make the locus f i t w i t h i n t h a t smal l reg ion , i t means .that your des i red re fe rence p lane l o c a t i o n i s beyond t h e range o f t h e RF€ adjustment. I n t h i s case, t h e bes t locus you can achieve w i l l be a rough s e m i c i r c l e l y i n g w i t h i n a r e g i o n such as t h a t shown here. The locus may have any angular l e n g t h and may be loca ted anywhere around t h e ou ter c i r c l e .

I I I I Cont i nue I I I I

In t h e l a t t e r case, t h e computer w i l l record da ta w i t h t h e snor tkng t e r m i n a t i o n i n p lace, and use these da ta to ' c o r r e c t ' t h e da ta obta ined l a t e r w i t h t h e t e s t load connected. Press ' A ' o r 'E' ( f o l l o w e d b y ENTER) t o t e l l t h e computer which case you have.

fiig. 4. Portion of tutorial GYMNASTICS illustrating reference plane extension.

its own without having to rely on continued interaction by the instructor, which would, of course, partially compro- mise the tutorial's purpose. The test was a valuable one in that it pointed out the need for the graphics-aided explanation of the reference-channel-extension question, as described above. Aside from that problem, however, the

students seemed to be able to work through the measurement sequence with very little interaction with the instructor. Further details on this trial of GYMNASTICS may be found in reference [SI.

During Fall semester 1988 a number of the students who used GYMNASTICS the previous semester were taking

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Fig. 5 . Data from tutorial GYMNASTICS in Smith-chart form, from pen plotter.

EE 413, Microwave Engineering, taught by yet another instructor. They were asked to make some reflection coefficient measurements on devices they had built in the laboratory. They requested GYMNASTICS for the purpose, and used it, again with much success. This brought up an idea for an eventual re-write of the program: possibly the user could select two “tracks,” one containing all the tutorial details and the other having just the prompts but without a lot of explanation so that the experienced user could work through a measurement sequence more quickly.

The author has tried to use two guiding principles in the writing of this and similar tutorials. First, the user should always be given the feeling that he or she is in complete control at all times. Some users, when they need to exit the tutorial and leave the laboratory, are made to feel almost like prisoners if there is no obvious way to exit; they are afraid that something will be damaged if they simply turn everything off. A second principle, mentioned earlier, is that once the user is operating the tutorial from the computer keyboard, it should not be necessary to refer back and forth between printed material and the screen; all further instructions should be presented on the computer screen. Printed notes or text material may be needed, but their role should be completed once the student is ready to go to the CBI tutorial.

V. CBI TUTORIAL ON THE SPECTRUM ANALYZER

A modem, high-performance spectrum analyzer is a complicated instrument and may be more intimidating to the user than an oscilloscope was the first time he or she used one. This makes familiarization with the spectrum analyzer another ideal candidate for a computer-based tutorial. After that task has been accomplished, the computer and spectrum analyzer working together can be of great value in introducing the principles of analysis of signals with different kinds of modulation.

The tutorial described here is named “SARCASTIC” (for spectrum analysis review and computer-aided system for teaching it conveniently). This tutorial is eventually to have at least three parts: 1) introduction to the HP 8569A Spectrum Analyzer; 2) spectral analysis of amplitude-modulated and pulse-modulated signals; and 3) spectral analysis of angle-modulated signals. Part 1 has been in use since 1984 and is the subject of this section of the paper. The first version of SARCASTIC was presented in more detail in an earlier paper [9].

The Hewlett-Packard Model 8569A, used in this tutorial, is interface-bus-compatible in a sort of “one-way” sense. The computer can obtain data from the spectrum display, and it can also interrogate the analyzer and read the analyzer’s control settings. But the analyzer’s controls

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must be set manually via knobs and switches; the computer cannot set the controls as it can do with the newest analyzers. Here again, this combination of capabilities makes the 8569A an ideal instrument for learning how to run a spectrum analyzer: the student learns by “hands- on” operation, but the computer can check to see whether the student has set the controls correctly, and if not, it can tell the student which control setting needs to be cor- rected.

It is also possible for the computer to write data (or even short messages) onto the display screen of the spectrum analyzer. This capability will be exploited in parts 2 and 3 of the tutorial, where the computer can, for example, generate and display an “ideal,” theoretical spectrum on the screen for comparison with a measured spectrum.

Part 1 of SARCASTIC first advises the student of pre- cautions to be observed due to the possibility of damage when too much signal power is applied to the R-F input. The tutorial then proceeds to familiarize him or her with the operation of the analyzer’s controls. The student is shown some spurious responses, and the effects of a sweep speed that is too high for the selected frequency span per division and resolution bandwidth.

For this part, we do not need a signal with a complex spectrum. The most convenient signal is the unmodulated 100 MHz calibrator signal generated within the analyzer itself. Fig. 6 shows how this signal is sent through a variable attenuator and fed back into the analyzer’s RF input. The only other connection needed is to the computer, via the interface bus.

Fig. 7 shows a table of contents frame. This frame is displayed at the start of each of the eleven steps in the tutorial, with the current step highlighted in capital letters to help the student keep track of his or her progress. At the bottom is the menu, which is on the screen at all times (except when the student is prompted to enter the answer to a question). Note that the student has several options, in addition to the normal “proceed” step: return to a table of contents (to “get one’s bearings”), re-start the entire tutorial, repeat the step (i.e., re-start at beginning of present step), skip either back or ahead to any other step, or exit the program. In this way the student has the feeling of complete control over what happens at all times, and can leave in the middle of the tutorial and return later to continue from any desired point.

A flowchart of the tutorial program appears in Fig. 8. This program is more interactive than GYMNASTICS. In several places the student is asked to enter one or more numbers in response to a question on the screen; the program then branches to whichever frame of text or instructions is appropriate to the user’s response. Fig. 9(a) shows an example of such a frame; Fig. 9(b) is the frame to which the program branches if the student selects the “don’t know” response in Fig. 9(a).

In many other places the student is directed to make certain changes in the analyzer’s control settings. In such cases the “frame” of data read from memory includes not

HP 8569A SPECTRUM ANALYZER

HP 849418495 VARIABLE AlTENUATOR

I I IEEE-488 INSTRUMENTATION BUS (HP-IB)

Fig. 6. Block diagram of the spectrum analyzer and computer configured for the tutorial SARCASTIC.

only the text to be displayed on the screen but also data telling the computer what the proper control settings should be. Then the program branches to a subroutine SACSI (Spectrum Analyzer Control-Setting Interrogator) where the computer checks to see whether the controls have been set correctly. If they have not, the computer signals audibly and displays on the screen an indication of which setting(s) are wrong and what they should be. After the student has made an adjustment, the program again invokes SACSI, and the process continues until the settings are correct. Fig. 10(a) shows such a frame of text, and Fig. 10(b) shows what is displayed if the CENTER FREQUENCY control was not set correctly.

The original and a revised version of SARCASTIC have been used by students in EE 411 since Spring semester 1984. The students are given a four-page handout which briefly discusses the generic swept-local-oscillator type of spectrum analyzer and some of the special features of the 8569A, and gives the instructions for loading and running the tutorial and turning on the equipment. Once the tutorial is started, no further reference to printed material is needed. The students are asked to sign up for a block of time when they will come to the lab and run the tutorial, individually or in groups of two at the most. A time block of one and one-half hours is recommended, with the provision that the student can quit at any time, come back later, and continue from where he or she left off previ- ously. Normally there is no instructor present. We have found that the students have no trouble loading and running the tutorial on their own. They find it a very appeal-

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*.E STEP 5. Step 4. Step 7. St.ep 8. Step 9.

Step 11. Step IC!.

Gerier-a1 comments and p r e c a u t i Tc.wn--on pt--ocedure f nr t.tie specxtt-c.tm anal y'xet- A f i r s t l o o k a t the spectrum analyzer- d i s p l a y tiow t h e spectrLi.m ana lyze r di .splays s i g n a l amp1 i tctde RESOLUT I UN EtANtbl I D T H Frequency span per- d i v i s i o n E + f ects o f sweep speed Spi.iri ous responses Siqnal harmonic:s and the " F u l l Band" feature The v ideo f i l t e t - Summar-y

-K -it

Fig. 7 . Typical table-of-contents frame from the tutorial SARCASTIC.

ACCEPT ANSWER FROM KEYBOARD

DISPLAY M E MENU (see bebw)

ON 'PROCEED' R

ASK FOR CHANGE YES IN ANALYZER CONTROLS

CALL-

RETURN

INCREMENT P TO FRAME CONTAINING

NEXT PAGE OF TEXT

RIGHT ANSWER WRONG

k B b l INCREMENT P

TO FRAME CONTAINING NEXT PAGE OF TEXT INCREMENT P TO FRAME

CONTAINING APPROPRIATE

DISPLAY INSTRUCTIONS MENU

RETURN

Fig. 8. Flowchart of the tutorial program SARCASTIC. The index "P" is used by the computer to identify frames of information to be selected and displayed on the screen.

158 IEEE TRANSACTIONS ON EDUCATION, VOL. 33. NO. I . FEBRUARY 1990

! S t e p 1 , c o n t ' d ! P a g e 4

As w e l e a r n how t u o p e r a t e t h e E3569A, w e w i l l b e o b s e r v i n g a v e r y s i m p l e s i g n a l : n a m e l y , a 1W:) tltiz s i n e wave. 'The s o u r c e o f t h i s s i g n a l is b u i l t i n t o t h e U S b Y A i e s e l f : t h e s i g n a l a p p e a r s a t t h e CAL OLJTPUT j a c k on t h e f r o n t p a n e l , a n d i t p r o v i d e s a n e a s y way t o v e r i f y t h e o p e r a t i o n o f t h e s p e c t r u m a n a l y z e r arid tc) c a l i b r a t e t h e i n s t r u m e n t . N o w y o u wou ld t h i n k t h a t H e w l e t t - f ' a c k a r d wou ld n o t b u i l d a s i g n a l s o u r c e i n t o t h e B 5 6 Y A t h a t is s o s t r o n g a s t o d a m a g e t h e ins- t rumeri t , when t h a t . 6ourc:e is c o n n e c t e d d i r e c t l y i n t o t h e RF INFUT jack: . . . a n d y o u ' r e r i g h t . Note t h a t t h e CAL OUTPUT s i g n a l l e v e l is - 1 0 dtim, w e l l u n d e r t h e .t3(> dBm Hf- 1NF'Ll r 1 j . m i t .

blow .for a p r e s i m i r i a r y e x e r c i s e : CALCULATE t h e RMS v o l t a g e across a 50-ohni Soad, i n MTL.LIVOL~TS, f u r a real power o f - 1 0 dHm. Round t o 2 s i g n i f i c a n t d : i . g i t s . T y p e i n t . h e number ( n o u n i t s , just t h e n u m b e r ) , a n d p r e s s ENTER.

i I-F you don ' t L:now? t y p e 1E20 a n d ENTER. )

( I t y o u h a v e t o exit. t h e t u t o r i a l a n d l e a v e , t y p e 1E30 a n d ENTER. )

I S t e p 1 , c o r i t ' d i

.lhe g i v e n s i q r i a l l e v e l , -1i3 dE3m (real or a v e r a g e p o w e r ) , eqi..i.aSs 10 times the 1c.q ( b a s e l i l ) o f t.he power i n m i l l i w a t t s , s v t h e power- is 0 - 1 milSI.watts ot- 0 . 1 :.: 10.'".(-3) w a t t s . S e t t h i s equal t o

I s q u a r e o-t. RMZ; v o l t a g e ) / 50

a n d s o l v e +o!- HPiS v o l t a g e . I t comes out t .o 70.71 x v o l t . s , o r , t o t.wo c i g n i f ica.n.1~ d i t j i t s , 7:l mW RMS.

Remember. t h a t power i n dEtm rquals 1 0 t i m e s t.he l o g o f t h e power i n i n i l l iwa t t s . Power i n dEiW e q u a l s 1 0 t i i n e s t h e l u g of t h e power i n wat ts .

(b) Fig. 9. Sample frames from SARCASTIC. (a) A frame that prompts the

student to enter a numerical answer. (b) The frame to which the program branches if the student selects the "don't know" response in the frame in part (a).

ing way to learn how to use the instrument, although part of the reason for this is undoubtedly the novelty of this approach.

Perhaps the biggest drawback at this point is that there is not yet a provision for "follow-through," i.e., no lab exercise follows this one that requires the students to use their newly-acquired skills in studying spectra. Parts 2 and 3 of the SARCASTIC will fill this void. It is possible that when the tutorial is fully developed, the entire sequence

will be transferred to the laboratory phase of another course, a senior-elective sequence in communications systems.

VI. FURTHER APPLICATIONS AND CONCLUSIONS In this paper we have described two applications of a

general technique in which computer-based instruction is integrated with computer-based instrumentation in order to do two things:

STEPHENSON: MICROWAVE MEASUREMENTS FOR SENIORS

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159

N o w l e t ' s see what we've g o t here. 'The s i g n a l i s s t i l l a 1 0 0 MHz s i n u s o i d , b u t we ' re observ ing i t on an analyzer t h a t has been s e t so t h a t t h e c e n t e r o f t h e screen repr-esen.ts 102 MHz. Wi th FREQUENCY SPAN/DIV s t i l l a t 1 MHz, t h e spec:tr"um appear ing on t h e screen 1s 97 t o 107 MHz. There fore w e would expect t h a t a 1 9 0 MHz s i g n a l would appear 2 d i v i s i o n s t o t h e l e f t o f cen ter .

P l a y around w i t h t h e TUNING c o n t r o l 5 (bo th coarse and f i n e ) ... and, if you l ike , t h e I-REQUENCY S F A N / D I V c o n t r o l as w e l l . Depending on how f a r you move .the Cc)nt.rols, y o u may f i n d a l l s o r t s o f i n t e r e s t i n g s tu f f showing LIP on t h e screen. W e ' l l t r -y t o e x p l a i n these t h i n g s l a t e r i n t h e t u t o r i a l . The p o i n t i s t h a t t h e s ig r ia l ' 5 spectrum i s ( t h e o r e t i . c a l l y ) an impulse f u n c t i o n o f f requency a t 1 0 0 MClz, and t h e p l a c e i i f any) where i t appears on t h e screen should always make wT'nse t.o y o u i n v iew of. where t h e cent.er f requency (CTR on t h e annunc ia to r ) and t h e frequency span per- d i v i s i o n (SPAN on t h e annunc ia to r ) happen t o be s e t .

Return t h e center f requency t o 1()0 MHz and t h e s p a n / d i v i s i o n t o 1 MHz.

l a b l ~ o f Cont. I R e - S t a r t F'rog. I I 1 PRDCEEIS 1icpeat Step IS1 Lp Back I S b i p Uliead Is top t E::1t I I *

Pat ience " . . contr -o l se t t ings ; o n spec:trum analyzer are t jeiny checked.

Llheck s e t t i n g of cenir.er f requ.ency ( t h e TUNING knob) . 11: shou1.d he \/pry c l o s e t o l .E+8 Her tz .

(a)

I ' -___---- -_ - -. -- I

(b) Fig. 10. Sample frames from SARCASTIC. (a) A frame that prompts the

student to set the spectrum analyzer controls in a specified way. (b) The frame that appears if the student, in response to the instructions in part (a), has set the CENTER FREQUENCY control incorrectly.

a) teach students how to operate advanced, complex instrumentation in a self-paced, hands-on environment;

b) lead students through the use of these instruments to obtain data on a test device or to study further applications of the equipment.

These CBI tutorials have been written for and used in a course in microwave measurements. This is an especially appropriate area in which to use this technique, because of the major differences between the techniques and instruments used in the microwave part of the spectrum and those used at lower frequencies, and because of the rapidly-growing sophistication of microwave instruments such as network and spectrum analyzers.

But it is apparent that this general idea need not be re- stricted to the discipline of microwave measurements. It can have widespread applications in any case where new, advanced instrumentation or experimental techniques are being introduced. Two such applications will be noted here; many others will suggest themselves.

First, a type of instrumentation rapidly coming into widespread use is "PC Instrumentation"-groups of instruments designed for use only through connection to a personal computer. Such laboratory systems are being introduced in university teaching laboratories [ 101. Clearly the integration of CBI tutorials with PC instrument systems is an idea that can be widely developed and used.

A second application stems from the fact that when an engineer in industry is assigned to learn how to operate a system of instrumentation for the first time, and to use it to obtain data, that person becomes just as much a student as anyone enrolled in a university laboratory course. Therefore, tutorials such as those described here can be just as beneficial in industry as in the university environment.

ACKNOWLEDGMENT The author is pleased to acknowledge with thanks the

support of the Westinghouse Educational Foundation.

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160 IEEE TRANSACTIONS ON EDUCATION. VOL. 33. NO. I , FEBRUARY 1990

Thanks are also due to graduate teaching assistant B. stevens, who has provided valuable feedback and sug-

nior microwave measurements course.

18) D. T. Stephenson, “Computer-based tutorial on microwave network analysis,” in Proc. 1988 Frontiers Educ. Cont . ASEEIIEEE, Oc- tober 23-25, 1988.

Educ. Tech. Sysr.. vol. 14. no. 1 , pp. 3-14, 1985-1986. gestions regarding these tutorials and their use in the se- 191 -, “Computer-aided instruction in automated instrumentation.” J .

[ IO1 J . J . Uhran and E. W. Henry, “PC instruments in an intermediate

REFERENCES ANSIIIEEE Standard 488- 1978, IEEE standard digital interface for programmable instrumentation, Inst. Elect. Electron. Eng., 1978. Revision of American National Standards Institute ANSIIIEEE Std. 488-1975. A. B. Arons, “Computer-based instructional dialogs in science courses,” Science, vol. 224, no. 4653, pp. 1051-1056, June 8, 1984. A. Bork, Learning wirh Compurers. Bedford, MA: Digital Press, 1981. G. Kontos, “Instructional computing: In search of better methods for the production of CAI lessons,” J . Educ. Tech. Sysr., vol. 13, no. 1 , pp. 3-14, 1984-1985. “Effective teaching with computers in engineering,” Eng. Educ. , vol. 76, no. 5 , Feb. 1986. J . P. Neal and D. V. Meller, “Computer-guided experimentation- A new system for laboratory instruction,” IEEE Trans. Educ., vol.

“Automating the HP 8410B microwave network analyzer,” Hewlett- Packard Appl. Note 221A, June 1980.

E-15, pp. 147-152, Aug. 1972.

electrical engineering laboratory,” in Proc. 1987 Annu. ASEE Cont . Reno, NV, June 1987. pp. 1440-1444.

David T. Stephenson (S’56-M’66) received the B S E E degree in 1958 from Washington State University, Pullman, and the M S E E and Ph D degrees in 1962 and 1965 from the University of Illinois. Urbana

In 1966 he joined the faculty of the Department of Electrical Engineering and Computer Engi- neering at Iowa State University. Ames, where he is now an Associate Professor His teaching inter- eTts are in electromagnetics, radio propagation. antenna engineering, and microwave measure-

ments His research has included antennas, propagation, and computer- aided instruction

Dr Stephenson is a member of ASEE, Eta Kappa Nu, Sigma XI, and Phi Mu Alpha Sinfonia

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Microwave measurements for seniors: applications of computer-based instruction

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