Project Number: P07305

Transcription

1 Multi-Disciplinary Engineering Design Conference Kate Gleason College of Engineering Rochester Institute of Technology Rochester, New York 463 Project Number: P07305 EE FRESHMAN PRACTICUM LEARNING MODULE - AM RECEIVER Hans-Christian Rotmann Christopher Urban Department of Electrical Engineering Kate Gleason College of Engineering Rochester Institute of Technology James E. Gleason Building 79 Lomb Memorial Drive Rochester, NY ABSTRACT This work describes a new two-week module on AM radio transmission and reception, to be implemented in the Electrical Engineering Freshman Practicum course in the Fall 007 term. An analysis of the benefits of a learning module on AM radio reception over similar projects is included. Preliminary testing of the learning module with a pilot class size of 7 freshman students showed that, on average, 8.5% of students completed the module on time. Average student rating of the learning module was 8 out of 0 (for both theoretical and practical components). Furthermore, 7.7% of students reportedly gained a basic level of understanding of the circuit operation. From the results obtained, it is clear that opportunities for students to engage in experiential learning are very well received. The results also support the thesis that the balance between theory and practical content of a learning module is a delicate one, especially if oriented towards first year students. The intended audience for this paper is individuals involved in developing similar course modules, who may be interested in gauging the level of circuit complexity that is suitable for a learning module of this nature. INTRODUCTION The goal of the Freshman Practicum course is to provide incoming students with an opportunity to sample various fields of study within the Electrical Engineering discipline through hands-on learning and studio-style teaching. The course was developed by Dr. Robert Bowman, and is structured into ten separate laboratory modules (one three-hour session every week). As stated by Dr. Bowman in [], the objectives behind Freshman Practicum are to bring relevance to physics and calculus classes, to create a laboratory haven for new students, and to allow interaction between them and their faculty advisors. In order to provide the necessary technical knowledge for upcoming classes, the course introduces students to the various instruments that are used in a typical laboratory setting. Therefore, the lab modules emphasize the use of instruments such as bench top power supplies, digital multi-meters, oscilloscopes, and function generators. Throughout the course, students are also introduced to various circuit simulation and computing packages (Cadence PSPICE and MATLAB). The course also aims at providing a chance for the students to make career decisions early on in their studies. Furthermore, Freshman Practicum also seeks to promote a learn by doing approach by allowing students to section and study a real circuit. Feedback provided by students suggests that more time be spent on tinkering with electronics while avoiding excessively canned exercises. This serves as the motivation for the introduction of a new learning module, in place of a two-week exercise on C programming. Students are already required to take a C programming class during the freshman year, and therefore the replacing of this exercise comes at no loss to them. Results from needs assessment Based on the research done on the course objectives of Freshman Practicum, it was determined that the learning module had to present the student with the 007 Rochester Institute of Technology

2 Proceedings of the Multi-Disciplinary Engineering Design Conference Page opportunity to work on a real-world circuit. The accompanying text had to include discussion on the theoretical aspects of the chosen topic in a predominantly qualitative way. Most importantly, the text had to be tailored to the average first year student with high-school level physics and mathematics knowledge, and presented in such a way that it highlighted the relevance of such fields in engineering. However, the accompanying text had to provide enough theory for the student to gain a basic understanding of circuit operation. From the hardware perspective, the learning module had to allow the student to make use of laboratory equipment, such as bench top power supplies, oscilloscopes, etc. The component count, circuit complexity had to be kept low enough so that prototyping was suitable for the skill level of a first year student with little experience in circuit assembly and debugging. However, using highly integrated components reduced the students possibilities for tinkering. Most importantly, the circuit had to be safe for the student. Since the new learning module was intended to replace a two-week exercise on C programming, all parts of it had to be completed in a total of 6 hours, split between two weekly 3-hour sessions. Enough time had to be allocated to discussions on theory and circuit operation, prototyping and testing. Finally, in order to avoid handling of cash within the Electrical Engineering department, the cost of the parts kit was originally included in the price of the course text. For this reason, it was important that the added module did not increase the total text and parts kit cost by more than US$ Concept selection and design specifications Two different alternatives were considered for the new course module. The first was the application of an RFID tagging system, based around a prepackaged RFID chip and interrogator module. The system would allow students to identify and keep track of their belongings, such as keys, a backpack, etc. However, this project did not seem to allow the student enough space to tinker with the RFID electronics, since most of the components that make RFID work were contained inside the chip. A radio receiver was the second alternative. Engineers have found a myriad of applications for radio waves, from the earliest forms of wireless telegraphy to cellular telephony and Bluetooth. Thus, radio communication is a phenomenon that all students have experienced, and yet may be rather obscure. The constraints placed on the nature of the project (circuit complexity and available time) ruled out the choice of a radio transmitter. Based on the modest circuit building experience of the average first year student, the best choice was a radio receiver (AM or FM). Ultimately the AM receiver prevailed over FM due to relative simplicity of AM modulation which can be explained in a very straight-forward manner (multiplication of sinusoid functions). In addition, the circuit structure of an AM receiver is highly systematic (functional blocks are clearly identifiable) and requires few components (hence a lower possibility for error in prototyping and quicker circuit debugging). An added advantage of using the AM receiver was the fact that it was small enough to be put on the PCB board already used in the course (Wein-Bridge Oscillator), without increasing board area (and hence, cost). Finally, it was a project that the students could keep and take home with them. Project deliverables It was agreed with the customer to deliver a complete, two-part, document to address the theoretical and practical requirements of the module, to be incorporated into the existing Freshman Practicum course text. The document had to include all circuit diagrams and pictures necessary for the student to complete the assembly of an AM receiver. Furthermore, the customer requested a complete bill of materials for the parts kit, with manufacturers, vendors and cost clearly indicated. THEORY AND ANALYSIS The learning module was divided into four sections that reflect the high-level layout of the module text. The approach taken was to have every component build upon the theory introduced in the previous one (as shown in Figure ) to provide the student with enough theory to back the circuit building activities. I. AM Transmission and Reception Students are introduced to AM transmission and reception through the concept of AM modulation. The module text first provides the reasons and motivations behind signal modulation. The way in which modulation is achieved is then outlined, by using the concept of signal multiplication. In highschool level mathematics courses, students are introduced to the multiplicative identities of sinusoids and should be familiar with the equation ( a ) cos( b) = sin( a b) + sin( a + b) sin, () which accompanies the diagram in Figure. Paper Number P07305

3 Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference Page 3 IV. EXERCISES III. CIRCUIT SCHEMATIC II. AM RECEIVER FUNCTIONAL BLOCKS I. AM TRANSMISSION AND RECEPTION Figure : Module structure In an effort to communicate the concepts relating to modulation in a non-intimidating manner, extensive use of visual aids was used. One such diagram is shown in Figure. Figure : AM Modulation By showing modulation as an input-process-output scheme, the student is presented with a handle to understand the principle behind modulation. Figure 4: Vibrating Fishing String Analogy. II. The AM Receiver Functional Blocks As stated previously, the main focus of the project is the construction of an AM receiver. Following in the divide and conquer a real circuit spirit of Freshman Practicum, the AM Receiver is presented as the combination of a series of functional blocks, each responsible for a particular step in the recovery of the original message signal. The block diagram included in the text is repeated here in Figure 3 to illustrate the material the student will encounter. The text goes on to describe the specific role each of the blocks that makes up an AM radio receiver. To explain abstract concepts, analogies are often utilized to illustrate the basic operation of a functional block instead of attempting to explain the process using overly complex theory. The operation of the antenna, for example, is explained by means of an analogy with ripples propagating through a lake, causing a nearby fishing string to vibrate, much like elections in an antenna when radio waves come in contact with it (Figure 4). Other aspects of the AM receiver are treated in a quantitative way, provided the equations involved did not go beyond high-school level mathematics. The operation of the LC tuning network is described qualitatively, but the students are also provided with the equation to calculate the resonant frequency for a given LC combination. Modulated Radio Signal Antenna Tuning Network Peak Detector Amplifier Speaker Audible Output Functions: Receives the modulated signal Tunes to a particular frequency Detects the original signal Amplifies the signal to an acceptable level Figure 3: Function Block Diagram of the AM Receiver Signal sent to a speaker. Copyright 007 by Rochester Institute of Technology

4 Proceedings of the Multi-Disciplinary Engineering Design Conference Page 4 This calculation is included as part of the laboratory notebook exercises, and is geared towards showing the students that they can modify the component values to tune to other frequencies. One of the most important aspects of the module is that it shows applications of devices that were introduced in the beginning of the Freshman Practicum course. The material in the third week of the course covered diode operation, and the students learned about its function as a rectifier. Using this knowledge as a platform, the peak detector is introduced, with the output wave riding the amplitude modulated signal (Figure 5). + A c m n (t)cos( c t) - Figure 5: Peak Detector. The students also review the concept of nodal analysis (which was introduced in the third week of the course) to find the gain of an inverting op-amp. The potentiometer (Figure 6) is then introduced to show that it could be used to vary the amplitude of the signal going into the op-amp. To explain the function of a potentiometer, the concept of voltage division (which was introduced in week ) is utilized to show that the output voltage varies according to V out = R 8 A R 8B + R 8B V where components R 8A and R 8B are shown in Figure 6. R 8A + + V R 8 V R in 8B out R 8 - Figure 6: Potentiometer used. for volume control. Voltage (V) in Time (sec), - III. AM Receiver Schematic Once all of the blocks are explained in terms of their individual functions, they are brought together to form the AM receiver which is shown in Figure 7. The topology chosen is that of an amplified crystal radio set, because of its simplicity and low component count. A short distance away from the university campus, local AM station W.H.A.M. broadcasts using a 50 kw transmitter at a frequency of 80 khz. The component values chosen for the LC tuning network were therefore selected with this frequency in mind, since the strong transmission would improve the reception volume. An inductor of value L = 80 H and a variable trimmer capacitor with range 0 pf C var 0 pf. By (), the theoretical tuning range for the AM radio is π LC f () π LC tuner max tuner 083 khz f 375 khz Since the stray capacitances introduced by the prototyping board were around 30 pf, a wide-ranging variable capacitor of higher center value was desired. However, the cost of including these in the updated parts kit was prohibitive, especially because they were not available for purchase from large distributors. In order to mitigate this negative effect, the prototyping exercise was geared toward studying the operation of the AM receiver by using a signal generator (already available at each of the workstations in the Freshman Laboratory) set up for amplitude modulation. This way, the students could clearly observe the processing along the signal path of the AM receiver. These stray capacitances are considerably smaller in a printed circuit board, and therefore it was expected that the PCB version of the AM receiver would work well. The Freshman Practicum parts kit already included a a TLV783 dual operational amplifier with a.5 V bipolar power supply. The op-amp remaining in the chip was used to drive the output of the circuit (a 3 speaker in the PCB version, or the scope probes in the prototype). For the prototype version, the gain of the amplifier was configured to be a set value of 5.7 V/V (5 db): relatively low due to the high amplitude of the incoming AM signal from the function generator. The PCB version, being a working AM receiver, required more gain in the amplifier stage. As an min Paper Number P07305

5 Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference Page 5 added feature, volume control was added, in the form of a potentiometer dividing the input signal voltage to the amplifier. The amplifier itself had a fixed gain of 0 V/V (40 db). The fixed resistance of the potentiometer was used as the R component of the RC network in the peak detector, to maintain a fixed time constant. The moving lead was connected to the positive input of the op-amp, as shown in Figure 9. Finally, the prototype version of the radio receiver included a k resistor in series at the output of the amplifier, to prevent damage to the IC by accidental short circuiting of the output. To aid the students in identifying the blocks found within the circuit schematic, each one was enclosed within a box corresponding to the functional diagram found in Figure 3. Figure 8: AM Modulated Signal (Top) and the Output of the Peak Detector (Bottom). Figure 7: The Prototype AM Receiver Circuit. IV. Exercises The students are asked to use the oscilloscope to probe various test points within the prototyped circuit in order to see if each of the functional blocks is operating as expected, and if their results match up to the theoretical predictions. Sample oscilloscope captures are shown in Figure 8. Figure 9: The PCB AM receiver schematic. Finally, students build the final version of the AM receiver using the printed circuit board (PCB) included in the parts kit that accompanies the Freshman Practicum course text. The schematic for this circuit is shown in Figure 9, and a PCB with the completed AM receiver is shown in Figure 0. Time allocation for the exercises is shown in Table. Table : Scheduled duration of learning module Planned Duration (minutes) Part I: Week 8 Prototyping and debugging 50 Instrument setup 30 Measurements 0 Lab notebook write up 0 Part II: Week 9 PCB assembly 60 Debugging 60 Lab notebook write up. 30 Figure 0: PCB with completed AM receiver. RESULTS A pilot version of the laboratory module was conducted with a class of 8 freshman students. A consensus was reached with Dr. Robert Bowman, the author of the course text, to tentatively introduce the new learning module during weeks 8 and 9 of the winter term. In order to successfully test the module, all students in the class were provided with a parts kit, which contained the necessary components to complete the prototype and final versions of the AM receiver, including the PCB. Copyright 007 by Rochester Institute of Technology

6 Proceedings of the Multi-Disciplinary Engineering Design Conference Page 6 Part I: Prototyping of AM Receiver. Survey Results: Overall Text Clarity The first part of the module took place on Tuesday February 6 th (during week 8 of the winter term). The students were expected to cover the theory relating to AM transmission and reception, build a working prototype of the AM receiver, obtain oscilloscope data showing correct operation, and write a brief report on the experiment conducted. For testing purposes, students were asked to complete a timesheet, detailing the start and end times for each of the four parts of the module (prototyping, instrument setup, measurements and notebook write up). The results obtained from the timesheets collected are shown in Table. Table : Timesheet results for Part I Exercise Time (minutes) Min. Max. Avg. Prototyping Instrument setup Measurements Notebook write up 5 40 Total Time At the end of the class period, students were given questionnaires to provide written (in the form of comments) and quantitative feedback (through ratingtype questions). A total of thirteen quantitative questions were included, a total of students answered the questions. The quantitative results from the questionnaire are summarized in Table 3. Figures Figure and Figure show the distribution of scores obtained for text and schematic clarity. Number of incidences Number of incidences Score Figure : Text clarity score distribution. Survey Results: Schematic Clarity Rating Score Figure : Schematic clarity score distribution. Table 3: Student ratings for Part I of learning module Area evaluated Rating (0: max; : min) Actual Expected Comment. Theory section of text (based on previous knowledge) 6 8 Failed. Relevance to background in physics and mathematics 7 8 Failed 3. Text clarity 8 7 Exceeded 4. Mathematical content 8 8 Met 5. Schematic diagram clarity 6 9 Failed 6. Amount of time allotted to discussion of theory 7 8 Failed 7. Amount of time allotted to circuit prototyping 7 6 Exceeded 8. Circuit prototyping difficulty 6 7 Exceeded 9. Circuit debugging simplicity 5 6 Failed 0. Basic understanding of circuit operation 7 8 Failed. Overall module quality 8 8 Met Paper Number P07305

7 Proceedings of the KGCOE Multi-Disciplinary Engineering Design Conference Page 7 Part II: PCB assembly of AM Receiver. The second part of the module took place on Tuesday February 3 th (during week 9 of the winter term). The freshman class who participated in part I of the module completed part II (this time, however, 8 students attended the session). In this portion of the exercise, the students were expected to assemble a working AM radio receiver using the printed circuit board and parts provided. The module text for week 9 provided a photograph of the completed PCB for reference. Component labeling in the PCB matched that of Figure 9. As in part I of the pilot session, students were given timesheets to keep track of their progress throughout the three hours scheduled for the completion of the activity. A brief questionnaire was handed out to gather feedback regarding the exercise. The results obtained from the student timesheets are summarized in Table 4. Student feedback on the overall quality of part II of the learning module is shown in Figure 3. Table 4: Timesheet results for Part II Exercise Time (minutes) Min. Max. Avg. PCB assembly Debugging Notebook write up 5 0 Total Time DISCUSSION OF RESULTS Based on the results obtained from the timesheets for both parts of the learning module, the durations predicted for each of the activities planned were accurate. On average, most students completed the assigned exercises in less time than needed. However, since students were in charge of keeping track of their own progress, it is highly likely that the data reflected in the timesheets may not be entirely accurate. For the week 8 test, 4 out of 7 students completed the prototype on time. For the week 9 test, 3 out of 8 students finished the PCB assembly. This discrepancy was mainly attributed to the shortage of available soldering irons in the freshman student laboratory. From the data obtained in both weeks, the learning module evidenced strength in the following areas:. Text clarity and content: several students reported that the accompanying text was clear and straightforward. Technical language on par to the level of background the students possessed.. The level of difficulty of the prototyping exercise was below the anticipated level, despite the fact that the students had, on average, little background on circuit prototyping. This result ties in with the fact that the majority of students believed that the amount of time available for prototyping was adequate. In overall, the module was rated with 8 out of 0 (0 being the best rating). However, the tests conducted revealed several weaknesses that need to be addressed before the final adoption of the learning module into the Freshman Practicum curriculum. The biggest weakness identified was related to the schematic diagram for the prototyping exercise in part I of the learning module. A majority of students were unfamiliar with the symbols used for an inductor and variable capacitor. Furthermore, they were unfamiliar with the physical appearance of these components. To address this issue, pictures of the inductor and variable capacitor used have been incorporated into the texts for parts I and II of the module. Figure 3: Part II overall quality score distribution. Due to the apparent complexity of the circuit, coupled with the limited experience of the average student in reading circuit schematics, the process of circuit troubleshooting was more difficult than anticipated. Nevertheless, these difficulties are correlated to the inductor and capacitor symbols issue discussed above. Therefore, by addressing that Copyright 007 by Rochester Institute of Technology

8 Proceedings of the Multi-Disciplinary Engineering Design Conference Page 8 problem, it is fair to assume that students will find circuit troubleshooting simpler. Even though the apparent relevance of the module content to physics and mathematics was lower than expected, it was still within marginally acceptable values. The same argument can be sustained for the basic understanding of circuit operation. When completing the assigned exercises for part I of the module, students were unable to tune into a second test frequency of 740 khz, mainly due to the dominant effect of stray capacitances in the prototyping board. As a result, this part of the exercise was eliminated. Variable capacitors with values several times above tens of pico-farads are expensive, but theoretically could avoid this problem. from start to finish and ensured its success. The authors would also like to thank Ken Snyder for his help gathering parts needed to test this module and James Stefano for his input and suggestions. Lastly, a big thanks to the 8 freshman students who leant their time to test and debug this module. REFERENCES [] Bowman, R. J., Getting Engineering into Engineering, Presentation given at the ASEE 005 Conference. Another observed shortcoming of the learning module was poor grounding on the PCB version of the AM receiver. By attaching the ground plane to a harder ground (such as that in a power supply or oscilloscope), the signal quality improved noticeably. CONCLUSIONS AND FUTURE WORK The learning module developed in the project and outlined in this text presents a number of positive aspects that will be of benefit to incoming first year electrical engineering students. The opportunity to work on a real-world circuit is an attractive idea for many incoming students, because they can relate to it. The feedback obtained from the students who tested this learning module support this claim. From a feature-focused perspective, the ability to tune into multiple stations (i.e. to improve channel selectivity) would greatly enhance the students experience with the learning module, as long as this improvement does not bring about a considerable increase in circuit complexity or cost. Furthermore, a solution to the ground integrity problem must be found, to improve the overall quality of the hardwarerelated portion of the AM receiver learning module. Finally, a revision of the components used (their value and physical size) will be beneficial in improving circuit prototyping and troubleshooting. ACKNOWLEDGMENTS Special thanks goes out to Dr. Robert J. Bowman and Prof. George Slack who helped guide this project Paper Number P07305