overcome with low-cost and portable hands-on experiments that can be done by students at their ... myDAQ data acquisition board, a student-owned laptop, and.
2014 American Control Conference (ACC) June 4-6, 2014. Portland, Oregon, USA
Embedding Low-Cost, Portable Experiments into a Lecture-Based Signals and Systems Course Aaron Lanterman, Michael Giardino, Bonnie Ferri, Jennifer E. Michaels, William Hunt, Aldo Ferri Georgia Institute of Technology; Atlanta, GA 30332 Abstract - This paper discusses the inclusion of low-cost, portable experiments into a lecture-based introductory Signals and Systems course. Most of the experiments can be completed in a regular lecture room during a lecture period by students working at their desks, while an acoustic-based experiment is completed in a large room by students using only their computers. The experiments are embedded into what is at most institutions a highly theoretical course, thereby giving students the integration of theory and experiments without the use of high cost, centralized laboratory facilities. 1. INTRODUCTION One of the recommended features of a good engineering educational laboratory experience, as determined by an ABET sponsored colloquy, is to have students relate the experimental results to theory [1]. The assumption in that paper, and in all engineering curricula, is that hands-on experiences such as those gained with labs are an important part of the educational process. It may be argued, in a parallel sense, that while a good laboratory should be tied to theory, a mark of a good engineering theory course is its connection to physical phenomena and experiments. However, many engineering programs relegate the laboratory experience to centralized facilities during specified laboratory hours. That separation in time and space from lecture/theoretical courses can be overcome with low-cost and portable hands-on experiments that can be done by students at their desk in a lecture room or in their dorm rooms or homes. Mobile labs, ones that can be done anytime, anywhere, are made possible by low-cost portable data acquisition boards such as the Mobile Studio [2], the Lab-in-a-Box [3], the National Instruments myDAQ board [4], and the Digilent Discovery board [5]. Priced similarly to a textbook, these devices are meant to be student-owned. A pedagogy has arisen around the effective use of these devices [6]. Along these lines, Georgia Tech has had two NSF grants to develop distributed laboratories, which supported the development of the TESSAL (Teaching Enhancement via Small-Scale Affordable Labs) Center [7,8]. The subject of Signals and Systems, in particular, is a good candidate course in which to embed portable experiments because the topic is very mathematical and theoretical. Many students struggle with the subject matter when the material is presented only as theory. With the inclusion of hands-on experiences, our hope is that the 978-1-4799-3274-0/$31.00 ©2014 AACC
students will retain a greater amount of knowledge and be able to more readily apply that knowledge, well beyond the completion of the course. This paper builds upon a previous study sponsored by NSF and conducted at Georgia Tech that examined the impact of hands-on experiences in the ECE 3085 Introduction to Systems and Controls lecture course [9,10]. That study developed LEGO-based experiments to examine concepts including aliasing, digital filtering, frequency response, and control systems. Assessment results showed that the experiments improved understanding of the material and the retention of knowledge [10]. Based on the success in ECE 3085 and other courses at Georgia Tech, portable hands-on experiments were purposely integrated into six core ECE theory courses, including the new course ECE 3084, Signals and Systems, which partially replaced ECE 3085. The new experiments developed for ECE 3084 utilize the National Instruments myDAQ data acquisition board, a student-owned laptop, and free downloadable software that implements a soft instrument panel on the student laptops. Starting in Fall 2013, all Georgia Tech ECE students and all students taking the circuits course for non-majors are required to purchase the myDAQ device. This paper discusses several of the experiments that have been developed for the 3 credit hour, junior-level lecture-based ECE 3084 Signals and Systems course. 2.0 DESCRIPTION OF EXPERIMENTS To maximize the benefits of incorporating experiments into a lecture course, laboratory modules should have certain features. They should fully support or demonstrate a fundamental principle that is hard to understand from theory alone. The experiments should not necessarily require faculty to change their standard evaluation methods, such as in-class tests. They should be able to be done in a classroom environment within 50 minutes. In order to be adopted widely, the experiments should be low-cost and have a very small learning curve for both the instructor and the students. The myDAQ data acquisition board was chosen because it has a very simple to use software interface (NI Instrument Launcher) that runs on student computers and provides soft instruments including a function generator, oscilloscope, frequency spectrum analyzer, and Bode plot generator. The myDAQ also interfaces with LabVIEW, which can be used to develop customized interfaces such as needed for feedback control. All the experiments described
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in this paper, with the exception of the Performance Hall Reverberation Time, use the myDAQ board. Five experiments have been developed and used in ECE 3084, although only three of the five are used in any one semester. The number of experiments to be used in a lecture-based course is an open question. We have found that using only one experiment in a course has limited benefit since there is an overhead for students learning the experimental procedure - they may lose sight of the fundamental concepts. For a second experiment, students are more used to the platform and can focus on the concepts. There is a limit at the upper end since most professors do not want to incorporate a large number experiments in the class due to loss of lecture time and the added logistics. Selecting three difficult concepts and providing hands-on experiences seems to provide a good compromise between the benefits versus the costs [11]. 2.1 Amplitude Modulation This lab explores the principles of amplitude modulation in both the time and frequency domains. Fundamental Concepts: The relation between multiplication in the time domain and shifting in the frequency domain; classic amplitude modulation communication schemes. Materials Needed: Two AD633 analog multiplier chips, data acquisition board, laptop, cable for connecting laptop audio output to the breadboard, headphone adapter for listening to signals on the breadboard. Cost $30.
The sinusoidal carrier is provided by the myDAQ, and the signal to be modulated is provided by a laptop, iPod, or similar device. The students are allowed their choice of speech and music for testing, usually drawn from their personal music library or from YouTube. The students can alternate between listening to and watching (using both the oscilloscope and frequency spectrum analyzer to see signals in the time and frequency domains, respectively) the original, modulated, and demodulated signals. An earlier version of the lab employed four-quadrant multipliers using the LM13700 operational transconductance amplifier, based on the National Semiconductor datasheet. While cheaper than the AD633 chips, this design required frequency recalibration. The AD633 is a relatively expensive integrated circuit (around $7 at the time of writing), but it rapidly “paid for itself” in terms of eliminating instructor and teaching assistant time dealing with calibration issues. In addition to alleviating the need for offset calibration, the AD633 is also well-calibrated in terms of signal amplitude. The output of the multiplier is the product of the two input signals, divided by 10 volts, so setting FGEN to its max output of 10 volts results in multiplying the signal by a unit-amplitude sinusoid. Setting the volts-per-division of the inputs to differ by a factor of 2 compensated for the factor of ½ that arises when multiplying by a cosine, allowing us to easily lay the original signal on top of the modulated signal on an oscilloscope plot, as shown Figure 2.
Description: The basic hardware setup for this lab may be one of the simplest of all of the TESSAL labs, since it involves two integrated circuits (AD633 four-quadrant multipliers) and no capacitors, resistors or inductors. One AD633 is used to modulate an audio signal, and the other is used to demodulate the signal, as shown in Figure 1. Using a carrier frequency in the audio range, instead of in the 5001600 kHz range employed by real AM radio broadcasts, allows students to hear and not just see the effects of frequency shifting. (Modulation at audio frequencies is known in the electronic music community as “ring modulation.”) Figure 2. Experimental results of amplitude modulation.
Figure 1. Circuit for amplitude modulation lab.
Our design deliberately omitted the lowpass filter needed to eliminate the high-frequency copies of the signal that appear after demodulation. With a carrier frequency of 15 kHz or above, these copies are at 30 kHz or above, which is well out of the range of both human hearing and what the headphones can reproduce. Although the original reason for leaving out the filter was to simplify the design, it turned out to be pedagogically intriguing, since lowering the carrier frequency enough allowed students to be able to hear the high-frequency copy. Depending on the source material, there are two carrier frequency ranges with interesting artifacts: a higher range where the high-frequency copy is the only major artifact, and a lower range that contains the 2544
higher-frequency copy as well as exhibiting distortion from spectral overlap in the initial modulation stage. This lab could be modified and extended in numerous ways. The most obvious extension would be to add the lowpass filter to eliminate the high-frequency image; students could observe and listen to the signal before and after this filter. The lab structure could easily support AM without suppressing the carrier, using a more traditional rectify-and-filter demodulation. The AD633 has an optional offset input, which could be exploited to implement for modulation with a carrier, and if a dual op amp is used, one could be used as a lowpass filter and the other and a couple of diodes could be used to form a rectifier. For listening to the modulated and demodulated signals, we created adapters that allow students to essentially plug their headphones directly into the breadboard. This is slightly problematic since the AD633 (as well as most op amps) have difficulty driving the reactive low impedance load of the headphones, which can result in distorted outputs. When observed, the effect can usually be alleviated by backing off slightly on the music player volume. A more robust approach would be to use a dedicated audio amplifier IC such as the LM386. 2.2 Filtering This lab requires students to build filters and examine the responses. Fundamental Concepts: Time and frequency responses of second-order systems, linear circuit building blocks, such as adders and integrators, based on op amps Materials Needed: Op amps, resistors, potentiometers, breadboard. Cost:
