Performance-Based Assessments for AC Circuit Competencies

Use an audio frequency impedance matching (1000:8 ohms) transformer for this exercise, using a load resistance great enough not to overload the signal generator powering it. I recommend using a 100 kΩ resistor for the load. With the audio transformer’s impedance ratio, this should appear as an 800 Ω load to the signal generator. Avoid frequencies below 1 kHz - power line frequency is much too low for this type of transformer.

An extension of this exercise is to incorporate troubleshooting questions. Whether using this exercise as a performance assessment or simply as a concept-building lab, you might want to follow up your students’ results by asking them to predict the consequences of certain circuit faults.

Lissajous figures are very informative if one understands how to interpret them. The purpose of this exercise is for students to learn exactly how to do that, in the context of measuring frequency (ratios). Please use sinusoidal AC sources (variable-frequency signal generators) for this exercise.

The instructor will determine the voltage of the two sources (set the same), and also will graphically specify the Lissajous figure by drawing the desired figure on the oscilloscope screen illustration. The student’s task is to figure out what frequency ratio will generate this pattern on the oscilloscope screen, and set the signal generators to that ratio (one set to 1 kHz, the other to the appropriate frequency). This works best if the student can only see the signal generator controls and frequency counter displays, and only the instructor can see the oscilloscope display.

Here are some sample Lissajous figures to draw for your students to duplicate:

By the word overtone, I imply the musical concept: the successive harmonics appearing in this particular non-sinusoidal waveform. If I were to say “1st harmonic,” “2nd harmonic,” and “3rd harmonic,” this would specifically refer to f_fundamental, 2f_fundamental, and 3f_fundamental, respectively. The first of these would be given, the second nonexistent in a square wave with 50% duty cycle, leaving only the third as something to actually predict and measure. By “overtones,” I mean the first, second, and third frequencies greater than the fundamental frequency found in a square wave, that happen to be harmonics of the fundamental.

If you lack a spectrum analyzer in your lab, fear not! There are free software packages in existence allowing you to use the audio input of a personal computer’s sound card as a (limited) spectrum analyzer and oscilloscope! You may find some of these packages by searching on the Internet. One that I’ve used (2002) successfully in my own class is called WinScope.