In my Applied Electronics for Bioengineers course, I had planned to spend the lecture time today talking about sampling and aliasing, but that is not what ended up happening.
I am making it a point to answer student questions (unless they are irrelevant) first, before doing whatever I prepared. The point of the lectures is to help students understand the reading and do the design work for the labs, and anything I have prepared is just a best guess at what the students need. Their questions address more directly what they perceive as their need. Most of the prepared lecture material is in the book (I wrote the book based on what I have covered in lectures), so answering questions from students who have read the book and are still confused is going to be better than my repeating what is in the book.
Today students had some logistic questions about what to write up for Lab 1 (not much, it was just soldering headers onto the Teensy boards and setting up PteroDAQ—I just asked for a description of what they did, whether anything went wrong, and what they did to fix the problem) and about prelab homework for Lab 2 (do it, but don’t turn it in, it is just setting up gnuplot so that they can use it for the lab). Those only took a couple of minutes.
The big question that diverted the entire flow of the lecture was a request for an explanation of the high-pass filter in Lab 2 that is used for recentering the function generator output at 1.65V. This lead to several things:
- Description of block diagrams as functional blocks connected by interfaces, and why this was an important concept in engineering. Frequency and voltage information was put on the block diagram connections.
- Capacitor symbol and DC-blocking property of capacitors.
- Resistor to Vref and why that would cause the output to become Vref, if there was no current through the output.
- Back to the block diagram to add the constraint that the analog-to-digital converter on the Teensy board couldn’t take any current from its input.
- Definition of “gain” as .
- Showing the high-pass filter Bode plot as two lines meeting at the corner frequency, and giving the corner frequency as , without derivation. I promised the students that we would derive that result in a few weeks, once we’ve had complex impedance.
- Replacing the resistor to Vref with a pair of resistors to 3.3V and Gnd.
- Introduction of the triangular ground symbol, and rejection of the chassis ground and earth ground symbols as not relevant for the class.
- Derivation of the voltage-divider formula from Ohm’s Law, using the important constraint that no current is taken from the output node of the voltage divider, so that the two resistors have identical currents. I had the students help with this, in order to elicit the most common mistake
- Assertion, without derivation or explanation, that the RC time constant for the high-pass filter should treat the two resistors as being “2R” rather than “R”.
For the last couple of minutes of class, I finally got to do the demo with the homemade stroboscope and pendulum of aliasing, but it was not very effective. Even with the lights off in the classroom, there was enough light through the windows to wash out the strobe. I could not easily keep the pendulum swinging with one hand and adjust the strobe with the other. If I do this again next year, I should make a panel with about 20 of the LED boards, for around 2.35A during the flash. At 1.64ms for the longest flash, that’s 3.85mC, which would drain 8.2V from the 470µF capacitor, if the power supply weren’t capable of delivering that much current (but I have a 6A 9V supply, so there should be no problem delivering full power). Hmm, maybe I should make up that panel for the Mini Maker Faire, instead of the wimpy 4-LED strobe I now have.
I’m actually pleased that I didn’t give the lecture I had planned—my book, which was based on my lectures, already covers the material adequately, and I’d much rather spend precious class time explaining the things that aren’t clear in the book. The only way I can know what the students need to hear is for them to ask for clarification where they are confused.