Gas station without pumps

2014 May 1

Sampling and aliasing lab

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 21:30
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Yesterday, I only briefly mentioned sampling and aliasing, which was the subject of today’s lab.  I was hoping they would read the handout, as that has some of the explanation missing from the lecture. That, as usual, turned out to be a forlorn hope. Some of them did read the handout, but not in a way that gained them any comprehension, and no one had done the prelab exercises, so the first hour (or more) of lab time was spent with students trying to figure out how to do a high-pass filter that did level shifting, so that the output would be in the 0–3.3V range of the analog-to-digital converter.  They all got to reasonable designs eventually (with capacitor values in the range 4.7µF to 470µF—I really like that the answer is not dictated by having only a small selection of components!).  I did have to re-teach Thévenin equivalents to some of the teams, as they were not getting the RC time constants that they claimed.

The level-shifting high-pass filter design will be useful again next week, for connecting the microphone to an op-amp audio amplifier.

I did not bring the stroboscope into lab as a demo—the demo had not worked all that well last year, and the students have all seen stroboscopes before.

PteroDAQ worked well for doing downsampling, and students recorded several waveforms.  I’ll see whether anything sensible is said about them in this week’s lab reports.  At least they had fun looking at the weird beat patterns you get if the signal you are looking at is close to the Nyquist frequency.

I also got the data today from the students who had a loudspeaker that behaved differently from everyone else’s on Tuesday (they tested another loudspeaker with the identical setup and got normal results, and I checked a few of their measurements—I believe they did just have a weird speaker).  There was a little metadata missing (like exactly what their fixed resistor was for converting current to voltage), but I was able to fit their data with just two more parameters on the model, a resistor and capacitor in parallel with each other, in series with the rest of the model:

    The bad loudspeaker has a higher than expected resistance at low frequency, then a 1/f-sloped region after the resonance peak, then a return to normal behavior. I modeled this loudspeaker by adding an extra R||C in series with the model we used for good loudspeakers.

The bad loudspeaker has a higher than expected resistance at low frequency, then a 1/f-sloped region after the resonance peak, then a return to normal behavior. I modeled this loudspeaker by adding and extra R||C in series with the model we used for good loudspeakers.

I have no explanation for the physical or electrical causes of an extra R||C in the loudspeaker.

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