Gas station without pumps

2012 December 19

Trying the oscilloscope practice lab

Filed under: Circuits course — gasstationwithoutpumps @ 19:14
Tags: , , , , , ,

The back of the electret mic, which looks just like the drawings of the CUI Inc part number CMA-4544PF-W, the microphone I’ll be including in the parts kit.

While my son worked on debugging the data logger software on Windows, I tried out the Oscilloscope practice lab, using the power supply, scope, and multimeter in the lab.  The first bench I sat down (Bench #1) had no power supplies, though I believe that problem will be remedied before classes start (returning the borrowed supplies from another lab or from other benches in the lab, since several had 2 power supplies).

The power supplies are quite nice, and provide very precise voltage control, but it will definitely be necessary for the students to  use the power supply together with the multimeter set up as an ammeter to measure the DC current through the microphone.

The current through the electret microphone as a function of the voltage across it.Note that the current measurements fluctuated by about ±0.3µA.

The current through the electret microphone as a function of the voltage across it.
Note that the current measurements fluctuated by about ±0.3µA.

I clearly needed to do more measurements between 0 and 1v, to get a better sense of the shape of the curve there, and the limitations on using the microphone at a low voltage. Had I been by myself, I might have spent more time in the lab, but we had to get my son home for an early dinner so that he could get to his theater practice on time. I’ll certainly be suggesting to the students in the write up that they use 0.1 v spacing up to 1 v, then 1v spacing up to 10v (the maximum rated voltage for the mic they’ll be using).

It was a good thing I went in before the course started to play with the equipment—I had expected some trouble with the oscilloscope, since those controls seem to be randomly organized on all digital scopes, but I had not expected trouble with the multimeter. The fancy ones in the lab have options for front or back connections of the leads, and for triggered or auto-running measurements. It took me a bit of random button pressing to figure out how to turn off the triggering and get continuous measurement.

I also put the microphone in series with a resistor and stuck a scope probe on it. It took me quite a while to get the voltage scaling and time base set reasonably on the digital scope, but I eventually managed to find all the controls I needed, and I think that I can show them to the students. Given the complexity of the user interface for the oscilloscope, I think that the simple measurements of the current vs. voltage curve and getting the scope to show both continuous waveforms (whistling or speaking) and triggering on a hand clap will be sufficient for the second lab. If students are done sooner, I could have them hook up the frequency meter and try to determine the pitch of their whistling or sustained vowel sounds.

The digital scope does allow seeing much lower frequency waveforms than my analog scope at home, and was clearly able to see 2Hz signals from moving my hand towards and away from the microphone (though 4Hz signals from faster hand movements were much clearer).

I still need to decide how many amplifier labs will use the microphone. I was figuring that the first op amp lab would do a gain of around 6 or 7 with AC coupling.
And that we’d follow that up with a power amp lab using an nFET and a pFET to make a class AB output stage after an op amp, but I’ve not wired up a power amp circuit yet, and I should do that before I order 20 copies of the transistors.

I think I’ve done all my orders except the Digikey one (and plastic bags to put the parts in). Unfortunately, it looks like WordPress.com lost 2 days worth of edits to the page of parts orders, which is going to be hellishly hard to replace. [Correction: the updates were in the autosave repository, and I was able to restore all but a few minutes work.  Hooray for autosave!]

4 Comments »

  1. I know what you mean about the random organization of scope controls. Sometimes there are faint lines around a group of controls to indicate the group that relates to Channel A amplitude, the groups that relates to Channel B amplitude, and the group that relates to time-base for both channels. It can be helpful to point out these categories to students so they can start to use a search pattern instead of randomly scanning across the front panel.

    When introducing the scope, I divide the controls into two groups: the ones I expect students to use/understand and the ones I expect students to set and ignore. To make this point, one of their tasks on the first day in the shop is to fill in a worksheet. The worksheet instructs them to set the “ignorable” controls to given settings, and instructs them to play with the other controls and write down what they do (I have them use the probe calibration test point as a signal generator at first). The worksheet them becomes a reference guide on future labs; inevitably, someone else’s class will change the scope controls, or my students will accidentally change a control, and the worksheet gives them a “known good” set of settings so that I don’t spend my entire shop period running around fixing people’s scope controls. I wonder if a similar approach would work for your students? The document should be available on Scribd in both PDF and Word formats, in case it’s useful.

    I have found that getting students to measure frequency by counting squares on the scope display is useful both for helping them solidify their understanding of what the display means, and also forces them to practice using scope controls (for example time/div, volts/div, vertical and horizontal position).

    FWIW, my students find triggering and AC vs. DC coupling to be the hardest things about scope operation to wrap their minds around.

    Comment by Mylène — 2012 December 20 @ 14:42 | Reply

    • We’ll work on AC vs. DC coupling in the first lab—the electret mic is good for that, since there is a DC signal (which they will be designing by choosing the series resistor) and an AC signal (which they will be trying to observe on the scope). The AC signal is much smaller than the DC bias (maybe 1% as big). I expect triggering to be a bit confusing for students, but that was one of the easier controls on the scope to figure out. I’ll look at your worksheet and see if something like that would help my students.

      Not only will I have to learn the layout of the controls for the Tektronix scopes in the lab, but I bought myself a Bitscope Pocket Analyzer USB scope, which has an even weirder set of controls (“ActiveTouch” in which buttons do different things depending where on the button you click—the most non-intuitive interface I’ve dealt with in a long time). There is something to be said for the simplicity of the old analog scopes, where each of the control knobs did one thing, and was labeled with what it did.

      Comment by gasstationwithoutpumps — 2012 December 20 @ 20:48 | Reply

    • Your worksheet won’t be much use to my students, as it assumes you have simple, sensible controls like an old-fashioned scope, where you can find the controls by looking at the scope. Our scopes are Tektronix 3000 series (3052 or 3054) and all necessary operations are hidden two or three levels deep in menus that require several button presses to select, with just a few knobs controlling all parameters. It’s not enough to know that you want to change the time base, you have to guess where they’ve hidden it and what mysterious other things you’ll accidentally change while looking for it.

      Comment by gasstationwithoutpumps — 2012 December 20 @ 20:55 | Reply

      • *laugh* Yes, there are lots of advantages to using simple analog scopes for students’ first point of contact. At the very least, perhaps a list of recommended settings would help them navigate back to a known-good position — especially, as you say, when nested menus make it highly likely that students will accidentally change things that they don’t even realize exist.

        Comment by Mylène — 2012 December 21 @ 16:29 | Reply


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