Oscilloscope practice lab

My thermistors and op amps were shipped yesterday morning, but did not come in today’s mail.  I hope to get them tomorrow. In the meantime, I’ve had a good conversation with Mylène in the comments on my previous posts.  She had some suggestions which are probably worth following up on:

• Do a stethoscope project for one of the labs, which would require simple audio amplification.
• Start student familiarization with the test equipment by having them use the multimeters to measure other multimeters.  What is the resistance of a multimeter that is measuring voltage?  of one that is measuring current? what current or voltage is used for the resistance measurement?
• Start oscilloscope familiarity by looking at the output of power supplies. What ripple can you see on the voltage output of a benchtop supply? of a cheap wall wart?  This requires the students to learn the difference between DC and AC input coupling for oscilloscopes.
• On the first day that students are using oscilloscopes, have them try looking at the output of a microphone.  Most already have some idea about what sound should look like as a time-varying signal, and it is fun for them to play with different sounds (singing vowels, clapping, playing music or test tones from their phones, … ).

Since my thermistors weren’t here, but I did have an electret microphone that I bought some time ago, I decided to try the oscilloscope labs.

The first thing I had to do was to adjust the scope probes to match the input impedance of the scope, using the square-wave calibration output.  I seem to need to do this every time I use the scope, even though I never change the leads. Perhaps I’m just impatient and don’t give the scope enough time to warm up, since I sometimes have to readjust the scope probes after using the scope for a while.  I assume that Steve has a handout on how to adjust the scope probes and why it is needed.  If not, we need to either find a good tutorial for the students on the web, or write one.

The first thing I should have done after adjusting the scope probes was to look at the wall wart output using AC coupling on the scope, to see how much ripple there is.  In fact, I did not do this until much later.

Ripple on the wall wart power supply with no load. The vertical scale is 20mv/division, so the ripple is about 40mV peak-to-peak.

If I filter the power supply with a simple RC low-pass filter (33kΩ and 470µF, for a 15.5 second time constant), the 100Hz ripple goes away (and it does seem to be 100Hz, not 60Hz—the trace is not steady if I use the 60Hz “line” trigger instead of the internal trigger).

Even after filtering, I’m still left with some high-frequency noise, decaying 23MHz bursts that seem to be somewhat irregularly spaced (I can’t seem to get a steady enough trigger to capture a second burst in a stable position, but they seem to be over 10µsec apart).  I assume that this is some sort of ringing in the input of the scope, as it is present even with the wall wart unplugged, and with the scope probe not connected to it. It gets bigger if I put a long loop of wire between the scope probe and the scope ground, but adding bypass capacitors seems to have little effect.   This signal is about 7mV peak-to-peak with the scope probe on the breadboard at the output of the RC circuit with or without the wall wart plugged into AC power.  Disconnecting the wall wart using the DC barrel plug reduced this high-frequency noise to about 2mV peak-to-peak, so I suspect that the wires connected to the scope probe make a difference.   The noise happens on both channels and seems to be the same on both.

The back of the electret mic. It looks just like the drawings of the CUI Inc part number CMA-4544PF-W, which is the cheapest microphone at DigiKey, so I assumed that it was that part.

I had bought the electret microphone without any spec sheet, and it is completely unlabeled. To figure out how to use it I had to guess and do a little web searching. I don’t remember where I got it (not DigiKey, since I don’t have their inventory label for it), so I pretended it was the cheapest mic from Digikey, the CUI Inc CMA-4544PF-W. While I was writing up this post, I remembered that I probably bought it from Sparkfun, as their electret microphone. But they give the spec sheet for the Knowles Acoustic MD9745APZ-F, which has very similar specs. But the pictures on the Sparkfun website and in the Knowles data sheet don’t match as well as the pictures in the CUI data sheet, so I think I have a CUI microphone, though not necessarily the one whose data sheet I used.

Circuit copied from the CUI CMA-4544PF-W datasheet. I used a 4.7kΩ resistor for the load, though the datasheet suggests 2.2kΩ—I didn’t have a 2.2kΩ resistor handy. I also used 0.56µF capacitor instead of a 1µF capacitor—again, using what I had on hand.

An electret microphone does not produce any electricity—you need to power it from a power supply using a series resistor.  The circuits shown in data sheets are all fairly similar.

With this circuit I had no trouble getting a good signal from the mic. My first attempts had just connected the wall wart power supply directly to +VS, but that produced a fairly large ripple signal (which lead me to look at the wall wart ripple directly with the scope).  Adding an RC filter with a fairly large resistor and capacitor (33kΩ and 470µF, for a 15.5 second time constant) cleaned up the power supply and no ripple was visible in the output.  The output of the electret microphone varied depending on how loud a sound was produced, but easily got 60mV peak-to-peak for a vowel sound and 150mV peak-to-peak for clapping.

A trace of rough vowel sound.

One interesting thing I observed is that I can get quite a large pulse just by moving my hand toward and away from the microphone, with pulses about 100msec wide. So it looks like the mic can go down to 10Hz, and possibly lower.

We would need an amplifier to use this as a stethoscope. Crude attempts (pressing the mic against my chest) were not very promising. I did not see any signal that seemed correlated with my heartbeat.