My son and I cycled up to campus today to turn off the computers in my office and in the lab (since the campus has a 2-day power shutdown starting tomorrow morning).
While we were on campus we checked out the latest version of his data logger code on the Windows machines there. We found that the installation was incomplete, and my son fixed the driver installation (testing with both an Arduino Uno and an Arduino Duemilanove, as they need different Windows drivers), Timer1 library renaming, and the Path variable on 6 of the machines (benches 7–12). These six are now all usable, except that the computer on bench 11 has only one USB port (the left port seems to be damaged). We did not update benches 1–6, but might go in next week to do it.
While my son was making sure that the drivers were installed and the other installation fixes, I checked out the function generator. This was a bit hard, as the output is from a BNC connector, and I had to hunt through the whole lab to find one cable with BNC connector on one end and clip leads on the other. I’m hoping that the cables are just in storage during break, because we need the function generator for the second lab. There was no problem driving a loudspeaker from the function generator, and the sine waves are clearly picked up by the microphone.
I also did a bunch more measurements of the I-vs-V DC characteristics of the electret mic, for lower voltages. This really changes my picture of the DC characteristics of the electret mic. Looking on a log-log plot, I now think we’ll need to piece together two very different models, a linear model for low drain-to-source voltages and a power-law fit for large drain-to-source voltages. The two-part fit is a common modeling trick for FETs (the linear region and the saturation region), but the saturation region is usually modeled as constant current.
Unfortunately, I think that this 2-part modeling is too complicated for the bioengineering students to discover on their own and we don’t want to go into such complicated models for FETs, so part of the microphone lab is now too messy for students to do. I mean, I can have them make the measurements, and see that it is a mess, and they can certainly figure out what size bias resistor to use to get a given DC output voltage given their measurements, but I don’t think I can have them do the more complicated fitting and they won’t come away from the lab with a simple model of the mic that they can use for design.
The usual operating conditions for the mic (with 3V across it) are well into the saturation region—I wonder how the mic behaves if the FET is operated in the linear region instead. Does this introduce a lot of distortion? If we want to keep the mic in the saturation region (and not linear or sublinear), we have to have at least 0.4V across it, which does put some constraints on the load resistor for converting the current to a voltage.
I’ll have to give some thought to how I’ll modify this lab to make it useful without being overwhelming.