Hi all, Again, thanks for the great ideas. I will try and keep up with them all. The original Tolman-Stewart used this rotating coil and braking; they re-di d their experiment about a decade later with a constantly rotating system a nd then everything moved to ac and was easier. The recent papers from the
80s used the ac system. The problem is, the physics is harder, and we want this to be for undergrads. So we like the braking experiment better.
Re: noise reduction. We can try and reduce the noise by turning off monito rs and running at night. Ideally, we?d like to be able to run this as part of our undergraduate advanced lab, so making it an extremely low-n oise environment is not ideal. But we can try. We?ve actually tri ed mu-metal, and it is not great at keeping out the magnetic fields. We? ??re currently considering just adding a 60 Hz notch filter.
We haven?t yet seen the signal we want, precisely because right now the noise swamps the TIA, and brings it to the rails.
George, The dc offset when it is just sitting there usually sits around a v olt or so. We?re working with low-noise op-amps, and so they are n ot trimmable. We were using the LT1792 and that one we could trim and that worked well. We can and have tested the circuit with an actual honest-to- goodness current source, and with two coils, where we send a pulse into one coil and pick it up in the second coil. The first experiment works well, the second is inconclusive so far.
As for connections to the coils, to contact the rotating coil we use a rota ry feedthrough that one of my students found on the web:
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c.com/html/110.html . The stationary coil, we just bring the contacts out and onto the bench. None of the electronics is attached to the coils, it i s all on the bench. We connect our two coils in series when the wires come to the breadboard.
Jan Panteltje, I am intrigued by your suggestion, as we initially wanted to integrate the current pulse. Our thought was that integrating would allow us to integrate over the noise and be left with only the slower 0.5 s puls e. But in order to get a large current, the input impedance of the measure ment system has to be small. This is why the TIA, with its virtual ground at the negative input is nice, acts as zero input impedance for the current pulse. What would be the input impedance of your transistor circuit be?
Whit3rd, We started first using a charge amplifier exactly as you described , but without the switch. So to prevent pinning the op-amp we added a feed back resistor also. The system we had was very unstable, even for short pe riods of time. But we did not try it with a switch to keep the capacitor f or charging. You are right that the math is easy if we can integrate the c urrent to get the charge. Then we only need to know the starting speed and the total charge.
John Larkin, I have limited experience with a differential amplifier. Can those be built to reject the 60 Hz noise?