GELLER Labs "Backyard Science"
Thoughts on a proton precession magnetometer design - a Proton Magnetometer Project. Build an Earth's field magnetometer.
The FDM MAGNETOMETER1 project is a low cost high performance proton magnetometer (a digital magnetometer) kit under development for universities and amateur scientists to be able to accurately measure and monitor changes in the Earth's total magnetic F field and to observe geomagnetic storms. Magnetic storms can cause large excursions in the field and are of concern to interests ranging from electrical power grids, radio communications, and satellite operations, to aurora watchers and amateur radio operators.
1 Filter Diagonalization Method "FDM" (harmonic inversion), see Jan 21 and Jan 23 entries, based on: Vladimir A. Mandelshtam, Howard S. Taylor, Harmonic inversion of time signals and its applications, Journal of Chemical Physics (1997), Volume 107, Issue 17, 1997, Pages 6756-6769
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I created a new Proton Magnetometer Group at Yahoo groups for those interested in discussing proton magnetometers with an emphasis on Earth's field measurements. I will try to keep it as open as possible without getting attacked by spam. I will also try first with no review of posts, let's see what happens. Please keep it friendly and professional. Use of real names is preferred, possibly required in the future.
Comments and observations related to the SWCTRL board:
On October 9, 2010 we noted that the precession (FID) envelopes showed more consistent peak amplitude values than had been observed to date with earlier prototypes. We accidently reversed the sense of the small signal relay in that last printed SWCTRL board (V 0.5), so the relay pulled in (energized) just before the digitization of the free induction (FID) signal following the polarization process. We corrected the "error" in the latest SWCTRL board (V 0.8) and now the amplitude variation shot-shot has deteriorated to what it was before. Apparenly the error was a bit of serendipity, since we now observe far less FID amplitude variation using the relay to "pull-in" for the digitization part of each measurement cycle. This is a relatively minor refinement from the system point of view, since the number of auto-retry operations is about the same, however, every improvment made without an increase in cost, is a good thing.
To see the difference, PDF the V 0.5 SWCTRL board was reinstalled and run for 25 measurements at about 7 seconds per measurement (blue curve , slightly higher amplifier gain). Then the V 0.8 board was installed and the test was repeated (red curve, slightly lower amplifier gain, so the peaks stayed below ~2 V). The results confirm what was informally noticed, that the amplitude variation shot-shot of the FID envelop is far superior (less variation) with the V 0.5 board. This graph shows peak FID amplitude for the twenty five shots using a 7 second measurement cycle for both boards: PDF We believe that the only significant difference is that the V 0.5 board couples the counter-wound sensor coil pair to the NBLNA amplifier when the relay pulls in, using the normally open contacts, while the V 0.8 board couples the counter-wound sensor coil pair to the NBLNA amplifier when the relay de-energized, using the normally closed contacts. Initially, I thought that it might be slightly more desirable to digitize with the relay de-energized, and that there might be some power savings (albeit very small), since the digitize cycle is so much shorted than the typical observatory overall measurement cycle time. However, it appears that superior results are had using the V 0.5 contact arrangement. To verify the difference, traces will be cut and replaced with soldered in wires to make the V0.8 layout have the reverse relay sense operation (there were other changes, believed not related to this observation).
The superior performance on relay closure (normally open contacts used for digitization) might be attributed to the rare earth magnet in this model of the Takamisawa small signal relay (NAS5W-K) which apparently improves the relay contact closure performance. The NAS5W-K is an SMT device, however, we are able to gently bend down the relatively long SMT leads. With the leads bent down, they fit remarkably well into our MillMax gold collet pin sockets.
Do not confuse the above discussion with the zero-current operation, which refers to only changing the state of the relay when there is zero current flowing in the contacts and the counter-wound sensor coils. That important aspect of the design remains unchanged.
I hacked up the new SWCTRL board to change the sense (normally open to normally closed for digitization). The FID amplitude (amplitude of the precession signal) results (green curve) look about the same as the V 0.5 board PDF. Also, the chassis ground for the SWCTRL box is the same analog ground as the chassis (box) ground for the analog module. That means that potentially the two boards can go in one box, perhaps with a shield between them, or the two boxes could mount to an extruded aluminum section for a portable instrument. The newer SWCTRL PCB has a much nicer layout for the resonating capacitor bank and a better FET layout that allows the FET to stand vertical or to be mounted flat on the board. The FET does not even get warm, so there are no heat sink considerations. (That's why I'm moding the Ver 0.8 board, instead of just going back to the V 0.5 board.) Will run on the hacked up board for a while before running (hopefully) the last prototype run.
Project Documentation (very early stages)
QUESTIONS/COMMENTS/notice of typos, etc. send email to joegeller at gellerlabs dot com
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