GELLER Labs "Backyard Science"
Thoughts on a proton precession magnetometer design - a Proton Magnetometer Project
The goal of this project is a low cost high performance proton magnetometer (a digital magnetometer) kit for amateur scientists to be able to accurately measure and monitor changes in the Earth's total magnetic F field and to observe geomagnetic storms. There is a regular daily (diurnal) variation in the Earth's magnetic field. During events related to solar activity, there can be sudden changes in the field (such as a sudden impulse) as well as large excursions in the field which can be more than ten times the regular diurnal variation caused by magnetic storms.
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Monday, February 1, 2010
Changed to one record on the graph every 2 minutes and changed the graphical presentation. A light colored line connects the points making it easier to distinguish actual trending from the effect of arriving and departing vehicles. Overnight run: PDF,TXT, USGS PDF. As before, auto-retry continues to make successive measurements at about once per 6 seconds until a measurement with a desired figure of merit (FOM) is achieved. Overnight, that process took place every two minutes.
Continued to run through much of today: PDF, TXT, USGS PDF (there is an interesting disturbance in the AK record at almost exactly 12 noon UTC which also showed to some degree in the records of many other stations).
Tuesday, February 2, 2010
Overnight run: PDF,TXT, USGS PDF. Moving vehicle suppression using the FOM filter (2e-6) and relatively slow mesurement rate (2 minutes) seems to be working well now. Most of the morning vehicle traffic has been rejected.
I am hoping to begin some I Q FDM studies today. It turns out that two channel data operations with the Keithley 194A is not so clear, forturnately we have two of them, so probably it will be easiest to set up one for the in-phase channel and the other for the quadrature channel with each having its own IEEE488 address. The K194A has trigger-out / trigger-in BNCs, so synchronization should be just a matter of daisy chaining the trigger signal from one ADC board of one unit to the the ADC board of the other. This is just proof of principle testing, a 194A will not be needed for the experiment.
Also, for this initial testing, the plan is to supply the quadrature (Q) signal by delaying the signal from the PAR 113 preamp by 90 degrees using one channel of a Krohn-Hite model 3202 filter, low pass, probably set to around 4 kHz.
Unfortunately the USB6008 multichannel operation is a single multiplexed ADC, so it is possible if we go that route, that two 6008s would be needed for an I/Q FDM magnetometer version. Or, there still could be a custom ADC board. The data is beginning to suggest that single channel operation might be good enough. Perhaps I/Q operation would be reserved for a more advanced experiment or for conditions where the highest artifact suppression is needed just to achieve routine operation.
Or, this might just be another diversion. I am very curious to see how I/Q operation affects typical FOM values in my present test setup.
Wednesday, February 3, 2010
I/Q FDM operation: Overnight run: PDF, TXT USGS PDF (3 hours later). Note that U.S. East Coast time is -5 hrs from UTC. I/Q FDM operation appears to also work well. Note that none of the morning vehicle traffic registered on the morning record. With the relatively slow measurement rate at 2 minute intervals, some vehicle registration might just be luck (such as missing the daily school bus). The slow rate is intentional for a cleaner graph. FDM has generally been under 1 second, however the K194A are relatively slow. Minimum cycle time with the two K194A is presently on the order of 10 to 15 seconds, including the present 1.8s polarzation time.
It occurs to me that apodization might be problematic in that FDM is less responsive to CW (e.g. power line pick up), therefore any improvment in figure of merit at the desired fundamental frequency for total field measurements might be a trade off with increased registration of CW interference.
Another open question is whether any of the many signals (much lower FDM amplitudes, not currently recorded) which are detected near the fundamental frequency relate to NMR spectroscopy (i.e. fluid type dependent) or whether they are all simply artifacts (EMI, RFI, aliased signals, etc.).
Thursday, February 4, 2010
Overnight run (using I/Q FDM) into Thursday afternoon: PDF, TXT, USGS PDF . Note the slight positive offset on the right side from a car leaving and returning to parking within about 200 feet of the sensor.
QUESTIONS/COMMENTS/notice of typos, etc. send email to joegeller @ gellerlabs dot com
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