Journal notes, Third Revision of SWCTRL
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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Project Documentation (very early stages)
Journal Notes:
Monday, November 1, 2010
Overnight: PDF, TXT, one small slow level shift after 1 am and one triangular shaped pulse before 4 am, otherwise the magnetogram shows continued quiet in the geomagnetic field overnight.
More minor changes to the SWCNTRL board (Ver. 0.8.6) before what hopefully will be the last prototype run. PDF Evening: Improved the FET ground return path back to the polarization power supply. Comments welcome.
Tuesday, November 2, 2010
Wednesday, November 3, 2010
Yet more minor changes to the SWCNTRL board (Ver. 0.8.7) before what hopefully will be the last prototype run. PDF On the right (analog) side, the labels Coil 1 and Coil 2 were reversed. Comments welcome.
Thursday, November 4, 2010
Friday, November 5, 2010
Overnight: PDF, TXT, still very quiet. Ordered prototype boards for the SWCNTRL board (Ver. 0.8.7). PDF
Sample spectra PDF, log spectra PDF. One of the experiments that I still would like to try is to synchronize the start of digitization with a power line cycle to see what affect that has on the consistency of the spectrum signals (other than the fundamental frequency ) of the digitized precession signal (the free induction or FID signal).
One of our readers had previously mentioned low noise amplifiers from THAT Corporation. One of their parts, the THAT 1510, is pin compatible with the ADI SSM2019. The ADI SSM2019 is performing well in our FDM magnetometer application, however, we should test a NBLNA using the 1510 to see if there is any significant improvement in the noise floor.
Saturday, November 6, 2010
Sunday, November 7, 2010
Observation:
Now that the relay is energized for the digitization cycle, I was thinking maybe it would make sense to open the relay (de-energize, coil off) after the signal processing part of the measurement cycle, before the relatively long measurement cycle delay. That works well (tried it this morning), however then I was reminded that when the amplifier is (partially) disconnected from the counter-wound pair, the NBLNA power supply current increases from 8 mils to 22 mils (+/- 6 V power supply voltages).
Also, I had been meaning to note to readers that a benefit of the SWCTRL (zero-current FET-Relay switch control board) PCB is that it makes it really easy to visualize the two sensor coil configurations, polarize and digitize. Recall that during polarization, the relay connects only one of the two coils to the polarization power supply and polarization FET switch. Then, during digitization, the counter-wound pair is re-configured as a center-tapped coil and connected to the differential input of the NBLNA input amplifier (via the RF/EMI common-mode filter). Now looking at the PDF , the three lines extending between the digital ground plane on the left side of the board and the analog ground plane on the right side of the board are the center-tapped coil (of the counter-wound sensor coil pair). You can also see the center-tap configuration by inspecting coil 2 connection at the counter-wound sensor coil terminal block. The center-tap trace is the middle trace to the right side analog common.
The reason these two thoughts tie together, is that now I think I see why the NBLNA current goes up when the relay is in the polarizaton (presently the coil off, or normally open position). I believe it is because while the coil to be polarized is totally disconnected from the NBLNA input, the remaining "noise cancelling sense coil" is still attached. Likely the DC bias (10 k bias resistor on the open side PDF, vs < 10 ohms on the sense coil side) and AC signal imbalance at the input (one coil full in band AC pickup without the counter balanced ambient pick up cancellation signal) is what is causing the higher current. The AC signal component is probably the dominant signal, saturating some of the amplifier stages. For now, it is not a problem, just another curiosity perhaps solved. Another possibility is the ability to sample ambient in band signals not related to the precession event (the FID signal) by making some sort of measurement based on the sense coil alone (i.e. with no noise cancelling), although that would probably call for another switch to reduce gain, to avoid saturation ... more interesting possibilities! There could also be a switch to short or disconnect the sense coil signal, however, for now there is no need for the increased complexity, and after many months of operation, no concern of harm to the NBLNA board. However, the DC current at the output transformer primary during the input imbalance needs to be checked.
Project Documentation (very early stages)
QUESTIONS/COMMENTS/notice of typos, etc. send email to joegeller at gellerlabs dot com
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