IRFI1310N as an alternative for the switch control (SWCTRL) module.
Build a Geomagnetic Observatory ! 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 magnet
ometer) 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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Journal Notes:
Tuesday, November 1, 2011
Overnight: PDF, TXT. The geomagnetic field was relatively active today PDF.
Wednesday, November 2, 2011
Thursday, November 3, 2011
Friday, November 4, 2011
Saturday, November 5, 2011
Overnight: PDF, TXT, picture perfect normal quiet diurnal cycle today PDF.
Instrument health (excellent): sample precession spectra PDF, Log spectra PDF. polarization controller PDF, fdm figure of merit (FOM) and amplitude PDF. The polarization controller automatically adjusts the time of polarization to achieve a relatively constant amplitude precession signal. Without the polarization controller, the precession signal amplitude would change with changes in temperature at the outdoor counter-wound coil sensor stand.
I am running the SWCTRL module on an IRFi1310 N channel FET today. The reverse avalanche voltage is 100 V, for a faster first stage discharge of the powered coil (according to the Varian-Packard method). Will run overnight. So far, there is no significant improvement, which probably means that we are already optimized at 55 V for the intial first stage powered coil discharge. The FDM figure of merit (FOM) statistical mode has been trending a little lower towards 1e-7 from our now normal 2e-7, however the average FOM is still about 5e-7 PDF. The 1310 is a somewhat more robust part (EAS 420 millijoules), however, at first look, I see no strong advantage to changing from the IRLIZ24N (71 mj, Vdss 55 V) of the present design. At our operating polarization current of 1.5 A, we store about 23 mj in the powered coil. Also, the 1310 has a lower turn on R at 36 milliohms compared with 70 mohms for the 24N. However, since most of our wasted R is in the cable run, that lower on-R is insignificant, especially considering that the 24N never gets warm. For those with possible repair issues some years in the future, the IRFI1310N certainly offers a viable substitute if the 24N is less available. Opening up the precession waveform with apodization graph to 5000 points shows, at times, a very nicely defined free induction decay (FID) or precession signal PDF, perhaps no more often than with the 24N though..
Also, in the next revision of the NBLNA board, the frequency adjustment in the multiple feedback bandpass filter has been moved from the relatively high value in the feedback path (200 k trimmer) to a low value of a 100 ohm trimmer in series with a 49.9 ohm resistor to common near the input. At this point, I see no significant change from a performance point of view, and I do not think that those with the earlier version of the NBLNA board would see any measureable improvement in performance by making this change.
Sunday, November 6, 2011
Overnight: PDF, TXT. Overnight run on the IRFI1310N in the switch control (SWCTRL) module (100 V first stage discharge of the powered coil), at first look, shows no performance improvement: polarization controller PDF, FDM FOM and FDM amplitude PDF. As can be seen in the front panel (magnetogram) plot, the % success rate remains at 82%, now a very typical performance number for a figure of merit (FOM) threshold setting of 2e-6. Sample FDM spectra PDF, log spectra PDF.
The IRFI1310N coil discharge voltage is impressive. This oscillograph (LeCroy LT344L) shows the voltage at the drain of the FET during coil discharge. Initial powered coil current is 1.5A. First stage discharge voltage is 115 V, first stage discharge time about 275 microseconds, second stage discharge time about 32 us.
According to the Varian-Packard method, we should discharge non-adiabatically (very fast) below about 2 or 3 Gauss. The desired non-adiabatic discharge rate is described as short compared to 1/ (gyromagnetic constant * 3G) =~ 1/ (2.7^4 1/(Gauss-second) * 3 Gauss) = ~12 microseconds. For our powered coil, this means we should discharge from about the last 20 milliamps to zero in less than about 12 us. Recall that discharge begins at about 1.5A with a -di/dt set by the inductance of the powered coil (~22 mH) and the first stage discharge voltage of 115V (E=L(di/dt)). Then, we finish the discharge (second stage) via a secondary dump resistor, presently 1.1 kohm (1 tau L/R about 20 us). If my numbers are correct, it looks like we are in that ball park of 12 us for the last 20 mA, this needs further study. Of course the system has been running flawlessly for many months, so this discussion is somewhat of academic interest only. Also, keep in mind, we are discussing the 275 us of coil discharge, following the polarization pulse (~.5 to 2 seconds). And, now we are looking into the last 12 us of the 32 us fall time of the drain voltage (lower right corner) JPG.
Checked the coil inductance today at the electronics side in the lab (cable disconected from the electronics) white/black 21.63 mH, Q 19.4, 6.25 ohms, red/black 21.63 mH, Q 19.4, 6.27 ohms. In circuit, relay closed, 45.6 mH (more than the total of the two, probably the mutual inductance between the closely spaced side by side coils, needs checking). Changed the resonating C to 104.7 nF, tuning in a fast cycle mode by watching the amplitude shot to shot in the fixed polarization controller mode at about 2282 Hz.
Want to build your own FDM Proton Precession Magnetometer?
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