Thinking about the FDM Proton Precession Magnetometer Magnetic Field Noise Floor
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:
Thursday, December 1, 2011
Overnight: PDF, TXT. Sample free induction decay signal (FID) spectra PDF, log spectra PDF.
Legend for our TXT files:
Date Time F Scalar Larmor FDM FDM FDM Env. Polr. Msrd. Air Fluid
Frequency Ampl. FOM NBS/N Ampl. Time Tau 2 Temp Temp
11/18/2011 9:44:22 AM 53616.50 2282.030 0.812 2E-7 21.4 0.818 0.609 0.763 34.26 35.40
11/18/2011 9:46:20 AM 53616.33 2282.023 0.808 1E-7 21.0 0.833 0.607 0.750 33.78 35.52
11/18/2011 9:48:18 AM 53615.88 2282.004 0.808 3E-7 13.7 0.831 0.603 0.737 33.11 35.70
FOM-figure of merit; NBS/N-narrow band signal to noise ratio; Ampl.-amplitude; Env.-envelope; Polr.-polarization; Msrd.-measured
Friday, December 2, 2011
Saturday, December 3, 2011
Monday, December 5, 2011
On the FDM Proton Precession Magnetometer noise floor: I have been wondering for some time how evaluate the noise floor of the magnetic field readings of our FDM PPM instrument. (Recall that these are "single-shot" measurements, not a running average of the field.) For example, one question is at the sub 1 nT, how can the ambient noise values be accounted for? The ambient noise values include the natural "noise floor" of the quiet day geomagnetic field as well as suburban factors such as distant vehicles and even trains at 1 km when discussing field variations at the 0.1 nT range. As a first attempt to say something about the noise floor of the instrument, I went back to some of our journal records which I had marked as especially quiet geomagnetic records, magnetograms of note because of how quiet the geomagnetic field was, typically during an overnight period.
Take as one example a period from our January 23, 2011 magnetogram PDF. Now that I look at the original magnetogram, I can see that there have been still "flatter" records, however, let's go with this one as a first example of one approach to look at the noise floor. First, I took a relatively quiet segment of just over 250 values (2 minute measurement rate, so we are looking at one point on an instrument magnetic noise spectra, ~.008 Hz) PDF. Then, a best line fit (linear regression) of the segment was taken to remove the relatively show change in the F scalar over that 250+ point period, yeilding differences (nanotesla) about zero PDF. Then within this period, there still can be seen a slow variation having two peaks, however leave that waveform intact for now as simply part of the "noise".
Googling for histogram programs I found a free histogram program download from the Department of Physics University of South Alabama. The first histogram, with bins of 0.1 nT is from -1.5 nT to +1.5 nT PDF. Next, I zoomed in a bit PDF to -0.5 nT to +0.5 nT (1 nT full X scale). Finally, the graph was zoomed further PDF to +/- 0.2 nT. Note that for this quiet field magnetogram segment about 41% of our data points fall within a range of +/- 0.2 nT. While this type of noise floor analysis is merely indicative of our noise floor, it does appear to indicate that our useful resolution is probably well below 1 nT.
There are certainly fundamental noise floor issues which will define an absolute noise floor for a "conventional" proton precession magnetometer. Then there is the natural "noisy" variation of the geomagnetic field still present on the quietest geomagnetic days, and most amateur scientists in residential neighborhoods will have a man made contribution as well. Note, that the analog noise floor of the narrow band high gain amplifier (the NBLNA module) which we have gone to great effort to measure, is a different measurement. The NBLNA noise floor in AC voltage (voltage amplitude) across our pass band is not totally unrelated to the current question (the noise floor (jitter in the determination of the Larmor freqeuncy) of the magnetic measurements in nT made very 2 minutes). However, the AC voltage "amplitude" noise floor of the analog electronics only contributes to the magnetic noise floor in the sense that it might indirectly cause some level of freqeuncy scatter in the values ultimately returned by the FDM frequency estimator (frequency noise).
Tuesday, December 6, 2011
More measurements related to the FDM PPM instrument noise floor at about .008 Hz (for our present 2 minute measurement rate): The histogram discussed Dec. 5, was repeated for a segment of about 81 points at 2 minutes per point (~2.7 hours) from the overnight record. As before, a linear fit was performed PDF, then the relatively slow DC like change attributed to actual (F scalar) field change was removed to yield the differences PDF. The differences TXT, include any actual variation of the geomagnetic field, any suburban man made field noise, plus the FDM PPM noise floor (all unknown). The noise histogram shows that the values for this period fell within a range of about -0.8 nT to +0.8 nT in 0.1 nT bins with one standard deviation at 0.2 nT PDF. A zoomed in view shows that some 38% of the values for this field segment fell within +/- 0.1 nT PDF.
The standard method of evaluating the noise floor for magnetic field measurement instrumentation is to place the sensor in a Mu metal shield, the shield typically having at least three shielding layers. In the case of sensors sensitive to vector direction (e.g. fluxgate sensors) the shield can be oriented magnetic E-W to further attenuate the Earth's field. Unfortunately the relatively large size of our sensor package seems to preclude such standard noise floor testing.
Wednesday, December 7, 2011
Thursday, December 8, 2011
Overnight: PDF, TXT, the geomagnetic field was very quiet overnight.
Friday, December 9, 2011
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