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→Fundamental noise limit estimates
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They don't seem to have special documents (or knowledge of how to obtain them) for selling and shipping the cells. I figured it would be easier to buy from Precision Glassblowing than go through all that.
== 08/26/2016 ==
=== Fundamental noise limit estimates ===
Finished the Mathematica models of the Mx in coated cell, SERF with buffer gas and SERF with OTS coating + 30 Torr Ne magnetometers. The models estimate the best achievable performance of each magnetometer in perfect circumstances (taking into account probe and pump de-pumping). Each magnetometer has a cubic cell of 1x1x1 cm. The magnetometers rank by sensitivity (from best to worst):
# SERF + OTS coating + 30 Torr N2 (0.1 fT/sqrtHz)
# SERF + 300 Torr N2 (0.5 fT/sqrtHz)
# Synchronously pumped magnetometer with a paraffin coated cell (1.5 fT/sqrtHz)
Mx magnetometers sensitivity is limited because they have lower alkali vapor concentration (can't heat coating above 60°C), and radiation trapping (no N2 in the cells). The SERF with OTS coating has narrower zero-field resonance line, so the probe can be detuned much closer than in the magnetometers with buffer gas. They also run at lower temperatures and so they have larger T1 due to decreased self-spin destruction collisions rate.
The fetal heartbeat largest feature size is of an order of 1 pT, so 1 fT/sqrtHz sensitivity seems to be still good enough.
Download here: [[File:Magnetometers fundamental limit 1cm3.zip]]
== 08/30/2016 ==
I, Mike and Zak have discussed what might be wrong with the LabVIEW code. We have identified the following issues:
# <s>In the demodulation circuit diagram: two 16-bit integers are multiplied and produce a 32-bit integer, which is then scaled back to 16 bit. The original v16 program scales is by a factor of 2^-14, while it really should be 2^-16, to pick out the high word of the product.</s> It should be 2^-15, and another factor of 2 seems to come from accounting for the demodulation with a sine, maybe because the integral of sin^2(x) is 1/2 ?
# In the data sampling: the FPGA always acquires the data at 500ksps. The data is demodulated and filtered at 250ksps, and both these frequencies are hardwired. Sampling rate set by the host VI only sets the rate at which the data is send to the computer. This rate is smaller than the data acquisition rate, and the VI seems to discard the other samples, rather than average them.
# The ADC with the 16-bit resolution and ±10V input range is only sufficient to resolve ~10-20 fT at 15 μV/fT gain
Apparently LabVIEW has an entire library to process biomedical signals, including ECGs. Here is the link http://sine.ni.com/psp/app/doc/p/id/psp-1141/lang/en
== NEXT ==
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