Lena/Sep 2016: Difference between revisions
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* [[File:2016.09.01 Group Meeting.pdf|Zack's notes]] |
* [[File:2016.09.01 Group Meeting.pdf|Zack's notes]] |
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* Thad suggested that we should have a logarithmic amplifier (so the signal is large enough for the ADC), and a PID loop at sub-Hz frequency to get rid of the DC fields around the sensor array. |
* Thad suggested that we should have a logarithmic amplifier (so the signal is large enough for the ADC), and a PID loop at sub-Hz frequency to get rid of the DC fields around the sensor array. |
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=== Fundamental noise limit estimates === |
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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): |
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# SERF + OTS coating + 30 Torr N2 (0.1 fT/sqrtHz) |
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# SERF + 300 Torr N2 (0.5 fT/sqrtHz) |
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# Synchronously pumped magnetometer with a paraffin coated cell (1.5 fT/sqrtHz) |
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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. |
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Download here: [[File:Magnetometers fundamental limit 1cm3.zip]] |
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== 09/01/2016 == |
== 09/01/2016 == |
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Revision as of 20:35, 8 September 2016
September 2016
BACK/NEXT
09/01/2016
Moving modulation/demodulation into LUT
Started on addressing the problems in the LabVIEW VIs. The first problem are the overflows in modulation/demodulation paths. The problem is that two 16-bit words are multiplied and they result in a 32 bit word, which is then scaled to fit back into a 16-bit word, and the scaling is not done correctly. I updated the VI, but it wouldn't compile because the compiler ran out of DSP48 blocks. The solution was to replace the Numeric multiplication blocks with High-throughput multiplication blocks and set the implementation to Look-Up Table, as described in this link http://forums.ni.com/t5/LabVIEW/Large-FPGA-vi-compiled-in-LV-2009-but-not-2010/m-p/1678820#M597625
Lab meeting
- File:2016.09.01 Group Meeting.pdf
- Thad suggested that we should have a logarithmic amplifier (so the signal is large enough for the ADC), and a PID loop at sub-Hz frequency to get rid of the DC fields around the sensor array.
09/01/2016
Talked to Mike about the insufficient bit resolution problem. Mike says the correct averaging should give us effectively more bits http://www.analog.com/library/analogDialogue/archives/40-02/adc_noise.html This should work as long as we increase the number of bits after the averaging.
09/08/2016
Implementing Mike's suggestions in LabVIEW (because it's easier to change the code than the hardware). The code mostly works, but still needs some debugging to make it compatible with the heartbeat recording VI.
Changes made compared to the previous version:
- Data is acquired at the maximum ADC rate of 500 ksps
- Raw ADC data is scaled up to int32 from int16 and anti-aliased by a rational resampler. The output data rate is ~20 ksps (2011 ticks at 40 MHz instead of expected 2000 ticks).
- Data is low-pass filtered at 250 Hz by a 6-pole Bessel filter.
- Data is sent to the PC with 32 bit resolution