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Lena/Oct 2016: Difference between revisions
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Expected attenuation with fc=20kHz (shielding + input filter + output filter) is 14 + 3 * (6 + 12) = 68 dB
Expected attenuation with fc=10kHz (shielding + input filter + output filter) is 14 + 4 * (6 + 12) = 86 dB
This noise will alias when the ADC samples at 20kHz, with a peak at 10 kHz, the digital low-pass filter should be able to take care of that.
Low-frequency gain of the differential polarimeter seems to be larger than the of SR570. It's unclear if it's a feature or a bug or a poor connection.
== 10/24/2016 ==
=== Testing the difference/sum mode ===
Tested how the difference only mode compares to the difference/sum mode in terms of field sensitivity. Both measurements were taken with the same polarimeter board.
When the polarimeter is balanced, the probe sensitivity in both modes is the same. When the polarimeter is misaligned, the noise in 0-10 Hz band increases for the difference-only polarimeter, but not for the difference/sum polarimeter. In overall, the noise floor for difference/sum polarimeter is really good and the probe noise level drops rapidly in 0-0.5 Hz band.
=== Small polarimeter imbalance ===
Difference between channels' light powers is 1.4 μW.
[[File:20161024SmallImbalance.png|300px]]
=== Large polarimeter imbalance ===
Difference between channels' light powers is 6 μW.
[[File:20161024LargeImbalance.png|300px]]
== 10/26/2016 ==
=== Pregnant patient in WIMR ===
Measured a pregnant patient in WIMR. Although all channels worked properly and probe sensitivity was 1 fT/sqrt(Hz), we didn't get any good data, and couldn't see the fMCG signal at all. Maybe there were large field gradients that we didn't properly get rid of, like from wires in a bra or other clothes. The patient returns on November, 14th.
Added a real-time fMCG preview into the FPGA so that we don't end up in this situation again.
== 10/27/2016 ==
=== Pulse-density modulation ===
Implemented delta-sigma PDM (pulse-density modulation) for the FPGA. Now the FPGA can output "analog" control signals from the digital outputs, which our FPGA has plenty of. This would allow us to control every sensor shell coil on all three axis, and permit us to feedback on the field to keep all the sensors in average zero field and thus extend the dynamic range of our measurements. The signal to noise ratio in 0-1 kHz range is surprisingly good, and is around 100 dB. The VI is configured to output the "analog" levels with 16 bit precision. The switching rate is 20 MHz, with the CLK output.
We want to drive the atoms with the high-frequency signal and have them act as the low-pass filter. Mike built a current supply for the coils that reverses the polarity of the coil current for 0 and 1 logical levels.
It would be great to replace the analog inputs with digital signals as well. This would permit us to use a bigger FPGA (NI PXIe-7971R) that could drive as many as 25 sensors. Texas Instruments has Verified Designs section, that explains how to design and layout ADC circuits. [[File:TI ADC design ECG.pdf]]
=== Group meeting ===
Thad is concerned that the excessive noise in our measurement could have been caused by the probe power noise. We need to modulate the pump power and see how in affects the measured field.
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