Lena/Aug 2016

August 2016

08/15/2016

NURI paper

Talked to Dima about the NURI magnetometer network paper. His comments:

• The device section describes a solution without a problem. Add to the introducion:
  • need for the accurate timing
  • need to correlate signals from the different sensors
  • magnetometer has to be portable

Mx magnetometer sensitivity

Talked to Thad about a possibility to have a synchronously pumped scalar Mx magnetometer in the fMCG project. The advantages would be:

• not having to cancel the field
• no calibration
• no 1/f electronic noise

Disadvantages:

• spin-exchange relaxation
• no vector sensitivity

Calculations for an Mx magnetometer built with a paraffin-coated 1cm3 cell show that it should be possible to get around 3 fT/sqrtHz per 1cm of the probe length.

• Download the Mathematica notebook - File:ScalarMagnetometerParaffinCells.zip

Alkali vapor cells sources

Velichansky's cells

Asked around to find who could make some paraffin-coated cells for us. Dima's sources:

• Misha Balabas ($500-$700 per cell)
• Antonie Weis (sells 20x20mm cells that they no longer use)
• Wenhao's group (can make cells or fill cells)
• Precision Glass Blowing (had poor T1)

Asked Vitaliy Shutov from Frequency Standards Lab in FIAN, Troitsk. They have a setup that produces the cells in small (~10) batches. The cells are baked and sealed under vacuum and don't have stems. Cells can be filled with any gas mixture, but they don't make coating. The cells come in two sizes

• 8x15mm
• 5x3.5mm

Velichansky's group also has a stash of paraffin-coated 20x20mm cells.

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.

08/30/2016

The problem

The magnetometer can take the vector measurements in two channels: X and Y. The Y is orthogonal to both pump and probe, and provides the sensitivity to the main direction at DC, while the X direction signal is demodulated at 1kHz. An LabVIEW filter in FPGA is setup to separate the two signals. The Y signal is obtained by low-pass filtering the signal with two 2-order Butterworth filters, while the X signal is obtained by multiplying the time signal by a reference and low-pass filtering it by the same filters.

Digitization noise

The problem is that the Y channel often has large digitization noise (see the picture). The digitization noise disappears when one of the Butterworth filters is deleted (but then the filtering is insufficient).

Investigating the filter response

We want to find how much the filtering affects the noise of the X and Y channels. To do that we take two measurements:

Technical noise

Zero volts from the frequency synthesizer is connected to the FPGA input. The data is stored in 2016.08.30, Run2, noise 6.

X noise (time domain)

X noise (frequency domain)

Y signal (time domain)

Y signal (frequency domain)

Known signal

Mike suggested that we apply a known signal to the FPGA input to measure the noise produced by the processing VI. We are sending a 50 mV 13 Hz AC signal into the FPGA from a frequency synthesizer DS345. The data is stored in 2016.08.30, Run2, noise 7.

X noise (time domain)

X noise (frequency domain)

Y signal (time domain)

Y signal (frequency domain)