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FMCG/Data Acquisition with LabVIEW Software: Difference between revisions
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FMCG/Data Acquisition with LabVIEW Software (view source)
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This part of the guide will cover the actual data acquisition with the LabVIEW software. It will assume the user has ''N'' operational magnetometers which have been field-nulled and optimized. The first part will explain how to use the program to acquire data in '''DC-SERF''' mode, the second part in '''Z-mode'''.
=DC-SERF mode
In DC-SERF mode, the magnetometers are sensitive to magnetic fields in the Y direction. The program is set up to calibrate the magnetometers' response using the shell-mounted Y coils, and then collect data.
==Data Acquisition==
# From the '''FPGA Magnetometer.lvproj''' project, under '''Main Programs''', open the '''Magnetometer_16.1_chirp-calibration.vi''' program.
# The front panel can be somewhat intimidating, but a lot of it is not used. We'll hit on the parts that are necessary to take a measurement.
## The next prompt will tell you to calibrate the '''MAGNETOMETER_Y'''. The chirp will be applied to the same current supply, except now the FPGA input should be connected to the magnetometer signals from the photodiodes (via the I-V converters). Flip the switches below the green ports '''DOWN''' to make that happen. Then click '''OK''' on the prompt. You should see the chirp signals appear on the scope. If the chirp signals are too large or a DC field has caused the magnetometer signal to swing such that the output rails during the calibration, it will have to be redone later. For now, just assume that it went fine.
## Lastly, a '''Noise/Heartbeat Measurement''' prompt will show up. No further switching is necessary (besides maybe adjusting the room or coil fields) before hitting '''OK'''. A time series of length '''Run Time''' will be collected.
# When the time series has been collected, the progress bar will be completely lit up and the message '''DONE''' will flash above it. Collection complete! Don't hit the '''STOP''' button yet, though!
==Data analysis==
Now is the time to look at the data that was collected. The program does a really nice job of analyzing all of it as well.
# First, click on the '''Calibration''' tab. This tab will have information about the '''OPAMP''' (or circuit) calibration data. The upper-right window shows a fourier spectrum of the circuit's response to the calibration chrip. Because the circuit doesn't filter the signal at all, this should be pretty flat, outside of peaks near 60 Hz and higher harmonics caused by noise during the acquisition. It's also likely that the voltage recorded is close to 80-85% or the applied voltage (ie, applying a '''.02 V''' chirp will generally give you a '''~.172''' monitor voltage output. Never quite figured out why. The upper-right window is the phase response, which should also be pretty flat. The red '''Y Applied Field''' sub-tab basically takes the voltage spectrum recorded and converts it to an applied field spectrum using the output resistor and coil calibration values specified in the setup. Clicking the tall blue '''NEXT''' button at the far right will allow you to scroll through the various channels of data. The current channel is displayed at the top next to the large red '''STOP''' button. '''Note: this NEXT button becomes disabled if you click the STOP button. So don't hit that until you're done looking at the analyzed data.'''
# The '''Response Waveforms''' tab plots the magnetometer responses to the calibration chirps in the top window, and individually the applied (white) and response (color) in the windows below. This is a good way to ensure the calibrations have sufficient signal to noise and that the I-V outputs did not rail during calibration. Again, use the '''NEXT''' button to scroll between channels.
# The '''Response Fits''' tab plots the amplitude and phase response of the magnetometer by Fourier-transforming the data from the previous tab. Note that the data in the '''Y Resp Fit''' sub-tab is in units of '''V/fT''', as it has taken into account the size of the applied field from the '''Calibration''' tab. The raw transformed voltage data can be found in the '''Y raw resp''' sub-tab.
# Under the '''Results''' tab you'll find the data taken during the '''Noise/Heartbeat Measurement''' part of the procedure. The top window is the calibrated noise data (in units of fT/rHz), and the bottom window is the raw data collected (in units of V). The cursors in the top window are helpful for determining baseline noise levels.
# The '''Processing''' tab takes the time series from the '''Results''' tab and calibrates it using the response.
# The '''misc.''' and '''changelog''' tabs are not used on a day-to-day basis.
=Taking more data=
To acquire more data simply click '''STOP''', and decide what you'd like to do.
# If there was an error during any of the calibration steps, simply click the '''Run''' arrow again. You'll start over from the beginning, except you'll be given prompts asking if you'd like to recalibrate either the circuit or magnetometer. The circuit only needs to be recalibrated if the chirp amplitude or output resistor is changed. The magnetometer should be recalibrated after any changes to the temperature, bias fields, or laser intensities/detunings. On the next run, the '''Run''' value at the top left will increment by 1, and the '''Noise''' value will reset to ''00''.
# If you're happy with your calibrations and would just like to acquire more data using those calibrations, click the button next to '''another noise run?''' at the top. You'll notice that the next time you click the '''RUN''' arrow, the '''Run''' number will not change, and the '''Noise''' number will increment by 1. When the '''RUN''' arrow is clicked, all of the calibration steps are skipped and the program jumps right to the '''Noise/Heartbeat Measurement''' prompt. Beyond simply taking more "Magnetic Noise" data, there are a couple of things you can do.
## Turning off the pump beam (almost completely) disables the device's sensitivity to magnetic fields. The noise collected during a measurement with the pump beam off is indicative of the '''technical noise''' (or '''probe noise''', since much of thise noise is due to polarization or intensity fluctuations in the probe beam) in the system.
## Turning off the probe beam leaves only noise produced in the electronics of the system. Generally a battery of 3 runs '''Magnetic Noise''', '''Probe Noise''', and '''Electronic Noise''' is taken to compare the levels of each.
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