Magnetic Field measurements: Weird FFT signals I cannot explain.

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Discussion Overview

The discussion revolves around the unexpected frequency peaks observed in magnetic field measurements taken with a Honeywell HMC2003 sensor. Participants explore potential causes for the observed signal characteristics, considering both physical and mathematical explanations, as well as the implications of measurement techniques and environmental factors.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant notes the presence of discrete peaks under an envelope in the FFT results and questions the physical or mathematical origins of these signals.
  • Another suggests that higher frequencies above 1Hz could be aliasing into the observed frequency range.
  • A participant mentions analyzing the spectrum for harmonic waves, specifically noting the presence of 50Hz signals, which should not be visible at a 1Hz sampling frequency.
  • Some participants propose that the source of the signals could be environmental factors, such as nearby equipment or the Earth itself, and suggest testing the setup in a quieter location.
  • Concerns are raised about the significance of the signal strength, with one participant indicating a measurement of 250µGauss and emphasizing the need to identify the source for discussion purposes.
  • There is a recommendation for using anti-aliasing low-pass filters, with some participants questioning their necessity for measurements at sampling rates below 1kHz.
  • A mathematical expression related to noise is presented, but its relevance to the discussion remains unclear.

Areas of Agreement / Disagreement

Participants express various hypotheses regarding the source of the signals and the measurement setup, but no consensus is reached on the cause or the significance of the observed peaks. Multiple competing views remain regarding the interpretation of the data and the appropriate measurement techniques.

Contextual Notes

The discussion highlights potential limitations in the measurement setup, including the influence of environmental noise, the need for filtering, and the effects of sampling rates on data interpretation. Specific assumptions about the measurement conditions and equipment are not fully resolved.

tboh
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Hi all!

First off: I don't know which forum to post this. I'm no grad student or PHD so academic forum seemed wrong, and it's not really just math, since it was the result of a measurment and the signals can come from environment, etc.

Ok. I measured the magnetic field using a sensor (HMC2003 of Honeywell) that gives me an analog output voltage dependent on the magnetic field. I have used a Agilent 34411A Multimeter to save the data on the computer.

Most important: Sampling rate was 1 data per second! 86400 measurements were made. Of the result I got I used the FFT algorithm (traing also different Windows just in case) of Origin 8.6. The result I got is shown in picture below.
http://postimage.org/image/j8jra66kl/

So my problem is basically this distribution of discrete peaks under an envelope. What can produce such a signal in my measurment? Also the number of peaks is different for the two envelopes at 0.2Hz and 0.4Hz and the difference of frequency the same, so the second should be no harmonic wave of the first. I found no signals of that frequency around my measurment location.

So at all those experienced physicists out there: What can cause these signals? Physically? Or some mathematical property?...
 
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Could be higher frequencies (above 1Hz, that you are not filtering) ailiasing as a frequency component in your band of interest.
 
I also analysed the spectrum in other frequency ranges. The only real peaks I found for the harmonic waves of 50Hz, the power line frequency here. They should not be visible in a 1Hz sampling frequency... Especially not like that :)
 
Some thoughts:

Maybe it is real. What is the thing that you are measuring? A refrigerator? The Earth? Do the spurs go away when you remove the source (good luck if Earth)?

Maybe the magnitude of these spurs is not significant. What would be the field strength of these spurs (in gauss, not "dB") ?

You should be using a battery to power the sensor.

Too bad you can't power your Agilent with a battery.

Maybe it's the proximity of your PC? Try data logging without being connected to PC.

You should still be using an anti-aliasing low-pass filter on the output of your sensor.
 
I'm measuring the magnetic field in a lab. So basically everything, from Earth to some remaining signals from neighbor-experiments.

The strength of those signals is not so high with 250muGauss. But still its not completely insignificant (high precision experiment) and I don't know where it comes from. Identifying a source would help because I have to discuss the signal.

50Hz Signals from sensor are also there with Battery, I tested.
And I never heard of an anti aliasing low-pass before ^.^ Should it be used in general for measurements like that with sampling rates unter 1kHz?
 
You should do some level of AA filtering, depending on your band of interest and what amount of non-interesting signals are present outside your band of interest. For background read:
http://www.ni.com/white-paper/3016/en#toc4

Try transporting your measuring system to a "quiet" place away from your lab. If you see the same thing, then the source of the mystery spurs is likely your measuring system, itself.
 
2^(100 / 6) = 104032
(~6 dB per bit in A/D)
noise
 

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