[time-nuts] Mini Magnetic Sensor May Have Biomedical, Security Applications

John Miles jmiles at pop.net
Sat Nov 10 17:49:39 EST 2007

There was an interesting homebrew proton-precession magnetometer in Circuit
Cellar magazine a few months back.  Not very exotic compared to Cs and Rb
tech, but somewhat more accessible to experimenters.  You fill two 16-ounce
plastic bottles with either water or kerosene, wind some wire around them
both, and connect the coils in series at the bottom to cancel external AC
fields.  Then you magnetize the coils for a short time depending on what
liquid you used.  When you cut the current off, the coil will ring down at a
frequency proportional to the local magnetic field.  Accuracy and resolution
(looking back at the article) both appear to be in the sub-nT range.

ObTimeNuts content: apparently most PPMs use reciprocal counters to measure
the ringing frequency, but the author of this article used a trick I wasn't
familiar with.  He oversamples the signal and divides the amplitude
difference observed between successive pairs of even-numbered sample points
by that observed at successive odd-numbered points.  Then, the atan() of
that ratio yields an absolute phase.  Given a fixed sampling clock, the
change in phase from one 4-sample set to the next yields the frequency of
the waveform, removing any DC offset in the bargain.

Since each cycle is measured with 4 points instead of 2 points with a
traditional zero-crossing detector, you apparently get better noise immunity
as well (I'd guess 6 dB worth.)

It reminded me of how the TSC-5120A data sheet says they measure phase with
an arctan function.  I don't immediately see why this is better than an FFT
in either the TSC-5120A's case or the magnetometer's... except that it's
more amenable to implementation on a low-powered microcontroller in the
Circuit Cellar project.

-- john, KE5FX

> NIST is making it small, which is new, but the technology is
> well-established.  See, for example:
> http://www.geometrics.com/magnetometers/magnetometers.html

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