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Speake magnetometers
I've been researching the building of a magnetometer based on the Speake FGM 3x sensors, and I remain thoroughly confused about the mechanism by which these sensors, and their ASICs, work. They are called fluxgate sensors, which they most certainly are not. Given a monopolar DC supply, they return a TTL signal with variable frequency of some tens of kHz, which is a quasi-linear function of the detected magnetic field strength. A true fluxgate would return a 2xf signal given a 1xf input. The latter signal would generally be buried in noise and the stimulating 1xf signal, so it would most likely be retrieved and measured with some sort of phase-sensitive detector and/or lock-in amplifier circuit. I have researched these sensors, and their signal analysis circuits, all over the place online, and have found no coherent explanation for how the sensor transduces a magnetic field into a variable frequency output. The reconstructed schematic I've seen looks like some sort of oscillator dependent on a pair of inductors. Does anyone know for certain what class of magnetometer this belongs to and how it works? I'd like to know so that I can evaluate and design an instrument rationally, which would require an understanding of the principles of operation. Any help, including a reference to my desired information would be much appreciated.
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I'm replying to my own thread because I did find some information on variable permeability devices, which do output a pulse train the frequency of which depends linearly with field strength. The link is found in Atul Asthena's comment, giving the link to Don Lancaster's archives, muse104.pdf. I don't know why I didn't see it earlier. -
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"Location: Corvallis, OR"
Interestingly, we share the same home town.Comment
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The Fisher FX3 is described as a "differential induction" magnetometer as opposed to fluxgate, and it sounds like it has the same "beat frequency" operation as with the Speake sensors. Personally, I'm not sure what the Speake sensors are; I'm not an expert on fluxgate methods and took them at their word that they are fluxgate.Comment
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The Fisher FX3 is two induction coils (20,000 turns of #40AWG wire on 8 inch ferrite cores) wired differentially. No excitation or AC signals are involved, when it's moved through a non-uniform field the field induces a differential quasi-DC signal which is amplified and used to modulate an audio frequency VCO for driving the speaker.
To my knowledge it is the only "outdoors use" differential induction magnetometer ever commercialized. We tried to farm out the inductors but got no-bids because companies in the inductor business were afraid to touch 20,000 turns of #40AWG. One company did bid-- $85 apiece, each machine requires two. So we said heck, we'll do it ourselves. Our tech Marv build a high-speed winding machine from a cheapie Sears lathe and set everything up so it didn't break the wire. And he and I also built a calibration system so that their antenna sensitivity could be matched in pairs. A few years later the company bought a real solenoid coil winding machine for the purpose. The original calibration system is still in service.
--Dave J.Comment
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Hello Dave,Originally posted by Dave J. View Post
Nice to read the “secret”from you , … wow……..can see the battle with that 20.000 turns copper wire 40 AWG(0.08 mm ?) and a lathe…!!!
Best regards.
Ap
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