I guess I'm diving down the rabbit hole on this one.
As I understand it, the output of a fluxgate transducer is really a combination of 3 things:
- fluxgate for accurate DC and low frequencies
- Conventional current transformer for high frequencies
- Hereward Transformer / amplifier to improve the low frequency response of the conventional current transformer.
Hereward Transformer: use feedback and a second coil to increase the effective magnetizing inductance, thereby reducing the cutoff frequency of a conventional current transformer.
https://cds.cern.ch/record/1068123/files/cer-002723994.pdfI attached a version of Lymex's schematic with major blocks identified. I think its missing some detail around the modulator section.