The secondary winding outputs a signal that is coupled through the core from the primary winding—see Figure 2. This
signal is affected by any change in the core permeability (slope of B-H curve) and appears as an amplitude
variation in the sense coil output. By using a phase
sensitive detector, the sense signal can be demodulated
and low pass filtered to retrieve the magnetic field
value. Another way of looking at the fluxgate operating
principle is to sense the ease, or resistance, of saturating
the core caused by the change in its magnetic
flux. The difference is due to the external magnetic field.