Figure 2.

Schematic description of data acquisition for the background correction procedure. B₀ is constant while the rapid scan field varies between +B_m/2 and −B_m/2. In scan 1, the external magnetic field, B₀, is along the +z direction. The current in the scan coils decreases for the first half cycle and increases in the second half cycle. The net field is the absolute value of the vector sum of B₀ and the rapid scan field. It decreases in the first half cycle and increases in the second half cycle. In scan 2, B₀ is along the −z axis. The data acquisition trigger is shifted by one half-cycle such that the current in the coils increases during the first half cycle and decreases in the second half cycle. The magnitudes of the net fields are the same for scans 1 and 2.