A dimmer control device is disclosed which determines when to switch a voltage level supplied to a light fixture comprised of a plurality of light bulbs by monitoring the phase relation between an input voltage and input current. The method includes confining movement of the body to a selected volume, inducing a static magnetic field in the volume, inducing a pulsed radio frequency magnetic field in the volume, and receiving nuclear magnetic resonance signals from the body. A common mode control circuit is connected to the common mode node, and exhibits a substantially zero output impedance.