 White argued that any wall interaction could only be the result of a well-formed resonance coupling between the device and wall and that the high frequency used imply the chances of this would be highly dependent on the device's geometry.As components get warmer due to thermal expansion, the device's geometry changes, shifting the resonance of the cavity.In order to counter this effect and keep the system in optimal resonance conditions, White used a phase-locked loop system (PLL).