Reference signal gives rotation rate and phase of the rotor Þ allows us to transform data from reference frame rotating with test cylinders to the laboratory (non rotating frame) where the expected signal is a constant displacement vector (for EP violation in the field of the Earth) or slowly following the daily motion of the Sun (for EP violation in the field of the Sun

Narrow band of raw data (in rotating frame) is filtered around spin frequency, and then transformed to non rotating frame Þ cleaner results (advantage of signal modulation)  

GGG prototype: the reference signal (II)