Overview of the LISA phasemeter

The LISA phasemeter is required to measure the phase of an electrical signal with an error less than 3μcycles/Hz over times scales from 1 to 1000 seconds. This phase sensitivity must be achieved in the presence of laser phase fluctuations 10 ⁸ times larger than the target sensitivity. Other challenging aspects of the measurement are that the heterodyne frequency varies from 2 to 20 MHz and the signal contains multiple frequency tones that must be measured. The phasemeter architecture uses high-speed analog to digital conversion followed by a digital phase locked loop. An overview of the phasemeter architecture is presented along with results for the breadboard LISA Phasemeter demonstrating that critical requirements are met.