Inter satellite laser interferometry is a central component of futurespace-borne gravity instruments like LISA, eLISA, NGO and future geodesymissions. The inherently small laser wavelength allows to measure distancevariations with extremely high precision by interfering a reference beam with ameasurement beam. The readout of such interferometers is often based ontracking phasemeters, able to measure the phase of an incoming beatnote withhigh precision over a wide range of frequencies. The implementation of suchphasemeters is based on all digital phase-locked loops, hosted in FPGAs. Herewe present a precise model of an all digital phase locked loop that allows todesign such a readout algorithm and we support our analysis by numericalperformance measurements and experiments with analog signals.
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