2pi low drift phase detector for high precision measurements

In modern particle accelerators RF phase detectors have to fulfill high demands on the stability and the accuracy to meet the goals of state-of-the-art synchronization systems. Especially challenges are those cases where the RF signal phase shift accuracy must be measured with fs-accuracy for several hours over the full 2π phase detection range. In these cases special measures on the components non-linearities and the RF channel isolation have to be taken. The long-term stability of the phase detector is mostly affected by temperature and humidity variations. To meet the synchronization goals we have built a phase detector that incorporates a high speed dual ADC with a special front-end for continuous phase drift calibration. In the front-end we successfully combined the RF signal with an RF calibration signal (second tone) to compensate common phase drift that occurs in the microstrip lines, the RF transformers and the ADC. The second-tone RF signals are directly converted to the digital domain by the fast ADC and based on signal processing in the FPGA used to calculate the RF signal phase shift correction caused by detector drifts. In this paper potential error sources of the analog and the digital part of the so-called two-tone calibration technique that limit the phase detector precision are discussed. Finally, the experiment results are presented showing a long-term phase stability better than 4 fs rms evaluated at an RF frequency of 1.3GHz.