Ágoston Schranz, who works as an assistant research fellow at the Department of Networked Systems and Services, Budapest University of Technology and Economics (Hungary), spent 22 days at the Faculty of Electrical Engineering, University of Ljubljana (Slovenia), under the supervision of Dr. Boštjan Batagelj. During this STSM, he measured the effect of ambient parameters, mainly temperature, on the phase drift of a periodic microwave signal modulated onto an optical carrier. As the precision of synchronization between remote locations is highly important in case of many applications, it is important to understand the origin of phase changes, as well as the expected amount of phase drift under given conditions. Several long-term measurements were conducted, using different optical link lengths between 0 and 17.5 km. The results showed that the phase drift is mainly due to temperature changes. The quantitative values are in good agreement with those predicted by theory.