Frequency Dependency of Phase Stability of RF Cables

During qualification tests of phase stable cables for the ACES ("atomic clock ensemble in space") flight segment, an unexpected frequency dependency of thermally induced phase drift of RF cables has been discovered. These observations led to a thorough test campaign involving high performance RF cables from a variety of manufactures. Tests have been performed at frequencies ranging from 5 MHz up to 10 GHz using dual-or single-mixer type phase detectors. Measurements show a general trend of reduced thermal sensitivity when the cables are operated at higher signal frequencies. In general, thermal sensitivities measured at 5 and 10 MHz are significantly higher compared to those found at 100 MHz or even at 1 GHz. Furthermore, thermal sensitivity data measured at low frequency appear to exceed manufacture's data significantly. This paper gives a presentation of the test results and a critical analysis of the measurement setups.