6/12-channel Synchronous Digital Phasemeter for Ultrastable Signal Characterization and Use

Nowadays, in primary time and frequency laboratories we can find high spectral purity signals in the 10 MHz - 10 GHz range generated from cryogenic oscillators or ultra-stable lasers together with frequency combs. Their short-term stability surpasses by one to two orders of magnitude the performances of active hydrogen masers (AHM), while in the long-term AHMs still have a better behavior. The new technology can be considered mature for what concern spectral purity, but we cannot say the same about complexity, power consumption and reliability. In this sense, it is important to measure ultra-stable sources with respect to AHMs. First, to test their spectral purity or, at least, to give it an upper bound; second to have a continuous monitoring; finally, to combine them in order to get the best of all in term of phase noise and frequency stability. All of these requirements can be satisfied by the system we are developing. It is a multi-channel synchronous and real-time phasemeter based on Tracking Direct Digital Synthesizer (TDDS) technique. The results related to the first prototype are presented.