Towards an all-digital time scale

The implementation of an all-digital time scale is under way at NIST, by use of a novel 8-channel, all-digital phase measurement system based on subsampling techniques. The phase measurement system is used to compare output signals from several commercial atomic frequency standards; the phase differences between these signals, at different measurement times, will then be input to the algorithm used to generate the digital time scale. The subsampling technique allows the elimination of analog mixers in the system's front end, with their noise contributions, and yields performance that is comparable to or better than the present state of the art. Long-term (up to 80 days) comparisons of both common-clock performance and fractional frequency measurements between the latest generation of the digital measurement system and the commercially available system used in the implementation of UTC(NIST) are presented.