Short and long term stability of HP Cs standards including environmental effects

The Finnish Centre for Metrology and Accreditation (MIKES) has two HP 5071A Cs-beam standards (standard version from 1997 and high stability version from 2000) and one CH1-76 Passive hydrogen maser (since February 2003). A special four-channel phase comparator with better than 0.3 ps single shot resolution is used to study the short-term stability of the Cs-standards, with hydrogen maser as a reference. Long term stability is measured with GPS as a reference. Allan stability plots indicate that the high stability version 5071A is approximately three times better than the standard version. The high stability version has only slightly smaller long term drift (ageing) than standard version but frequency retrace after clock failure or power blackout is significantly better in high stability version.