On an Improved Method of Resolving the Frequency Difference Between Two Very Accurate and Stable Frequency Signals

Richard Percival, Clive Green

Abstract:	With the advance in accuracy and stability of modern atomic clocks, the need for a very precise method of detecting instabilities in their signals has arisen. For trample, with two hydrogen masers with a I in 10^15 frequency difference, the drift rate of phase due to the frequency difference will be 1fs per second or 3.6ps per hour. No ordinary universal counter would be capable of resolving the difference between the two signals. In response to this need, a state-of-the-art, computer-based very high resolution frequency and time interval measurement system has been developed. The measurement system integrates the most advanced phase Comparators with modern PC time-interval counting techniques. Software consists of Stable32(TM) routines capable of measuring both first- and second-difference variances. Internally the system also has a high stability rubidium oscillator and a high isolation 4 output distribution amplifier. With a hydrogen maser reference in a temperature-controlled room, the A7 specifications state short-term stability (Allan variance) of 1.5E-13/gate time, resulting in 1.5E-13, 1.5E-14: and 1.5E-15 _for 1s, 10s, and 100s gate times (r). Initial results suggest even better performance than this with Allan variances of 5E-14, 8E-15, 9E-l6and 3.5E-16 for Is, 10s, 100s,and 1000s gate times (r). The Noise Floor is reached after about 1,000s. Constant ambient temperature ensures typical drift of 2ps/hr. A 1'C temperature change adds less than 10ps. A single-shot rms resolution of 0.3~s was also measured, enabling the A7 to easiIy resolve the 3.6ps/hr drift rate between two hydrogen masers. The primary benefits of the A7 are improved accuracy and reduced measurement time. Fast measurements with high accuracy permit greater knowledge of the stability of the signal. The applications for a measurement instrument capable of such resolution are anticipated to be numerous, ranging from national standards and calibration laboratories, through cesium, rubidium, and quartz production to time-transfer measurements. Further work on reducing the size and improving the resolution is being carried out.
Published in:	Proceedings of the 31th Annual Precise Time and Time Interval Systems and Applications Meeting December 7 - 9, 1999 Marriott's Laguna Cliffs Resort Dana Point, California
Pages:	637 - 648
Cite this article:	Percival, Richard, Green, Clive, "On an Improved Method of Resolving the Frequency Difference Between Two Very Accurate and Stable Frequency Signals," Proceedings of the 31th Annual Precise Time and Time Interval Systems and Applications Meeting, Dana Point, California, December 1999, pp. 637-648.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In