Walter A. Johnson

Design, development, and initial measurements of a 1.4-mm radiometric system

The development of a 1.4-mm heterodyne radiometric system and the initial measurements taken with it are described. The \Delta T of the system at the antenna terminals was approximately 18.5\deg K for a 0.25-Hz post-correlation noise bandwidth. Measurements show that the attenuation through the atmosphere at this wavelength is primarily due to water vapor, and an estimate of the zenith attenuation is given by A (dB)= 2.8w , where w is the precipitable water in centimeters. Measurements of the antenna half-power beamwidths using the sun as a source show that at 1.4 mm the atmospheric turbulence effects are not appreciable for observations through the atmosphere at zenith angles less than 45\deg with a 15-foot antenna.

Self-Monitoring and Self-Assessing Atomic Clocks

From digital communications to satellite navigation, remotely synchronized clocks play a role of primary importance. The failure of these clocks will lead to not only service interruptions, but also, in some cases involving satellite navigation, more dire consequences with potential loss of life. Consequently, ensuring the integrity of remote clocks is now an issue of considerable import. In this paper, we demonstrate that an atomic clock can autonomously assess its own frequency stability and integrity by comparing the phase of its output signal to a delayed version of itself in what is essentially an interferometric technique. Using a high-quality crystal oscillator, we demonstrate that fractional frequency jumps of 10-11 are easily observed and that a cesium atomic clocks short-term Allan deviation can be measured without reference to another standard in a fully autonomous manner.

Autonomously measuring an atomic clock's allan variance

In this work, we demonstrate that an atomic clock can autonomously assess its own frequency stability and integrity by comparing the phase of its output signal to a delayed version of itself in what is essentially an interferometric technique. Using a high-quality crystal oscillator as a surrogate delay line, we demonstrate that fractional frequency variations at the level of 10−12 are detectable, and that a Cs clocks short-term Allan deviation can be measured without reference to another standard. The paper concludes with a discussion of how an ambiguity in the method might be resolved, and how the method might be employed in the optical domain.

Fine-structure artifact of the velocity distribution of Cs beam tubes as measured by the pulsed microwave power technique (atomic frequency standard)

An artifact of the pulsed microwave power technique used to measure the velocity distributions of Cs beam tubes is shown. To predict these results, a simple theory is used that considers the Ramsey response function (i.e. output tube current vs. microwave frequency) as being a simple electrical-filter transfer function for the input of the pulse-modulated microwave power. Calculations that assume the principle of superposition are used to give predictions that are in close agreement with the measured results. This theory predicts how to set the pulse width so that the fine-structure artifact is minimized.<<ETX>>