Leonard S. Cutler

Thermalization of199Hg ion macromotion by a light background gas in an RF quadrupole trap

The largest systematic uncertainty in the performance of atomic frequency standards using a cloud of ions stored in an rf quadrupole trap is the second-order Doppler shift which depends on ion temperature and trapping parameters. This paper presents evidence that cooling the ions by collisions with atoms of a background gas light compared to the ions results in the condensation of the ions into a cloud of almost uniform density determined by space charge versus potential well forces. In this condition the second-order Doppler shift is simple to calculate and is found to depend only on readily measured characteristics of the ion cloud. This along with already observed good signal-to-noise ratio shows that the frequency standard we have constructed using the hyperfine splitting of singly ionized199Hg, with helium cooling can have an order of magnitude better performance in accuracy, stability, and reproducibility than presently available commercial cesium beam standards.

Doppler effects due to thermal macromotion of ions in an rf quadrupole trap

The second-order Doppler shift is an important source of systematic error in rf quadrupole trapped ion frequency standards. This shift can be reduced by cooling the secular motion of the ion cloud with a light background gas at low pressure. Using a thermalized ion cloud model, it is possible to relate the Doppler shift to the temperature of the ion cloud. It is shown that, in practice, the measured frequency of the first-order Doppler sidebands can be used to determine the ion cloud temperature.

Draft revision of IEEE STD 1139-1988 standard definitions of physical quantities for fundamental, frequency and time metrology-random instabilities

This is a draft revision of IEEE Std 1139-1988 Standard Definitions of Physical Quantities for Fundamental Frequency and Time Metrology. This draft standard covers the fundamental metrology for describing random instabilities of importance to frequency and time metrology. Quantities covered include frequency, amplitude, and phase instabilities; spectral densities of frequency, amplitude, and phase fluctuations; variances of frequency and phase fluctuations; time prediction; and confidence limits when estimating the variance from a finite data set. The standard unit of measure for characterizing phase and frequency instabilities in the frequency domain is L(f), defined as one half of the double-sideband spectral density of phase fluctuations. In the time domain, the standard unit of measure of frequency and phase instabilities is the fully overlapped Allan deviation /spl sigma//sub y/(/spl tau/) or the fully overlapped modified Allan deviation Mod /sub /spl sigma//(/spl tau/).

A low-profile high-performance crystal oscillator for timekeeping applications

A crystal oscillator is described that uses various unusual techniques to achieve double-oven class temperature stability with a single oven. Stability of better than 1 part in 10/sup 11/ over a temperature range of -40/spl deg/ to +85/spl deg/ C has been demonstrated. The use of the single stage oven allows for a form factor with a relatively low height (19 mm. or 3/4 inch) for an oscillator of this performance class. The low profile facilitates card-based designs. In timekeeping applications such as wireless and telecom synchronization, the temperature coefficient of frequency (tempco) is more important than ever. The temperature extremes involved in wireless base stations increase the error due to temperature while the use of GPS timing receivers to discipline quartz oscillators greatly reduces the error due to aging, leaving tempco as the major component. Numerous design innovations are described such as a hermetic oven mass assembly and a non-ovenized digital temperature controller. This controller permits automated optimization of the oven set point and thermal gain. An overview of a novel zero-gradient oven technique used to achieve the double-oven performance is given, with more details in a related paper. Frequency pulling due to the oscillator circuit components is greatly reduced by a novel balanced-bridge controlled oscillator circuit that is described briefly here and in more detail in another related paper.

Mercury-199 Trapped Ion Frequency Standard: Recent Theoretical Progress and Experimental Results

Abstract : The 40.5 GHz hyperfine resonance of 199 Hg ions stored in an rf quadrupole trap has shown to have a very small fractional linewidth, suggesting its use as a high-precision frequency standard. The most significant offset in such a standard would be the second-order Doppler shift resulting from the motion of the stored ions. We have recently analyzed the situation in which the secular motion is cooled to a temperature of about 300 K by the presence of a light background gas at low pressure, resulting in an ion cloud whose density is almost completely determined by the balance of pseudopotential and space-charge forces. Under these circumstances we have found that the second-order Doppler shift can be calculated accurately from the trapping parameters, the temperature, and the total number and mass of the stored ions.

High-temperature superconducting resonators

Preliminary measurements on high-T/sub c/ superconducting resonators are reported and why they are attractive candidates for incorporation in low-noise oscillators is discussed. Some of the important contributions to oscillator noise are reviewed and how they depend on the resonator parameters is shown. A preliminary YBaCu/sub 3/O/sub 7//LaAlO/sub 3/ resonator with a Q of 9*10/sup 4/ at 6.9 GHz and 7*10/sup 4/ at 3.5 GHz has been fabricated. The temperature sensitivity, power dependence, and residual phase noise are discussed. An upper-limit on the coefficient of the 1/f component of fractional-frequency fluctuations has been measured to be -204 dB at 60 K.<<ETX>>

The theory of zero-gradient crystal ovens

Crystal ovens designed to have zero or near-zero thermal gradients within the oven mass are described both from a theoretical viewpoint, and with actual examples. Thermal gains of well over 100,000 in a single oven with a height of only 19 mm. ( 3/4 inch) have been demonstrated experimentally. Conventional ovens often have one or two point sources of heat and rely on high thermal conductance in the oven mass to achieve a temperature profile that is roughly isothermal. Unavoidable residual thermal gradients limit the achievable thermal gain to a few thousand, and in only a small area. By using distributed heaters that match the heat loss distribution, the overt mass can be maintained arbitrarily close to a zero gradient state with resultant high thermal gain over most of its volume. An idealized strawman oven with spherical symmetry is used to establish the basic principles of zero gradient ovens. These are then applied to a practical oven configuration having cylindrical symmetry. A finite element analysis of this oven is presented. Experimental results of actual ovens are given.

A no-drift and less than 1/spl times/10/sup -13/ long-term stability quartz oscillator using a GPS SA filter

Recently the time-domain spectrum of the Global Positioning Satellite (GPS) Selective Availability (SA) modulation has been characterized. The continued growth of GPS, its utility, and the technology to receive and utilize the GPS signals have resulted in stable, multi-channel GPS timing receivers. These are now available from several vendors. Further, the GPS satellite constellation is essentially complete and has been declared operational, at least in its initial phase.<<ETX>>