Accurate Frequency References Research Articles

Cs Fountain Clocks for Commercial Realizations-An Improved and Robust Design.

We report on the design, assembly, testing, and delivery of a series of new cesium fountain primary frequency standards built through commercial and scientific collaboration with international users. The new design, based on proven National Physical Laboratory solutions, improves reliability, simplicity of operation, and transportability. The complete system consists of a novel physics package, a specially developed optical package, and dedicated electronics for system control. We present results showing that despite their simplified and more compact design, the new fountains have state-of-the-art performance in terms of signal-to-noise ratio and robust long-term operation. With a sufficiently low-noise local oscillator, they are capable of reaching a short-term stability below 3×10-14 (1 s) and have potential accuracy in the low 10-16 range, similar to the best cesium fountains currently in operation. This cost-effective solution could be used to increase the availability of accurate frequency references and timescales and provide redundancy in critical locations.

A System-on-Chip Crystal-Less Wireless Sub-GHz Transmitter

This paper presents a fully integrated system-on-chip wireless transmitter that has no need for an external quartz crystal frequency reference. Instead, a nontrimmable $LC$ oscillator (LCO) is implemented, operating at a fixed frequency of roughly 3.2 GHz. This LCO serves as the accurate frequency reference of the transmitter and is used to derive a frequency in the sub-GHz range for wireless applications via a fractional phase-locked loop. The digital functionality required for the frequency generation and thus for the wireless operation is performed by an 8-b microcontroller. The calibration data as well as the application specific firmware are stored in an integrated EEPROM. An initial frequency accuracy of ±52 ppm over a temperature of −20 °C to 85 °C is achieved using only two temperature insertions for the calibration. The device covers a continuous frequency range of 10–960 MHz and provides an ASK and GFSK operation. A low harmonic power amplifier was implemented to overcome design issues due to injection modulation effects. The chip is implemented in a 130 nm standard CMOS process and constitutes a new approach for a fully integrated wireless sub-GHz transmitter that is competitive to state-of-the-art quartz crystal-based transmitters as well as to already existing crystal-less architectures.

Precision Packet-Based Frequency Transfer Based on Oversampling

Frequency synchronization of a distributed measurement system requires the transfer of an accurate frequency reference to all nodes. The use of a general-purpose packet-based network for this aim is analyzed in this paper, where oversampling is considered as a means to counter the effects of packet delay variation on time accuracy. A comprehensive analysis that includes the stability of the local clock is presented and shows that frequency transfer through a packet network of this kind is feasible, with an accuracy level that can be of interest to a number of distributed measurement applications.

Comparison of resonance frequencies of major atomic lines in 398–423 nm

We have demonstrated spectroscopy of Ca, Rb, In, K, Ga, and Yb atomic lines in 398–423 nm. Using an etalon of an ultralow-expansion coefficient, we have determined ratios of the resonance frequencies of these atoms. The etalon has small group-delay-dispersion mirrors to be an accurate frequency reference over a wavelength span of a few tens of nanometer. The etalon resonance frequencies are calibrated with accurately known transition frequencies of Ca at 423 nm and Rb at 422 nm. Based on this calibration, the absolute frequencies are also determined for some atomic lines with smaller uncertainties than earlier reports.

Frequency locking of a 399-nm laser referenced to fluorescence spectrum of an ytterbium atomic beam

The laser cooling of ytterbium (Yb) atoms needs a 399-nm laser which operates on the strong S-1(0)-P-1(1) transition and can be locked at the desired frequencies for different Yb isotopes. We demonstrate a frequency locking method using the fluorescence spectrum of an Yb atomic beam as a frequency reference. For unresolved fluorescence peaks, we make the spectrum of the even isotopes vanish by using the strong angular-dependence of the fluorescence radiations; the remained closely-spaced peaks are thus clearly resolved and able to serve as accurate frequency references. A computer-controlled servo system is used to lock the laser frequency to a single fluorescence peak of interest, and a frequency stability of 304 kHz is achieved. This frequency-locked laser enables us to realize stable blue magneto-optic-traps (MOT) for all abundant Yb isotopes.

Doppler-free saturation fluorescence spectroscopy of Na atoms for atmospheric application

The well-known theory of absorption and fluorescence is briefly reviewed in a systematic manner for the Na D transitions. The resulting formalism is applied to simulation of Doppler-free saturation fluorescence spectra. With only one adjusting parameter, the nonradiative rate chosen to represent the time a thermal atom takes to move across the laser beams, the simulated Doppler-free spectra match the measured ones well for both D(1) and D(2) transitions over one decade of excitation intensities. Relative to the weighted center of the six D(2) hyperfine transition lines, the frequencies of the dominant Doppler-free features have been determined from a simulated spectrum to within ±0.1 MHz to be -651.4, 187.8, and 1068.0 MHz, respectively, for D(2a), crossover, and D(2b) resonances. These features may be used as accurate frequency references for atmospheric spectroscopy. They are essential for the operation of the newly developed narrow-band Na fluorescence lidar for wind and temperature measurements in the mesopause region.

The AR6A Single-Sideband Microwave Radio System: Frequency Control

The AR6A Radio System requires precise control of the local oscillator frequency at repeaters and main stations, so that the IF equalizer pilots will be accurately located with respect to the narrowband crystal pick-off filters. In addition, the accumulated frequency error from each repeater and main station must be limited to a value that can be corrected by the tracking receiver of the multimastergroup terminal. Precise frequency control is maintained along the route by locking local oscillators to a stable and accurate frequency reference provided at each repeater station by the Microwave Carrier Synchronization Supply (MCSS). The MCSS provides a source and a backup for this signal. Extensive monitoring circuitry in the MCSS detects failed conditions. The accuracy of the two oscillators is continuously monitored by making frequency comparisons of their output. These frequency-monitoring circuits are also used to make yearly adjustments to the oscillators using a rubidium frequency standard as reference. This paper describes the MCSS System that has been developed for the AR6A Radio System.

Heterodyne frequency measurements of CO 2 laser hot-band transitions

Careful measurements by means of stabilized CO2 lasers are presented for the frequencies of 28 lines in the P-branch of the 01 0 0-(11 1 0, 03 1 0)I band of (C-13)(0-16)2, observed in laser emission, and three lines in the R-branch, observed in absorption with a diode laser. A significant improvement in the ro-vibrational constants has been obtained from a least squares fit to these data, demonstrating that the laser lines stabilized by natural absorption techniques provide convenient, accurate frequency references near 11.7 microns with an uncertainty of less than 0.1 MHz.