John P. Lowe

The accuracy evaluation of NIST-7

We have performed evaluations of the major systematic errors in NIST-7 with an overall uncertainty of less than a part in 10/sup 14/. The complete evaluation process has been separated into two parts. With a computer-controlled, digital servo system and some new measurement techniques, we now perform core evaluations (second-order Zeeman and Doppler shifts, cavity pulling and phase shift, line overlap and some electronic shifts) with an overall uncertainty of less than one part in 10/sup 14/ in just a few days of measurements. The complete evaluation of all small and subtle effects in both the physics and electronics requires a few hundred days of data. But, these small effects are not variable at the 10/sup -14/ level and their infrequent evaluation does not detract from the operational accuracy of the standard. >

Recent progress in laser-pumped rubidium gas-cell frequency standards

This paper presents the current results of our development of a laser-pumped passive rubidium frequency standard. With a vapor cell containing isotopic Rb/sup 87/ and a mixture of buffer gas we obtained a double resonance signal compatible with a short-term stability of 2.10/sup -13/ /spl tau//sup -1/2/ (shot noise limit). Measurements of the effect of the interrogating phase noise demonstrated that our microwave synthesizer did not limit this potential short-term stability. Two types of monochromatic light source lasers have been used: broad-band solitary lasers and extended cavity lasers. We found that their main limitation on the frequency stability was due to the AM noise detected by the photocell. In order to improve the S/N of the clock, an all-electronic AM noise cancellation technique has been successfully employed. Light-shift measurements allowed tuning of the laser frequency to the zero light-shift point. Presently, our clock has a short-term stability of 7.10/sup -13/ /spl tau//sup -1/2/ (2</spl tau/<40 s) with the solitary laser and 5.10/sup -13/ /spl tau//sup -1/2/ (4</spl tau/<40 s) with the extended cavity laser. These are the best reported performances for passive rubidium clocks.

Error analysis of the NIST optically pumped primary frequency standard

The major sources of systematic error in the NIST optically pumped primary frequency standard has been evaluated with an uncertainty of a few parts in 10/sup 14/. The cavity end-to-end phase shift is the only item which differed markedly from expectations based on the design and/or measurements made during assembly. The authors think the difference is attributable to the physical dimensions of the cavity and not to its asymmetry. The device will begin to function as the US primary frequency standard, even before planned extensions to the analysis presented and improvement to the laser and servo electronics allow evaluation at its full design accuracy of one part in 10/sup 14/. >

WWVB Radio Controlled Clocks: Recommended Practices for Manufacturers and Consumers

Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose. iii FOREWORD Radio controlled clocks represent a true revolution in timekeeping. Clocks that synchronize to NIST radio station WWVB now number in the millions in the United States, and new sales records are being established every year. As a result, many of us are now accustomed to having clocks in our homes, offices, and on our wrists that always display the correct time and that never require adjustment. This NIST Recommended Practice Guide was written to provide guidance to both manufacturers and consumers of radio controlled clocks. Through voluntary compliance with the recommended practices listed here, manufacturers can benefit by continuing to develop more reliable and usable radio controlled products, increasing both consumer confidence and sales. Consumers can benefit by using this guide to help them select and purchase radio controlled clock products, to learn how the products work, and to help troubleshoot reception problems.

Reducing local oscillator phase noise limitations on the frequency stability of passive frequency standards: tests of a new concept

We report on the experimental test of a new concept for reducing the limitation on short-term frequency stability of passive frequency standards due to local oscillator phase noise. This concept is general and can be applied to many passive frequency standards. Systems that use sinewave modulation to interrogate a stable resonance are limited in short-term frequency stability by phase noise at the second harmonic of the modulation, f/sub m/. This effect limits the fractional frequency stability to approximately /spl sigma//sub v/(/spl tau/7)=0.9(f/sub m///spl nu//sub 0/) (S/sub /spl phi//(2f/sub m/))/sup 1/2//spl tau//sup -1/2/, where /spl nu//sub 0/ is the carrier frequency and S/sub /spl phi//(2f/sub m/) is the phase noise at twice the modulation frequency. (Contributions from higher even harmonics of the modulation generally can be neglected). This new concept uses notch filters at /spl plusmn/2f/sub m/ from the carrier to reduce this effect. Tests on a modified passive rubidium standard demonstrate an improvement of approximately 18 in /spl sigma//sub y/(/spl tau/). The dual notch filters proved to be feasible and were obtained commercially. Measurements suggest that ultimate performances of less than 2/spl times/10/sup -14//spl tau//sup -1/2/ are possible if the atomic resonance has sufficient quality.<<ETX>>

NIST-7, the new US primary frequency standard

NIST-7, an optically pumped, cesium-beam frequency standard, has replaced NBS-6 as the official US primary frequency standard. The present short-term stability of the standard, measured with respect to an active hydrogen maser, is characterized by /spl sigma///sub t/(/spl tau/) /spl ap/88 /spl times/ 10/sup -13/ /spl tau//sup -1/2//. A first evaluation has resulted in an uncertainty of 4 /spl times/ 10/sup -14/. An improved servo-electronic system is being developed which should improve stability and allow more precise evaluation of the various systematic errors.<<ETX>>

Reducing the effect of local oscillator phase noise on the frequency stability of passive frequency standards

The authors report on the experimental test of a new concept for reducing the limitation on the short-term frequency stability of passive frequency standards due to local oscillator phase noise. Systems that use sine-wave modulation to interrogate a stable resonance are limited in short-term frequency stability by phase noise at the second harmonic of the modulation, f/sub m/. The new concept uses notch filters at /spl plusmn/2f/sub m/ from the carrier to reduce this effect. Tests on a modified passive rubidium standard demonstrate an improvement of approximately 18 in /spl sigma//sub y/(/spl tau/). The dual notch filters proved to be feasible and were obtained commercially. Measurements suggest that ultimate performances of approximately 2 /spl times/ 10/sup -14/ /spl tau//sup -1/2/ are possible if the atomic resonance has sufficient quality.<<ETX>>

Increasing the Modulation Depth of the WWVB Time Code to Improve the Performance of Radio Controlled Clocks

The National Institute of Standards and Technology radio station WWVB has officially changed its broadcast format. As of January 1, 2006 the WWVB broadcast signal has increased the depth of the time code modulation from 10 dB to 17 dB. The increase in modulation depth has been implemented to improve the performance of commercial radio controlled clocks in areas of low signal strength. The increase in modulation depth effectively appears to a matched-filter receiver as if the transmitted power has increased by 2.0 dB, thus extending the coverage area over which WWVB controlled clocks will work properly. This is demonstrated by an amplitude shift keyed (ASK) analysis. The results given by this analysis do not depend on the noise level or the bit error rate (BER)

Systematic errors in NIST-7

We describe a continuing, in-depth evaluation of NISTs new optically pumped frequency standard, in which all known sources of systematic error are investigated; most by two or more independent techniques. Additionally, we have used both analog (fast sine-wave modulation) and digital (slow, square-wave modulation) servo systems during the evaluation.<<ETX>>