1974 TO 1984

NBS 6 was the U.S. primary frequency standard from 1973 to 1992. NBS 7 replaced the magnetic state selectors, one shown as the large cylinder wrapped in heating tape, with optical state selectors.

Howe transferred to the Frequency and Time Standards group in 1974. The work in this group involves an advanced understanding of atomic physics and quantum electronics. In particular, work was with the NIST primary cesium (Cs) beam standards at that time, NBS-4 and NBS-6, and later with hydrogen maser development. This required a working knowledge in the field of atomic physics, specifically atomic beam spectroscopy. Additionally, he did research in oscillator phase noise measurements and spectral analysis. He has been engaged in a number of different programs as outlined below.

ERROR AND ACCURACY ANALYSIS OF U.S. ATOMIC TIME STANDARD

The standard of time interval is the “second.” It is currently defined by the frequency of a particular quantum energy transition in an unperturbed cesium atom. In 1983, Howe was asked by NIST to complete an evaluation of the reference clock, which is a highly specialized atomic spectrometer and the source of U.S. time interval based on the current definition of the second. This required a responsibility, discipline, and background reserved for the highest caliber of people in the division. Usually, this work was done by either scientific management, or teams of two physicists. A change in management in the prior year, together with confidence in him on the part of the atomic standards group, led to the request that he manage this evaluation. During this evaluation, new breakthroughs occurred in collaboration with Italian scientists which yielded new accuracy to 7 x 10-14. The success of his studies has led to better understanding of the physical principles of atomic spectroscopy and precise time and frequency generation using atomic quantum energy transitions.

FREQUENCY AND TIME STANDARD RESEARCH

Howe has experience and proficiency in many areas of physics. These include:

• Atomic and molecular spectroscopy
• Atomic transitions, quantum energy states, and Lorentzian transitions and line shape
• Electron-oscillator model
• Magnetic dipole, hexapole and optical state selection
• Maser and laser theory
• Saturated absorption
• Magnetic field and gyromagnetic ratio measurements
• Optical pumping
• Polarization and induced dipole moments
• Atomic resonance, radiative damping, and resonance Q
• Lifetime and collision broadening and spin-exchange cross section
• First- and second-order Doppler shifts, reduction and cancellation schemes
• Doppler broadening, thermal atomic and molecular effects
• Cavity design, cavity Q, cavity (reactive) frequency pulling, line pulling, background pulling
• Laser diodes
• Atomic rate equations
• Evaluation of coupling coefficients
• Atomic and molecular beam velocity distributions, thermal equilibrium and Boltzman derivation
• Field inhomogeneity effects
• Optical resonators
• Waveguide and waveguide coupling theory

Related publications:

• Some Recent Progress in Microwave Frequency and Time Standards at the NBS
• Prospects for Cesium Primary Standards at the NBS
• A Digital 5.00688 MHz Synthesizer and Squarewave FM Servo System for Cesium Standards

HYDROGEN MASER DEVELOPMENT

The first compact H-maser was built in 1982.  There were 6 prototypes.  Shown, Dave Howe (standing) and Fred Walls

Howe’s work here was in the development of a new kind of precision frequency standard, a compact passive hydrogen maser. This development work represented an entirely new approach to maser technology and required a very broad background in both physics and electronics fundamentals. Howe and one other co-developer were fully responsible for this four-year effort. The purpose of this project was to develop hardware for potential use in the next generation of clocks in outer-space satellite probes, future military work related to the Milstar program, and civilian navigation needs, noteably the Global Positioning System. Furthermore, five hydrogen masers are combined to be the ”flywheel” oscillators in NIST’s realization of UTC (universal coordinated time) since NIST’s very accurate primary Cs frequency standard (F-1) cannot be run continuously as a clock and is not as stable as the hydrogen masers. For this work, he was awarded the distinguished Bronze Medal, which is the third highest award given in the Department of Commerce. It also led to a promotion.

Specific tasks that were performed during the hydrogen maser work are:

1. He designed and built a pure atomic hydrogen beam source using an RF plasma discharge of molecular (tank) hydrogen. The discharge is electrostatically induced on the inside of a Pyrex bulb. Atomic hydrogen is collimated through a hexapole state-selector magnet in which the lower magnetic dipole energy level in the ground state of the atom is focused into a microwave cavity.

2. He designed and built a new microwave cavity which is a high-Q TE011 mode type at the hydrogen magnetic transition frequency of 1.420 GHz. This cavity has a wall coating of low-loss alumina to reduce its size by dielectric loading of the electric component in the RF field.

3. He developed a new technique for applying a highly clean and inert wall coating of Teflon on the inside of the microwave cavity. The atomic wall interaction between the volume of hydrogen and the Teflon coating determines to a very large extent the quality factor of the maser. With his technique, he was able to reach the known best limit of performance of the atom-wall interaction. The breakthrough in understanding the Teflon wall coating technique is expected to affect a very wide variety of Teflon users.

4.   He also contributed to the first development of optical state selection in the Cs beam physics package for reducing size and weight of Cs tubes. See “OPTICALLY PUMPED SMALL CESIUM BEAM STANDARDS: A STATUS REPORT”.

Related publications:

• Test of Wall Coatings for Controlled Moist Air Experiments

5. He designed and built a multitude of peripheral aspects to the maser including temperature servo systems, vacuum hardware (in the 10-12 Torr region), large magnetic and electrostatic shields, HF to microwave multipliers, high resolution frequency synthesizers, and precision machining work of various kinds.

Related publications:

• A Passive Hydrogen Maser Frequency Standard
• A Small, Passively Operated Hydrogen Maser
• Precision Timekeeping Using a Small Passive Hydrogen Maser
• Timekeeping Potentials Using Passive Hydrogen Masers
• A Compact Hydrogen Maser with Exceptional Long-Term Stability

AMMONIA UTILITY STANDARD

From 1976 to 1978, Howe was responsible for the development of the electronic control system (commonly referred to as a frequency control servo system) for a portable frequency standard based on an ammonia vapor electric dipole resonance. Most cesium frequency standards are large, expensive, and weigh nearly 100 lbs with batteries. This device was to fill a need for an accuracy level of 10-9 and was designed to be small enough to be easily carried by an individual, to be low in cost (one-tenth the cost of cesium standards), to have quick start-up capability (within 15 seconds for full accuracy), and to be extremely rugged.

Having fun at 50’s theme party: L-to-R John Bollinger, Dave Wineland, Dave Howe, Jim Bergquist, all at NIST, and Lyndon Lewis, now CTO for Ball Aerospace Corp., and John Prestige, now chief scientist for NASA JPL Deep Space Atomic Clock (DSAC) project.  Dave Wineland was hired by Hellwig and joined th Section in 1977.  The Special-Purpose Ammonia paper was contained our first reported results on an ammonia NH3 standard. This was also our first project for ARPA (now DARPA).

This work, sponsored by a U.S. Air Force R&D contract, when successfully completed, also produced several highly innovative outcomes. One of the most notable was the development of a high-stability internal reference oscillator having a miniature printed-circuit, high-Q, transmission line resonator (commonly referred to as a microstrip resonator). Quartz crystal oscillators are used most often, but their narrow tunability and shock sensitivity made them unsuitable as a reference in this ruggedized atomic standard. In the development of the microstrip oscillator, a breakthrough occurred in the fundamental understanding of the mechanism that controls the intrinsic stability of all oscillators. Prior theory held that a particular kind of noise process (instability) depended on resonator Q, or quality factor. This was proven false by data from his stripline design which showed that the prior noise process could be directly controlled by oscillator drive (which is limited in quartz oscillators) and active-device impurities. Specific noise processes were largely independent of Q.

A new kind of servo system was also developed out of the ammonia standards work. Rather than frequency-locking at a fundamental resonance rate, higher harmonics were also sampled and this greatly reduced errors in the accuracy of the lock. This work ultimately led to a promotion for me.

Related publications:

• Results with the Special-Purpose Ammonia Frequency Standard
• Special Purpose Ammonia Frequency Standard – A Feasibility Study
• Special Purpose Atomic (Molecular) Standard

ATOMIC INTERFEROMETRY AND BEAM VELOCITY SELECTION

Pulsing a split microwave-RF interrogation of the Cs beam provided a means of Ramsey-Interferometric velocity selection of the thermal beam.  The time interval of RF pulses between the cavities selected a particular atomic-velocity group.

Howe was assigned to design and build a complete system which would interface to a cesium beam tube and determine the distribution of atomic velocities in the beam. This determination is a crucial part of a complete evaluation which the beam tube must undergo in order to calculate the accuracy limit of the tube in a frequency standard configuration. The measurement of the velocity distribution is a complex task requiring the aid of a large number of probes and a computer if the measurement is to be done efficiently. Furthermore, the technique for this measurement had never been used prior to his work.

By 1975, a complete velocity distribution measurement system was assembled under his supervision. He developed not only the hardware and interface aspects, but also designed the software and operating system. The system was based on a Data General frame computer with reel-to-reel 9-track tape mass storage units, best available at the time. The development of this system led to a novel digital servo system design for finding the exact center frequency of the cesium atomic transition. The measurement of velocity distribution was performed and documented on over 20 Cs tubes, all of which were maintained at the NIST and U.S. Naval Observatory in Washington, DC. The system is capable of measuring the velocity distributions of literally all cesium beam tubes in existence. The wealth of data available about cesium standards tested by his system has generated considerably more understanding of the nature of the noise which limits accuracy in the case of atomic spectroscopy measurements.

First continuous-Ramsey pattern using pulsed interrogation.

Related publications:

• Preliminary Research and Development of the Cesium Tube Accuracy Evaluation System
• Results in Operation, Research, and Development of Atomic Clocks at the National Bureau of Standards
• Velocity Distribution Measurements of Cesium Beam Tubes
• Some Causes and Cures of Frequency Instabilities (Drift and Noise) in Cesium Beam Frequency Standards
• Optically Pumped Small Cesium Beam Standards: A Status Report
• A Beam Reversal Experiment on NBS-6 Primary Cs Standard

First programmable Ramsey velocity-selection electronics with Cs tube in the foam on the right (Howe 1973).

Howe was responsible for measurement of the velocity distribution using a velocity selector based on pulsed microwave interrogation.  The basic scheme is shown above.

First Measurement of velocity distribution using pulsed interrogation (Howe 1974)

Dave Howe is shown with NBS 6 during its evaluation.  The largest uncertainty in the evaluation of NBS 6 was the uncertainty of the beam velocity distribution

PRECISION POWER SPECTRUM, PHASE-NOISE, ELECTRICAL, AND TIME INTERVAL MEASUREMENTS

Howe has done extensive work related to making precise signal and noise measurements, particularly phase-noise measurements, for signal carriers ranging from below 1 Hz to 40 GHz. This work requires expertise in many topics related to fundamental circuit design and electronic and eletromagnetic theory and measurements. He is proficient in:

• Network and circuits analysis, and equivalent circuits
• Pulse power measurements, pulse amplification, and pulse distribution
• Filter design (Butterworth, Chebyshev, FIR and IIR, recursive, constant-delay, multi-tap, etc.)
• Scattering parameter estimation, Bode plots, Smith charts
• Fast rise-time detection and related noise uncertainty analysis
• Wideband analog-to-digital (ADC) and digital-to-analog (DAC) conversion schemes
• Dielectric coefficent measurements, dielectric loss, and loss tangent
• Laplace and z transforms, Maxwell’s equations
• Windowing and sidelobe leakage suppression
• Digital filter design and characterization
• Time-frequency domain, and related convolution-multiplication and scaling effects
• Spectral aliasing
• Frequency synthesis (using direct digital, PLL, multiplication-division, programmable array logic, etc.)
• Noise suppression (using cross-correlation spectral analysis, negative feedback, synchronous lock-in methodologies, etc.)
• Multiresolution signal decomposition and signal reconstruction
• Design of all-pass equalizers, delay-line filters, and complex-conjugate matched filters
• Impedance transforms and reactance and resistance matching network design
• Analysis and reduction of media phase and frequency dispersion, standing waves, multipath, and transmission-line loss
• Electrical-length measurements (for example, accurate phase and group velocity estimation)
• Uncertainty analysis of volt and resistance standards
• Essentially all signal modulation schemes
• Single and multi-mode optical fiber delay measurements
• Directional couplers, bridge couplers, and circulators

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