Frequency generation and control - Atomic hydrogen frequency standard evaluation
Method for evaluating traveling wave tube stability and atomic hydrogen frequency standard
HYDROGEN ATOM↗
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Aeroelastic Simulation of Transonic Truss-Braced Wing Aircraft for Flight Dynamic Stability Analysis
This paper presents aeroelastic simulation of the Mach 0.8 Transonic Truss-Braced Wing (TTBW) aeroelastic model using CFD solver FUN3D for the flight dynamic and control stability analysis. A jig twist optimization is performed to obtain a flight optimized jig twist for the flexible Mach 0.8 TTBW model. The developed aeroelastic model steady-state stability derivatives with respect to the angle of attack, angle of sideslip, and airspeed are calculated with perturbations in the angle of attack, angle of sideslip, and Mach number, respectively. A series of unsteady simulations is conducted for the developed aeroelastic model in pitch oscillation at various reduced frequencies. The dynamic stability derivatives are estimated using a frequency domain estimation method. The control derivatives of the elevator, rudder, and ailerons of the developed aeroelastic model are estimated for control stability analysis.
TTBW↗
On stability theory
It is found that under mild assumptions, feedback system stability can be concluded if one can 'topologically separate' the infinite-dimensional function space containing the system's dynamical input-output relations into two regions, one region containing the dynamical input-output relation of the 'feedforward' element of the system and the other region containing the dynamical output-input relation of the 'feedback' element. Nonlinear system stability criteria of both the input-output type and the state-space (Liapunov) type are interpreted in this context. The abstract generality and conceptual simplicity afforded by the topological separation perspective clarifies some of the basic issues underlying stability theory and serves to suggest improvements in existing stability criteria. A generalization of Zames' (1966) conic-relation stability criterion is proved, laying the foundation for improved multivariable generalizations of the frequency-domain circle stability criterion for nonlinear systems.
Safonov, M. G.↗
Maximum likelihood method for estimating airplane stability and control parameters from flight data in frequency domain
A frequency domain maximum likelihood method is developed for the estimation of airplane stability and control parameters from measured data. The model of an airplane is represented by a discrete-type steady state Kalman filter with time variables replaced by their Fourier series expansions. The likelihood function of innovations is formulated, and by its maximization with respect to unknown parameters the estimation algorithm is obtained. This algorithm is then simplified to the output error estimation method with the data in the form of transformed time histories, frequency response curves, or spectral and cross-spectral densities. The development is followed by a discussion on the equivalence of the cost function in the time and frequency domains, and on advantages and disadvantages of the frequency domain approach. The algorithm developed is applied in four examples to the estimation of longitudinal parameters of a general aviation airplane using computer generated and measured data in turbulent and still air. The cost functions in the time and frequency domains are shown to be equivalent; therefore, both approaches are complementary and not contradictory. Despite some computational advantages of parameter estimation in the frequency domain, this approach is limited to linear equations of motion with constant coefficients.
Klein, V.↗
Flight Phasemeter on the Laser Ranging Interferometer on the GRACE Follow-On Mission
As the first inter-spacecraft laser interferometer, the Laser Ranging Interferometer (LRI) on the GRACE Follow-On Mission will demonstrate interferometry technology relevant to the LISA mission. This paper focuses on the completed LRI Laser Ranging Processor (LRP), which includes heterodyne signal phase tracking at µcycle/vHz precision, differential wavefront sensing, offset frequency phase locking and Pound-Drever-Hall laser stabilization. The LRI design has characteristics that are similar to those for LISA: 1064nm NPRO laser source, science bandwidth in the mHz range, MHz-range intermediate frequency and Doppler shift, detected optical power of tens of picoWatts. Laser frequency stabilization has been demonstrated at a level below 30 Hz/vHz, better than the LISA requirement of 300 Hz/vHz. The LRP has completed all performance testing and environmental qualification and has been delivered to the GRACE Follow-On spacecraft. The LRI is poised to test the LISA techniques of tone-assisted time delay interferometry and arm-locking. GRACE Follow-On launches in 2017.
Bachman, B↗
Improved Performance of a Temperature Compensated LN2 Cooled Sapphire Oscillator
We report on improved stability in a whispering gallery sapphire resonator for which the dominant WGHn11 microwave mode family shows frequency-stable, compensated operation for temperatures above 77K. Several modifications during the past year have led to significant improvements in performance. Current tests with improved thermal stability provide Allan Deviation of frequency of 2.6 - 4 ? 10-13 for measurement times of 1#t#100 seconds. We project a frequency stability of 1014 for this resonator with stabilized housing temperature and with a mode Q of 107.!.
Temperature Compensated LN2 Cooled Sapphire Oscill↗
Linear ion trap based atomic frequency standard
In order to develop a trapped ion-based fieldable frequency standard with stability 1 x 10 to the -13th/sq rt tau for averaging times tau greater than 10,000 s, a hybrid RF/DC linear ion trap was developed which permits storage of large numbers of ions with reduced susceptibility to the second-order Doppler effect caused by the RF confining fields. The authors have confined Hg-199(+) ions in this trap and have measured very high Q transitions with good SNRs. In preliminary measurements they obtained stabilities of 1.6 x 10 to the -13th/sq rt tau (tau between 50 and 800 s) with a 160-mHz wide atomic resonance linewidth and a signal-to-noise ratio of 40 for each measurement cycle. Atomic resonance lines as narrow as 30 mHz on the 40.5-GHz clock transition have been measured with no appreciable reduction in the ion signal. A stability of 7 x 10 to the -14th/sq rt tau is made possible by the signal-to-noise and line Q of this measured transition. Analysis of fundamental sources of frequency instability indicates that a long-term stability of 2 x 10 to the -16th is feasible for this device with existing technology for tau = 10 to the 6th s or more.
Prestage, John D.↗
Determination of lateral-stability derivatives and transfer-function coefficients from frequency-response data for lateral motions
A method is presented for determining the lateral-stability derivatives, transfer-function coefficients, and the modes for lateral motion from frequency-response data for a rigid aircraft. The method is based on the application of the vector technique to the equations of lateral motion, so that the three equations of lateral motion can be separated into six equations. The method of least squares is then applied to the data for each of these equations to yield the coefficients of the equations of lateral motion from which the lateral-stability derivatives and lateral transfer-function coefficients are computed. Two numerical examples are given to demonstrate the use of the method.
Donegan, James J↗
Application of very long baseline interferometry to Astrometry and Geodesy: effects of frequency standard instability on accuracy
The accuracy of geodetic and astrometric information obtained from very long baseline interferometry (VLBI) observations is dependent upon the stability of the frequency standard, or clock, used at each site of VLBI array. The sensitivities of two hydrogen maser frequency standards of different design to pressure, temperature, and magnetic field variations were measured; and, for one of the standards, sensitivity was found to be severe enough to degrade the information content of VLBI measurements. However, the effect on the geometric and astrometric information of such clock instabilities, with time scales of hours or greater, can be sharply reduced through the use of differencing techniques.
Clark, T. A.↗
Generation of mechanical oscillation applicable to vibratory rate gyroscopes
To achieve a drive-axis oscillation with improved frequency and amplitude stability, additional feedback loops are used to adjust force-feedback loop parameters. An amplitude-control loop measures oscillation amplitude, compares this value to the desired level, and adjusts damping of the mechanical sense-element to grow or shrink oscillation amplitude as appropriate. A frequency-tuning loop measures the oscillation frequency, compares this value with a highly stable reference, and adjusts the gain in the force-feedback loop to keep the drive-axis oscillation frequency at the reference value. The combined topology simultaneously controls both amplitude and frequency. Advantages of the combined topology include improved stability, fast oscillation start-up, low power consumption, and excellent shock rejection.
Lemkin, Mark A.↗
Bi-stability and period-doubling cascade of frequency combs in exceptional-point lasers
Abstract Recent studies have demonstrated that a laser can self-generate frequency combs when tuned near an exceptional point (EP), where two cavity modes coalesce. These EP combs induce periodic modulation of the population inversion in the gain medium, and their repetition rate is independent of the laser cavity’s free spectral range. In this work, we perform a stability analysis that reveals two notable properties of EP combs, bi-stability and a period-doubling cascade. The period-doubling cascade enables halving of the repetition rate while maintaining the comb’s total bandwidth, presenting opportunities for the design of highly compact frequency comb generators.
exceptional points↗
Recent Stability Comparisons with the JPL Linear Trapped Ion Frequency Standards
The Hg research frequency standards LITS-1 and LITS-2 were developed to provide continuous, reliable, high stability performance. For simplicity, a Hg lamp is used for state selection and a helium buffer gas for ion cooling.
Helium buffer gas ion cooling LITS-2 ion microwave↗
The superconducting cavity stability ruby maser oscillator
Analysis of an application of the rudy maser to a superconducting Cavity Stabilized oscillator shows many attractive features. These derive from the mechancial stability inherent in an all-cryogenic design and from the properties of the ruby maser itself. A multiple-cavity design has been developed to allow physical separation of the high-Q superconducting cavity and the ruby element with its requried applied magnetic field. Mode selection is accomplished in this design by tuning the ruby by means of the applied field. We conclude that such an oscillator would perform well, even with cavity Q's as low as 10 to the 8th power allowing the use of a superconductor-on-sapphire resonator with its greater rigidity and lower thermal expansion. A first test of the Superconducting Cavity Stabilized Maser Oscillator (SCSMO) confirms the efficacy of the multiple-cavity design and the applicability of the ruby maser. Frequency variation less than 4x10 to the minus 11th power was measured in the stabilized mode and is attributed to the reference oscillator and to instabilities in the pump source. Variation of 10 to the minus 10th power was observed in the low-Q unstabilized mode, again attributable to pump fluctuations. Even so, direct scaling to a Q of 10 the 9th power predicts a stability better than 10 to the minus 15th power. Together with results showing the lowest losses to date in sapphire at microwave frequencies, and preliminary experiments on superconductor-on-sapphire resonators, frequency stability, levels as low as 10 to the minus 17th power are indicated.
Dick, G. J.↗
Initial Ares I Bending Filter Design
The Ares-I launch vehicle represents a challenging flex-body structural environment for control system design. Software filtering of the inertial sensor output will be required to ensure control system stability and adequate performance. This paper presents a design methodology employing numerical optimization to develop the Ares-I bending filters. The filter design methodology was based on a numerical constrained optimization approach to maximize stability margins while meeting performance requirements. The resulting bending filter designs achieved stability by adding lag to the first structural frequency and hence phase stabilizing the first Ares-I flex mode. To minimize rigid body performance impacts, a priority was placed via constraints in the optimization algorithm to minimize bandwidth decrease with the addition of the bending filters. The bending filters provided here have been demonstrated to provide a stable first stage control system in both the frequency domain and the MSFC MAVERIC time domain simulation.
Jang, Jiann-Woei↗
Temperature Sensors Based on WGM Optical Resonators
A proposed technique for measuring temperature would exploit differences between the temperature dependences of the frequencies of two different electromagnetic modes of a whispering gallery-mode (WGM) optical resonator. An apparatus based on this technique was originally intended to be part of a control system for stabilizing a laser frequency in the face of temperature fluctuations. When suitably calibrated, apparatuses based on this technique could also serve as precise temperature sensors for purposes other than stabilization of lasers. A sensor according to the proposal would include (1) a transparent WGM dielectric resonator having at least two different sets of modes characterized by different thermo-optical constants and (2) optoelectronic instrumentation for measuring the difference between the temperature-dependent shifts of the resonance frequencies of the two sets of modes.
Savchenkov, Anatoliy↗
Small-signal characteristics of the diode- stabilized linear integrated devices.
Small signal characteristics and frequency response of diode-stabilized integrated linear circuits, discussing feedback, impedance, bias- diode, etc
Yang, E. S.↗
Analysis of of Injection-Velocity Effects on Rocket Motor Dynamics and Stability
A concept of combustion time lag that includes dependency on injection velocity is introduced. The concept is used in the formulation of chamber transfer functions and in an analysis of low-frequency combustion instability. Theoretical frequency responses and stability boundaries are compared with those obtained when the injection-velocity effect on the time lag to be an important consideration, in the theory of chamber dynamics and combustion instability
Hurrell, Herbert G.↗
Cable delay compensator for microwave signal distribution over optical fibers
The basic principles of microwave fiber-optic systems are outlined with emphasis on fiber-optic cable delay compensators (CDC). Degradation of frequency and phase stability is considered, and it is pointed out that the long-term stability of a fiber-optic link is degraded by group delay variations due to temperature fluctuations in the optical fiber and low-frequency noise characteristics of the laser. A CDC employing a voltage-controlled oscillator to correct for phase variations in the optical fiber is presented, and the static as well as dynamic closed-loop analyses of the fiber-optic CDC are discussed. A constructed narrow-band fiber-optic CDC is shown to reduce phase variations caused by temperature fluctuations by a factor of 400. A wide-band CDC utilizing a temperature-controlled coil of fiber to compensate for phase delay is also proposed.
Primas, Lori E.↗