Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “frequency”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,585 records · Page 88
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

A low frequency radio array for space

This paper considers the need for and the possibility of constructing and operating a low-frequency array in space, the Low Frequency Space Array (LFSA), which is presently in its developmental phase, to form an entirely space-based synthesis interferometer for high-resolution high-sensitivity sky surveying and source imaging over the frequency range from about 1 to about 30 MHz. It is emphasized that there is a wealth of new astronomical information to be found in the as yet unexplored frequency range below 30 MHz, the wavelengths at which only interferometry is practicable. A possible instrumental concept for an LFSA spacecraft is described together with orbits, hardware, and subsystems.

Weiler, K. W.↗
NASA NTRS1988

Frequency stabilization of diode-laser-pumped solid state lasers

The goal of the NASA Sunlite program is to fly two diode-laser-pumped solid-state lasers on the space shuttle and while doing so to perform a measurement of their frequency stability and temporal coherence. These measurements will be made by combining the outputs of the two lasers on an optical radiation detector and spectrally analyzing the beat note. Diode-laser-pumped solid-state lasers have several characteristics that will make them useful in space borne experiments. First, this laser has high electrical efficiency. Second, it is of a technology that enables scaling to higher powers in the future. Third, the laser can be made extremely reliable, which is crucial for many space based applications. Fourth, they are frequency and amplitude stable and have high temporal coherence. Diode-laser-pumped solid-state lasers are inherently efficient. Recent results have shown 59 percent slope efficiency for a diode-laser-pumped solid-state laser. As for reliability, the laser proposed should be capable of continuous operation. This is possible because the diode lasers can be remote from the solid state gain medium by coupling through optical fibers. Diode lasers are constructed with optical detectors for monitoring their output power built into their mounting case. A computer can actively monitor the output of each diode laser. If it sees any variation in the output power that might indicate a problem, the computer can turn off that diode laser and turn on a backup diode laser. As for stability requirements, it is now generally believed that any laser can be stabilized if the laser has a frequency actuator capable of tuning the laser frequency as far as it is likely to drift in a measurement time.

Byer, Robert L.↗
NASA NTRS1988

Initial high-degree p-mode frequency splittings from the 1988 Mt. Wilson 60-foot Tower Solar Oscillation Program

The initial frequency splitting results of solar p-mode oscillations obtained from the 1988 helioseismology program at the Mt. Wilson Observatory are presented. The frequency splittings correspond to the rotational splittings of sectoral harmonics which range in degree between 10 and 598. They were obtained from a cross-correlation analysis of the prograde and retrograde portions of a two-dimensional (t - v) power spectrum. This power spectrum was computed from an eight-hour sequence of full-disk Dopplergrams obtained on July 2, 1988, at the 60-foot tower telescope with a Na magneto-optical filter and a 1024x1024 pixel CCD camera. These frequency splittings have an inherently larger scatter than did the splittings obtained from earlier 16-day power spectra. These splittings are consistent with an internal solar rotational velocity which is independent of radius along the equatorial plane. The normalized frequency splittings averaged 449 + or - 3 nHz, a value which is very close to the observed equatorial rotation rate of the photospheric gas of 451.7 nHz.

Rhodes, Edward J., Jr.↗
NASA NTRS1989

Stabilized fiber-optic frequency distribution system

A technique for stabilizing reference frequencies transmitted over fiber-optic cable in a frequency distribution system is discussed. The distribution system utilizes fiber-optic cable as the transmission medium to distribute precise reference signals from a frequency standard to remote users. The stability goal of the distribution system is to transmit a 100-MHz signal over a 22-km fiber-optic cable and maintain a stability of 1 part in 10(17) for 1000-second averaging times. Active stabilization of the link is required to reduce phase variations produced by environmental effects, and is achieved by transmitting the reference signal from the frequency standard to the remote unit and then reflecting back to the reference unit over the same optical fiber. By comparing the phase of the transmitted and reflected signals at the reference unit, phase variations of the remote signal can be measured. An error voltage derived from the phase difference between the two signals is used to add correction phase.

Primas, L. E.↗
NASA NTRS1988

Effect of aerodynamic detuning on supersonic rotor discrete frequency noise generation

A mathematical model was developed to predict the effect of alternate blade circumferential aerodynamic detuning on the discrete frequency noise generation of a supersonic rotor. Aerodynamic detuning was shown to have a small beneficial effect on the noise generation for reduced frequencies less than 3. For reduced frequencies greater than 3, however, the aerodynamic detuning either increased or decreased the noise generated, depending on the value of the reduced frequency.

Hoyniak, D.↗
NASA NTRS1989

Frequency-selective reflection and transmission by a periodic dielectric layer

The feasibility of using a periodic dielectric layer, composed of alternating bars having dielectric constants epsilon sub 1 and epsilon sub 2, as a frequency selective subreflector in order to permit feed separation in large aperture reflecting antenna systems was examined. For oblique incidence, it is found that total transmission and total reflection can be obtained at different frequencies for proper choices of epsilon sub 1, epsilon sub 2, and the geometric parameters. The frequencies of total reflection and transmission can be estimated from wave phenomena occurring in a layer of uniform dielectric constant equal to the average value for the periodic layers. About some of the frequencies of total transmission, the bandwidth for 90 percent transmission is found to be 40 percent. However, the bandwidth for 90 percent reflection is always found to be much narrower; the greatest value found being 2.5 percent.

Bertoni, Henry L.↗
NASA NTRS1989

A frequency domain analysis for damped space structures

A method is presented for the analysis of damped structural systems in which the structural components are represented by impedance models and analyzed in the frequency domain. Methods are presented to assemble and condense system impedance matrices, and then to identify approximate mass, stiffness, and damping matrices for systems whose impedances are complicated functions of frequency. Formulas are derived for determination of approximate values for system natural frequencies and damping using frequency domain quantities. The sensitivities of these approximate values to system parameter changes are analyzed. The implementation of these analysis tools is discussed and applied to a simple mechanical system.

Hagood, Nesbitt W.↗
NASA NTRS1989

Frequency fluctuations of a diode-pumped Nd:YAG ring laser

The spectral density of the frequency fluctuations of a diode-pumped single-mode monolithic Nd:YAG ring laser was measured by locking a Fabry-Perot resonator to the laser frequency. The fluctuations approach the limit due to spontaneous emission (the Schawlow-Townes limit) at frequencies above 80 kHz. The inherent frequency stability of these lasers makes them attractive as a potential light source for gravitational-wave interferometers.

Fritschel, Peter↗
NASA NTRS1990

Frequency-Tracking-Error Detector

Frequency-tracking-error detector compares average period of output signal from band-pass tracking filter with average period of signal of frequency 100 f(sub 0) that controls center frequency f(sub 0) of tracking filter. Measures difference between f(sub 0) and frequency of one of periodic components in output of bearing sensor. Bearing sensor is accelerometer, strain gauge, or deflectometer mounted on bearing housing. Detector part of system of electronic equipment used to measure vibrations in bearings in rotating machinery.

Randall, Richard L.↗
NASA NTRS1989

High-frequency instability of the sheath-plasma resonance

Coherent high frequency oscillations near the electron plasma frequency (omega approx. less than omega sub p) are generated by electrodes with positive dc bias immersed in a uniform Maxwellian afterglow plasma. The instability occurs at the sheath-plasma resonance and is driven by a negative RF sheath resistance associated with the electron inertia in the diode-like electron-rich sheath. With increasing dc bias, i.e., electron transit time, the instability exhibits a hard threshold, downward frequency pulling, line broadening and copious harmonics. The fundamental instability is a bounded oscillation due to wave evanescence, but the harmonics are radiated as electromagnetic waves from the electrodes acting like antennas. Wavelength and polarization measurements confirm the emission process. Electromagnetic waves are excited by electrodes of various geometries (planes, cylinders, spheres) which excludes other radiation mechanisms such as orbitrons or beam-plasma instabilities. The line broadening mechanism was identified as a frequency modulation via the electron transit time by dynamic ions. Ion oscillations at the sheath edge give rise to burst-like RF emissions. These laboratory observations of a new instability are important for antennas in space plasmas, generation of coherent beams with diodes, and plasma diagnostics.

Stenzel, R. L.↗
NASA NTRS1989

Frequency response modeling and control of flexible structures: Computational methods

The dynamics of vibrations in flexible structures can be conventiently modeled in terms of frequency response models. For structural control such models capture the distributed parameter dynamics of the elastic structural response as an irrational transfer function. For most flexible structures arising in aerospace applications the irrational transfer functions which arise are of a special class of pseudo-meromorphic functions which have only a finite number of right half place poles. Computational algorithms are demonstrated for design of multiloop control laws for such models based on optimal Wiener-Hopf control of the frequency responses. The algorithms employ a sampled-data representation of irrational transfer functions which is particularly attractive for numerical computation. One key algorithm for the solution of the optimal control problem is the spectral factorization of an irrational transfer function. The basis for the spectral factorization algorithm is highlighted together with associated computational issues arising in optimal regulator design. Options for implementation of wide band vibration control for flexible structures based on the sampled-data frequency response models is also highlighted. A simple flexible structure control example is considered to demonstrate the combined frequency response modeling and control algorithms.

Bennett, William H.↗
NASA NTRS1989

High-frequency instability of the sheath-plasma resonance

Coherent high-frequency oscillations near the electron plasma frequency are generated by electrodes with positive dc bias immersed in a uniform Maxwellian afterglow plasma. The instability occurs at the sheath-plasma resonance and is driven by a negative RF sheath resistance associated with the electron inertia in the diodelike electron-rich sheath. With increasing dc bias, i.e., electron transit time, the instability exhibits a hard threshold, downward frequency pulling, line broadening, and copious harmonics. The fundamental instability is a bounded oscillation caused by wave evanescence, but the harmonics are radiated as electromagnetic waves from the electrodes acting like antennas. Wavelength and polarization measurements confirm the emission process. Electromagnetic waves are excited by electrodes of various geometries (planes, cylinders, spheres), which excludes other radiation mechanisms such as orbitrons or beam-plasma instabilities. The line-broadening mechanism has been identified as a frequency modulation via the electron transit time by dynamic ions. Ion oscillations at the sheath edge give rise to burstlike RF emissions. These laboratory observations of a new instability are important for antennas in space plasmas, generation of coherent beams with diodes, and plasma diagnostics.

Stenzel, R. L.↗
NASA NTRS1989

Simultaneous multi-frequency imaging observations of solar microwave bursts

The results of simultaneous two-frequency imaging observations of solar microwave bursts with the Very Large Array are reviewed. Simultaneous 2 and 6 cm observations have been made of bursts which are optically thin at both frequencies, or optically thick at the lower frequency. In the latter case, the source structure may differ at the two frequencies, but the two sources usually seem to be related. However, this is not always true of simultaneous 6 and 20 cm observations. The results have implications for the analysis of nonimaging radio data of solar and stellar flares.

Kundu, M. R.↗
NASA NTRS1989

Two novel automatic frequency tracking loops

Two automatic-frequency-control (AFC) loops are introduced and analyzed in detail. The algorithms are generalizations of the well known cross-product AFC loop with improved performance. The first estimator uses running overlapping discrete Fourier transforms to create a discriminator curve proportional to the frequency estimation error, whereas the second one preprocesses the received data and then uses an extended Kalman filter to estimate the input frequency. The algorithms are tested by computer simulations in a highly dynamic environment at low carrier/noise ratio (CNR). The algorithms are suboptimum tracking schemes with a larger frequency-error variance compared to an optimum strategy, but they offer simplicity of mechanization and a CNR with a very low operating threshold.

Aguirre, Sergio↗
NASA NTRS1990

The vibrational frequencies of ozone

The singles and doubles coupled-cluster (CCSD) method is used, together with the recently proposed CCSD(T) and CCSDT-1 methods, to determine the equilibrium structure and harmonic frequencies of ozone at various levels, on the basis of large one-particle basis sets. It is demonstrated that f-type functions are required in order to obtain quantitative accuracy for the equilibrium structure and harmonic frequencies. The CCSD(T) level of theory, in conjunction with a (4s 3p 2d 1f) atomic natural orbital set, yields an equilibrium geometry and harmonic frequencies that are in excellent agreement with experimental results; an effort has accordingly been made to determine the cubic and quartic force constants, and evaluate the anharmonic corrections to the fundamental vibrational frequencies.

Lee, Timothy J.↗
NASA NTRS1990

Millimeter-wave monolithic diode-grid frequency multiplier

A semiconductor diode structure useful for harmonic generation of millimeter or submillimeter wave radiation from a fundamental input wave is fabricated on a GaAs substrate. A heavily doped layer of n(sup ++) GaAs is produced on the substrate and then a layer of intrinsic GaAs on said heavily doped layer on top of which a sheet of heavy doping (++) is produced. A thin layer of intrinsic GaAs grown over the sheet is capped with two metal contacts separated by a gap to produce two diodes connected back to back through the n(sup ++) layer for multiplication of frequency by an odd multiple. If only one metal contact caps the thin layer of intrinsic GaAs, the second diode contact is produced to connect to the n(sup ++) layer for multiplication of frequency by an even number. The odd or even frequency multiple is selected by a filter. A phased array of diodes in a grid will increase the power of the higher frequency generated.

Maserjian, Joseph↗
NASA NTRS1990

Digital frequency synthesizer for radar astronomy

The digital frequency synthesizer (DFS) is an integral part of the programmable local oscillator (PLO) which is being developed for the NASA's Deep Space Network (DSN) and radar astronomy. Here, the theory of operation and the design of the DFS are discussed, and the design parameters in application for the Goldstone Solar System Radar (GSSR) are specified. The spectral purity of the DFS is evaluated by analytically evaluating the output spectrum of the DFS. A novel architecture is proposed for the design of the DFS with a frequency resolution of 1/2(exp 48) of the clock frequency (0.35 mu Hz at 100 MHz), a phase resolution of 0.0056 degrees (16 bits), and a frequency spur attenuation of -96 dBc.

Sadr, R.↗
NASA NTRS1990

Design and industrial production of frequency standards in the USSR

Some aspects of research development and production of quantum frequency standards, carried out in QUARTZ Research and Production Association (RPA), Gorky, U.S.S.R., were investigated for the last 25 to 30 years. During this period a number of rubidium and hydrogen frequency standards, based on the active maser, were developed and put into production. The first industrial model of a passive hydrogen maser was designed in the last years. Besides frequency standards for a wide application range, RPA QUARTZ investigates metrological frequency standards--cesium standards with cavity length 1.9 m and hydrogen masers with a flexible storage bulb.

Demidov, Nikolai A.↗