Search NASA⌕ Search

SEARCH · Search NASA

Results for “frequency variation”

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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Supersonic rectangular jet impingement noise experiments

The discrete frequency sound produced by jets issuing from a convergent, rectangular nozzle of aspect ratio 4.24 was investigated. Experiments were performed both with the free jet and with the jet impinging on a hard ground surface. The impingement tones that dominate the impinging jet spectra show a definite staging behavior which appears to be biased toward the free jet screech frequency once the separation distance exceeds the region of substantial shock cell development. The frequency variation of the impingement tone stages fit the details of a feedback cycle if the disturbance convection velocity is chosen to be 20 percent higher than that necessary to satisfy the screech feedback loop. Phase locked optical records show a flapping mode of jet oscillation with tones at or near the screech frequency, with superimposed symmetric oscillations when a second dominant tone of unrelated frequency appears in the spectrum.

Norum, Thomas D.↗
NASA NTRS1989

Input preshaping with frequency domain information for flexible-link manipulator control

The application of an input preshaping scheme to flexible manipulators is considered. The resulting control corresponds to a feedforward term that convolves in real-time the desired reference input with a sequence of impulses and produces a vibration free output. The robustness of the algorithm with respect to injected disturbances and modal frequency variations is not satisfactory and can be improved by convolving the input with a longer sequence of impulses. The incorporation of the preshaping scheme to a closed-loop plant, using acceleration feedback, offers satisfactory disturbance rejection due to feedback and cancellation of the flexible mode effects due to the preshaping. A frequency domain identification scheme is used to estimate the modal frequencies on-line and subsequently update the spacing between the impulses. The combined adaptive input preshaping scheme provides the fastest possible slew that results in a vibration free output.

Tzes, Anthony↗
NASA NTRS1985

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.↗
NASA NTRS1991

An evaluation of preliminary Doppler global velocimetry measurements

A review of Doppler Global Velocimetry (DGV) data obtained during wind tunnel tests on a 75 degree swept delta wing was performed. High frequency variations observed in normalized data files are attributed to image alignment problems. Unfortunately, initial DGV velocity data compared poorly with baseline reference data. Nonlinear DGV system operation during the tests is the likely source of this problem. Corrected data compares much more favorably and suggests that DGV is a valid measurement technique. Future DGV investigations should include a method or means for monitoring laser frequency relative to the ALF transfer function behavior.

Miller, L. Scott↗
NASA NTRS1976

Vortex noise from nonrotating cylinders and airfoils

An experimental study of vortex-shedding noise was conducted in an acoustic research tunnel over a Reynolds-number range applicable to full-scale helicopter tail-rotor blades. Two-dimensional tapered-chord nonrotating models were tested to simulate the effect of spanwise frequency variation on the vortex-shedding mechanism. Both a tapered circular cylinder and tapered airfoils were investigated. The results were compared with data for constant-diameter cylinder and constant-chord airfoil models also tested during this study. Far-field noise, surface pressure fluctuations, and spanwise correlation lengths were measured for each configuration. Vortex-shedding noise for tapered cylinders and airfoils was found to contain many narrowband-random peaks which occurred within a range of frequencies corresponding to a predictable Strouhal number referenced to the maximum and minimum chord. The noise was observed to depend on surface roughness and Reynolds number.

Schlinker, R. H.↗
NASA NTRS1990

The relationship of extratropical outgoing longwave radiation to monthly geopotential teleconnection patterns

The relationship between low frequency variations in extratropical fields of outgoing longwave radiation (OLR) and geopotential teleconnection patterns as determined by rotated principal components analysis of the NMC 500-mb heights is investigated in the Northern Hemisphere. The monthly broadband OLR is obtained from the Nimbus-6 and Nimbus-7 Wide-Field-Of-View radiometer record. Each of the main 500-mb teleconnection patterns has a characteristic signal in the OLR field for the month in which the 500-mb pattern occurs. The OLR signals mark cloud and diabatic heating events that are associated with the teleconnection patterns. A demonstration is given of correlation between extratropical monthly OLR and geopotential height. Coupled with the expected tropospheric response to radiation on monthly time scale. This demonstration stresses the importance of the radiation simulation in model studies of the low frequency variability of atmospheric circulation. The extratropical OLR does not appear to be a useful predictor for the 500-mb teleconnection patterns on a monthly time scale.

Charlock, Thomas P.↗
NASA NTRS1967

Temperature-stabilized, triggerable microelectronic astable multivibrator starts reliably

Multiple chip custom block, MIC construction is used to fabricate an ultracompact, low-power astable multivibrator. The design provides a multivibrator that free runs, eliminating lockup, is triggerable, pulling into synchronization with an external signal source, and permits design flexibility for controlling the frequency variations with temperature.

Stebbins, W. J.↗
NASA NTRS1970

Telemetry receiver

Communications system maintains phase lock of weak telemetry signals with a minimal expenditure of power and bandwidth. An estimate of the frequency variation as a function of time is used to achieve coherent phase demodulation.

Gilchriest, C.↗
NASA NTRS1972

Ionospheric disturbances caused by long period sound waves generated by Saturn-Apollo launches

Wavelike disturbances were observed in the ionosphere following several nuclear explosions in early 1960's. Supersonic shock waves within the atmosphere generated by large rockets can cause ionospheric electron density perturbations. A CW phase path Doppler array in the New York area was operated during the Saturn-Apollo 12 and 13 launches and recorded Doppler frequency fluctuations due to rocket launchings. Cross correlation and power spectral analyses of the phase path-path Doppler frequency variation records showed that the phase velocities of the signal arrivals were from south of the array with 700 - 800 m/sec corresponding to periods in the range of 2 to 4 minutes. Ionograms taken every 60 seconds from Wallops Islands showed clearly ionospheric disturbances due to rockets. The group velocities were estimated to be of the order of 450 m/sec 1 obtained from the earliest visible disturbances seen on CW phase path Doppler records and ionograms together with the rocket trajectory data.

Rao, G. L.↗
NASA NTRS1973

NASA hydrogen maser accuracy and stability in relation to world standards

Frequency comparisons were made among five NASA hydrogen masers in 1969 and again in 1972 to a precision of one part in 10 to the 13th power. Frequency comparisons were also made between these masers and the cesium-beam ensembles of several international standards laboratories. The hydrogen maser frequency stabilities as related to IAT were comparable to the frequency stabilities of individual time scales with respect to IAT. The relative frequency variations among the NASA masers, measured after the three-year interval, were 2 + or - 2 parts in 10 to the 13th power. Thus time scales based on hydrogen masers would have excellent long-term stability and uniformity.

Peters, H. E.↗
NASA NTRS1974

Photoelectric observations of Krzeminski's star, the companion of Cen X-3

Centaurus X-3 was the first discovered X-ray source whose intensity is pulsed and eclipsed. Though the X-ray properties of the Cen X-3 system have been intensively studied, the optical properties remained unstudied until Krzeminski's identification of the optical counterpart of Cen X-3. Observations of the X-ray eclipse duration, pulsation frequency variation, and pulse arrival time variation have been used to place limits on the nature of the system and to predict the nature of the optical companion and X-ray sources. An observational determination is presented of the several system parameters based on a preliminary interpretation of the light curve of Krzeminski's star.

Petro, L. D.↗
NASA NTRS1972

NASA hydrogen maser accuracy and stability in relation to world standards

Frequency comparisons were made among five NASA hydrogen masers to a precision of one part in 10 to the 13th power. Frequency comparisons were also made between these masers and the cesium-beam ensembles of several international standards laboratories. The hydrogen maser frequency stabilities as related to the international time scale (IAT) were comparable to the frequency stabilities of individual time scales with respect to IAT. The relative frequency variations among the NASA masers were 2 plus or minus 2 parts in 10 to the 13th power after a three year interval. Charts are developed to show the results of the accuracy tests.

Peters, H. E.↗
NASA NTRS1974

Wideband infrared receiver backend

The design, operation, and configuration of the receiver backend for a wideband infrared tracking receiver are described. A squaring loop is incorporated into the receiver design to enable it to track the Doppler shifted carrier which is Phase Shift Keyed modulated. The receiver has a 400 MHz instantaneous bandwidth and tracks signals whose carrier frequency at the backend input is between 200 and 900 MHz with frequency variation rates greater than 20 MHz/second. The output data is compatible with MECL 3 logic.

Flattau, T.↗
NASA NTRS1974

A stable, linear frequency-modulated oscillator, part 1

The development of a push-pull frequency-modulated oscillator employing field-effect transistors is described. The advantages of field-effect transistors for use in a frequency-stable oscillator are presented. Linearization of the frequency deviation was accomplished by utilizing the square-law characteristic of an FET used as a modulating amplifier. The push-pull oscillator model produced a linear frequency deviation of more than 10 MHz at a center frequency of approximately 100 MHz. Output power is within 0.6 db of a nominal +8.5 dBm over the desired frequency range, and the modulation bandwidth is dc to 10 MHz. Frequency variation with temperature after compensation with a negative-temperature-coefficient capacitor is within + or - 0.05% from 0 to 60 C.

Honnell, M. A.↗
NASA NTRS1977

CO2 laser communication systems for near-earth space applications

Projections of the growth of earth-sensing systems for the latter half of the 1980's show a data-transmission requirement of 300 Mbit/s and above. The most efficient technique to return the data from a sensing satellite to a ground station is through a geosynchronous data relay satellite. A laser system is most attractive for the space-to-space link. A five-year program was conducted which has shown the basic technical feasibility of accomplishing this link with a CO2 laser system operating at a wavelength near 10 microns. The system uses a mercury-cadmium-telluride infrared mixer cooled to approximately 100 K by a radiative cooler. The laser local oscillator and coupling-modulated laser transmitter use conductively cooled beryllium oxide construction. The optical antenna for transmission and reception has a gain of nominally 92 dB with a 3-dB beamwidth of 80 microrad. Tracking jitter is less than ten microrad, and signal acquisition occurs in less than 1 min. The receiver subsystem has a sensitivity of 10 to the -19th power W/Hz, accommodates a 300-Mbit/s data rate, and can track Doppler frequency variations over a + or - 700-MHz range.

Mcelroy, J. H.↗
NASA NTRS1977

An antenna for limited scan in one plane - Design criteria and numerical simulation

Aperture illumination is synthesized through the use of overlapping subarrays with independently controlled excitations. Detailed analysis and design criteria are provided for a specific limited-scan antenna configuration scanning in one plane. The number of phase shifts is the theoretical minimum corresponding to specified aperture size and field of view. Beam pointing and width are approximately constant for moderate frequency variation, and ultralow sidelobes outside the field of view can be produced at the cost of a slight loss of illumination efficiency. Accurate aperture illumination control makes for excellent independently specified sum and difference patterns in beamforming.

Borgiotti, G. V.↗