Digital data processor- pfm sexadecimal-to- decimal converter
Digital data processor pulse frequency modulation sexadecimal-to-decimal converter for use in development of PFM encoders for satellites
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Results for “pulsing 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.
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Reducing Field Distortion in Magnetic Resonance Imaging
A concept for a magnetic resonance imaging (MRI) system that would utilize a relatively weak magnetic field provides for several design features that differ significantly from the corresponding features of conventional MRI systems. Notable among these features are a magnetic-field configuration that reduces (relative to the conventional configuration) distortion and blurring of the image, the use of a superconducting quantum interference device (SQUID) magnetometer as the detector, and an imaging procedure suited for the unconventional field configuration and sensor. In a typical application of MRI, a radio-frequency pulse is used to excite precession of the magnetic moments of protons in an applied magnetic field, and the decaying precession is detected for a short time following the pulse. The precession occurs at a resonance frequency proportional to the strengths of the magnetic field and the proton magnetic moment. The magnetic field is configured to vary with position in a known way; hence, by virtue of the aforesaid proportionality, the resonance frequency varies with position in a known way. In other words, position is encoded as resonance frequency. MRI using magnetic fields weaker than those of conventional MRI offers several advantages, including cheaper and smaller equipment, greater compatibility with metallic objects, and higher image quality because of low susceptibility distortion and enhanced spin-lattice-relaxation- time contrast. SQUID MRI is being developed into a practical MRI method for applied magnetic flux densities of the order of only 100 T
Eom, Byeong Ho↗
Discovery of Weak EXO 2030+375 Outbursts With BATSE
EXO 2030+375 is a 42-second X-ray pulsar orbiting a Be star every 46 days. Previous work using epoch-folded frequency searches over 1-day intervals of BATSE data [1] indicated that EXO 2030+375 underwent a 2.5 year period of quiescence from 1993 September-1996 March. Improvements in the search method that reduced systematic errors due to (1) aperiodic noise from the nearby black hole candidate Cyg X-1 and (2) sources undergoing Earth occultation, have allowed longer time intervals to be searched, hence increasing BATSE's sensitivity. Using the improved method with 4-day intervals, we detect EXO 2030+375 near most of its periastron passages during 1993 September-1996 March and for most periastron passages during BATSE's 9 years in orbit. Earth occultation measurements in the 20-100 keV band, selected when Cyg X-1 was below the Earth's horizon and epoch-folded at EXO 2030+375's orbital period of 46 days, also indicate that EXO 2030+375 was active for most of the mission. We will present histories of EXO 2030+375's pulse frequency and pulsed flux. In addition, we will present results of a pulse timing analysis and evidence that EXO 2030+375's outbursts shift in orbital phase.
Wilson, Colleen A.↗
Cheap, Easy-To-Read Frequency Monitor For Pulsed Laser
Electronic circuit provides bar-graph display of difference between carrier frequency of pulsed laser transmitter and frequency of another laser serving as local oscillator in receiver. Display device linear array of light-emitting diodes (LED's), each representing 1-MHz portion of beat-frequency range from 20 to 40 MHz. Middle LED and neighbors green; LED's representing edges of passband yellow; LED's of frequencies outside passband red. Operator determines approximate relative frequency of transmitter at a glance by observing color and position of illuminated LED.
Esporoles, Carlos↗
Coherent and incoherent lidar - An overview
Characteristics of coherent and incoherent lidars are described. The carrier-to-noise ratio (CNR), SNR relationship, coherence diameter, speckle averaging, multiple pulse averaging, and the possibility of single-frequency pulse generation from the laser transmitter for the two lidar types are compared. Diagrams of the incoherent lidar's direct detection system and the coherent lidar's heterodyne detection system are presented. The transverse coherence diameter is discussed and equations for backscatter from a distributed medium, transverse coherence length, and return path refractive index variations are provided. The importance of transverse coherence length in the coherence lidar system design is considered. The effectiveness of the lidar systems is evaluated in terms of CNR and SNR. The differences in generation of noise-equivalent power are studied and it is observed that the coherent lidar system achieves higher CNR values. The relation between signal fluctuation due to speckle effects and SNR is analyzed; equations for SNR in coherent and incoherent lidar systems are given.
Menzies, R. T.↗
Development and Characterization Testing of an Air Pulsation Valve for a Pulse Detonation Engine Supersonic Parametric Inlet Test Section
In pulse detonation engines, the potential exists for gas pulses from the combustor to travel upstream and adversely affect the inlet performance of the engine. In order to determine the effect of these high frequency pulses on the inlet performance, an air pulsation valve was developed to provide air pulses downstream of a supersonic parametric inlet test section. The purpose of this report is to document the design and characterization tests that were performed on a pulsation valve that was tested at the NASA Glenn Research Center 1x1 Supersonic Wind Tunnel (SWT) test facility. The high air flow pulsation valve design philosophy and analyses performed are discussed and characterization test results are presented. The pulsation valve model was devised based on the concept of using a free spinning ball valve driven from a variable speed electric motor to generate air flow pulses at preset frequencies. In order to deliver the proper flow rate, the flow port was contoured to maximize flow rate and minimize pressure drop. To obtain sharp pressure spikes the valve flow port was designed to be as narrow as possible to minimize port dwell time.
Tornabene, Robert↗
New evidence on the nature of the neutron star and accretion flow in Vela X-1 from pulse timing observations
It is believed that variations in the intrinsic pulse frequencies of neutron star X-ray sources reflect changes in the rotation rate of the stellar crust produced by torque originating outside and inside the star. The external torque depends on the flow pattern of the accreting plasma, while the internal torque depends on the state of the interior and its coupling to the crust. Information about the accretion flow and the star itself can, therefore, be obtained on the basis of a study of intrinsic frequency variations. The present investigation is concerned with the results of a detailed study of the 283 s pulsing X-ray star Vela X-1 using pulse timing techniques. It is found that the frequency variations in this source are consistent with the occurrence of a succession of temporally unresolved events which can be characterized mathematically as second-order red noise in the pulse phase.
Boynton, P. E.↗
High-Frequency Metal-Oxide Varistor Modeling Response to Early-time Electromagnetic Pulses.
Abstract not provided.
Bowman, Tyler↗
XTE J1946+274 = GRO J1944+26
C. A. Wilson, M. H. Finger, R. S. Wilson, and D. M. Scott, Marshall Space Flight Center, NASA, report for the BATSE team: "Pulsations from a previously-uncatalogued x-ray pulsar in the Vul-Cyg region have been detected with BATSE in the energy band 20-50 keV from Sept. 8 through Sept. 14. A preliminary location (95-percent confidence) is bounded by R.A. = 19h 34m to 19h 54m and Decl. = +22o.5 to +30o.5 (equinox 2000.0). GRO J1944+26 rose from a 20-50-keV rms pulsed flux of < 15 mCrab on Sept. 7 to 27 +/- 3 mCrab on Sept. 13. Observations between Sept. 8 and 14 are consistent with a barycentric pulse frequency of 0.06315331 (6) Hz on Sept. 11.0 UT, with a frequency derivative of 2.2(2) x 10(exp -12) Hz/s. The pulse profile in the band 20-50 keV has a single asymmetric peak.
Smith, D. A.↗
Limits on an optical pulsar in supernova 1987A
Since March 1987 the optical flux from supernova 1987A for periodic pulsations has been sought. As of August 1988, after 38 separate observations, no pulsar has been detected. The typical upper limit placed on the pulsed fraction optical light from the supernova is 0.0002, for pulse frequencies in the range 0.03-5000 Hz. The best limit on the pulsed fraction of supernova light is 7 x 10 to the -6th, on January 22, 1988. On August 28, 1988 the faintest limit for the magnitude of the pulsar, dimmer than 20th mag is reached. These limits are based on Fourier transforms of up to 67 million points, covering a range of spindown rates.
Pennypacker, C. R.↗
Serendipitous Detections of XTE J1906+09 with the Rossi X-Ray Timing Explorer
The 89 s X-ray pulsar XTE J1906+09 was discovered during Rossi X-Ray Timing Explorer (RXTE) observations of SGR 1900+14 in 1996. Because of monitoring campaigns of SGR 1900+14, XTE J1906+09 was also monitored regularly in 1996 September, 1998 May-June, 1998 August-1999 July, and 2000 March-2001 January. A search for pulsations resulted in detections of only the two previously reported outbursts in 1996 September and 1998 August-September. Pulsed flux upper limits for the rest of the observations show that XTE J1906+09 is a transient X-ray pulsar and likely has a Be star companion. The RXTE all-sky monitor did not reveal XTE J1906+09. Pulse-timing analysis of the second outburst discovered a sinusoidal signature in the pulse frequencies that is likely produced by an orbital periastron passage. Fits to pulse phases using an orbital model and quadratic phase model have chi(exp 2) minima at orbital periods of 26-30 days for fixed mass functions of 5, 10, 15, and 20 solar masses. The pulse shape showed energy- and intensity-dependent variations. Pulse-phase spectroscopy quantified the energy-dependent variations. The phase-averaged spectrum used the pulse minimum spectrum as the background spectrum to eliminate effects from SGR 1900+14 and the Galactic ridge and was well fitted by an absorbed power law with a high-energy cutoff with column density N(sub H) = 6 +/- 1 x 10(exp 22)/sq cm, a photon index of 1.01 +/- 0.08, cutoff energy E(sub cut) = 11 +/- 1 keV, and e-folding energy E(sub fold) = 19 +/- 4 keV. Estimated 2-10 keV peak fluxes, corrected for contributions from the Galactic ridge and SGR 1900+14, are 6 x l0(exp -12) and 1.1 x 10(exp -10) ergs/sq cm/s for the 1996 and 1998 outbursts, respectively. XTE J1906+09 may be part of an unusual class of Be/X-ray binaries that do not lie on the general spin period versus orbital period correlation with the majority of Be/X-ray binaries.
Wilson, Colleen A.↗
Generating Submillimeter-Wave Frequencies From Laser Pulses
Semiconductor photoconductive switches generate electrical pulses containing submillimeter-wavelength carrier signals (frequency between 300 and 3,000 GHz) and harmonics thereof when illuminated with short-rise-time pulses from lasers. Device of this type used as local oscilator in heterodyne submillimeter-wave receiver. Electrical output of device coupled via transmission line, waveguide, or antenna to mixer circuitry of receiver. Phase delays between optically activated semiconductor switches determine output carrier frequencies. N electrical pulses generated by each laser pulse. Thus, fundamental output frequency is N times laser-pulse-repetition rate.
Spencer, Michael G.↗
Optical and infrared pulsations from the HZ Herculis binary system during the 1983 prolonged X-ray low state
Pulsed infrared and optical flux from HZ Her has been detected during the 1983 prolonged X-ray low state by simultaneous observations with the Lick Observatory Shane 3.1 m and Crossley 91 cm telescopes. The pulsed fluxes in the 1.0-2.5-micron bandpass and the 3200-7500 A bandpass agree in pulse frequency and phase and were measured to be 31 and 13 microJy, respectively. These pulsed fluxes indicate that the reprocessed pulsation spectrum is consistent with optically thin thermal bremsstrahlung radiation with a characteristic temperature of 25,000 K (+25,000; -12,500) which is modulated in intensity. An apparent earth-approaching Doppler velocity of about 65 km/s is observed for these pulsed fluxes; this velocity can be interpreted as pulsed X-ray reprocessing by material in the mass transfer stream between L1 and the outer edge of the accretion disk. Other optical observations made during and before the 1983 prolonged X-ray low state show that the optical pulsations may have been subtly affected by the obscuration responsible for the X-ray low.
Middleditch, J.↗
Dual-Ion-Trap Frequency Standards With Overlapping Cycles
Proposed scheme for enhancing performances of atomic frequency-standard apparatuses calls for two or more ion traps per apparatus interrogated in alternation by radio-frequency pulses. Provides nearly constant feedback gain, thereby providing nearly constant corrections for fluctuations in frequency of local oscillator. Degradation of performance by fluctuations in local oscillators reduced.
Dick, G. John↗
Field emission microscopy of carbon nanotube fibers: Evaluating and interpreting spatial emission
In this work, the authors quantify field emission properties of cathodes made from carbon nanotube (CNT) fibers. The cathodes were arranged in different configurations to determine the effect of cathode geometry on the emission properties. Various geometries were investigated including (1) flat cut fiber tip, (2) folded fiber, (3) looped fiber, and (4) fibers wound around a cylinder. The authors employ a custom field emission microscope to quantify I-V characteristics in combination with laterally resolved field-dependent electron emission area. Additionally, they look at the very early emission stages, first when a CNT fiber is turned on for the first time, which is then followed by multiple ramp-up/down runs. Upon the first turn on, all fibers demonstrated limited and discrete emission area. During ramping runs, all CNT fibers underwent multiple (minor and/or major) breakdowns, which improved emission properties in that turn-on field decreased and field enhancement factor and emission area both increased. It is proposed that breakdowns are responsible for removing initially undesirable emission sites caused by stray fibers higher than average. This initial breakdown process gives way to a larger emission area that is created when the CNT fiber subcomponents unfold and align with the electric field. Overall, the authors' results form the basis for careful evaluation of CNT fiber cathodes for dc or low frequency pulsed power systems in which large uniform area emission is required or for narrow beam high frequency applications in which high brightness is a must.
Image processing↗
Characterization of metal-polymer boundaries using Stoneley and Leaky waves
Optical and acoustooptical techniques for bonded surface inspection and interface wave detection are discussed. Interface wave propagation models based on viscous, elastic, and viscoelastic mechanical properties are reviewed. Gross boundary anomalies such as large defects and localized disbond regions were measured using conventional high frequency pulse echo and pitch catch ultratechniques. Bond durabilty is estimated by comparing detected wave attenuation versus frequency as a function of prebond surface preparation with the experimental limit obtained for perfect welded contact. Sensitivity of interface acoustic field periodicity to both residual and dc applied stress at the boundary surface is observed as well as hysteresis due to viscoelastic polymeric behavior as the applied stress is cycled.
Claus, R. O.↗
Numerical Simulations of the Steady and Unsteady Aerodynamic Characteristics of a Circulation Control Wing Airfoil
The aerodynamic characteristics of a Circulation Control Wing (CCW) airfoil have been numerically investigated, and comparisons with experimental data have been made. The configuration chosen was a supercritical airfoil with a 30 degree dual-radius CCW flap. Steady and pulsed jet calculations were performed. It was found that the use of steady jets, even at very small mass flow rates, yielded a lift coefficient that is comparable or superior to conventional high-lift systems. The attached flow over the flap also gave rise to lower drag coefficients, and high L/D ratios. Pulsed jets with a 50% duty cycle were also studied. It was found that they were effective in generating lift at lower reduced mass flow rates compared to a steady jet, provided the pulse frequency was sufficiently high. This benefit was attributable to the fact that the momentum coefficient of the pulsed jet, during the portions of the cycle when the jet was on, was typically twice as much as that of a steady jet.
Liu, Yi↗
The NANOGrav 12.5 yr Data Set: The Frequency Dependence of Pulse Jitter in Precision Millisecond Pulsars
Low-frequency gravitational-wave experiments require the highest timing precision from an array of the moststable millisecond pulsars. Several known sources of noise on short timescales in single radio pulsar observationsare well described by a simple model of three components: template fitting from a finite signal-to-noise ratio, pulsephase/amplitude jitter from single-pulse stochasticity, and scintillation errors from short-timescale interstellarscattering variations. Currently template-fitting errors dominate, but as radio telescopes push toward higher signalto-noise ratios, jitter becomes the next dominant term for most millisecond pulsars. Understanding the statistics ofjitter becomes crucial for properly characterizing arrival time uncertainties. We characterize the radio frequencydependence of jitter using data on 48 pulsars in the North American Nanohertz Observatory for GravitationalWaves timing program. We detect significant jitter in 43 of the pulsars and test several functional forms for itsfrequency dependence; we find significant frequency dependence for 30 pulsars. We find moderate correlations ofrms jitter with pulse width (R = 0.62) and number of profile components (R = 0.40); the single-pulse rms jitter istypically 1% of pulse phase. The average frequency dependence for all pulsars using a power-law model hasindex 0.42. We investigate the jitter variations for the interpulse of PSR B1937+21 and find no significantdeviations from the main pulse rms jitter. We also test the time variation of jitter in two pulsars and find thatsystematics likely bias the results for high-precision pulsars. Pulsar timing array analyses must properly modeljitter as a significant component of the noise within the detector.
Lam, M. T.↗