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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

Measurement System Analysis of a Novel Phase-Based Ultrasonic NDE Technique for Bond Strength Measurement

Fiber reinforced polymer parts have shown tremendous benefits in aerospace structural applications, but their qualification and certification for use in safety critical areas are currently hindered by the lack of a capable non-destructive evaluation (NDE) method or technique for the inspection of these adhesively bonded parts. Conventional NDE methods and techniques typically detect gross bond defects in a qualitative (Pass/Fail) manner. These techniques struggle to detect weak or kissing bonds. Also, there are no widely adopted NDE methods or techniques for measuring interfacial bond strength or detecting kissing bonds. Bond strength is currently ensured by process control and semi-destructive testing. Results from recent research from other authors, including but not limited to mechanical testing, have shown an excellent correlation between interfacial stiffness of an adhesively bonded joint and the adhesive bond strength of that joint. In this paper, a measurement system analysis (MSA) of a novel phase-based ultrasonic NDE Technique, developed at NASA Langley Research Center, is presented for bond strength measurement to assess at an increased level the measurement process and identify components of variation in that measurement process.

Ultrasound↗
NASA NTRS1978

The false lock performance of Costas loops with hard-limited in-phase channel

The ability of a Costas loop to false lock on a data sideband is a problem which must be dealt with in the design of suppressed-carrier receivers which employ such loops for carrier reconstruction. For conventional Costas loops wherein the error signal is formed from the product of two analog signals, the false lock problem has recently been investigated by the author and others and is now well understood. For implementation reasons associated with the reduction of dc offsets, it is often desirable to hard-limit the output of the in-phase channel and replace the analog multiplier which forms the above product with a chopper-type device. The false lock behavior of such a Costas loop with hard-limited in-phase channel is quite different from that of the conventional Costas loop and is the subject of investigation in this paper. Results are also presented for a modified version of the Costas loop wherein the quadrature arm filter is removed.

Simon, M. K.↗
NASA NTRS2004

Reduction of Phase Ambiguity in an Offset-QPSK Receiver

Proposed modifications of an offset-quadri-phase-shift keying (offset-QPSK) transmitter and receiver would reduce the amount of signal processing that must be done in the receiver to resolve the QPSK fourfold phase ambiguity. Resolution of the phase ambiguity is necessary in order to synchronize, with the received carrier signal, the signal generated by a local oscillator in a carrier-tracking loop in the receiver. Without resolution of the fourfold phase ambiguity, the loop could lock to any of four possible phase points, only one of which has the proper phase relationship with the carrier. The proposal applies, more specifically, to an offset-QPSK receiver that contains a carrier-tracking loop like that shown in Figure 1. This carrier-tracking loop does not resolve or reduce the phase ambiguity. A carrier-tracking loop of a different design optimized for the reception of offset QPSK could reduce the phase ambiguity from fourfold to twofold, but would be more complex. Alternatively, one could resolve the fourfold phase ambiguity by use of differential coding in the transmitter, at a cost of reduced power efficiency. The proposed modifications would make it possible to reduce the fourfold phase ambiguity to twofold, with no loss in power efficiency and only relatively simple additional signal-processing steps in the transmitter and receiver. The twofold phase ambiguity would then be resolved by use of a unique synchronization word, as is commonly done in binary phase-shift keying (BPSK). Although the mathematical and signal-processing principles underlying the modifications are too complex to explain in detail here, the modifications themselves would be relatively simple and are best described with the help of simple block diagrams (see Figure 2). In the transmitter, one would add a unit that would periodically invert bits going into the QPSK modulator; in the receiver, one would add a unit that would effect different but corresponding inversions of bits coming out of the QPSK demodulator. The net effect of all the inversions would be that depending on which lock point the carrier-tracking loop had selected, all the output bits would be either inverted or non-inverted together; hence, the ambiguity would be reduced from fourfold to twofold, as desired.

Berner, Jeff↗
NASA NTRS1980

The performance of suppressed carrier receivers in a pulsed RFI environment

This paper investigates the tracking performance of a Costas loop in the presence of single and multiple in-band pulsed RFI signals and receiver additive noise. It is shown that the loop's tracking performance is very sensitive to the pulse durations of the RFI signals, as well as their repetition rates and peak powers relative to those of the desired signal. The presence of in-band RFI can affect the loop's tracking performance in basically three ways. 1) The loop tracks the composite phase of the RFI instead of the phase of the desired signal when the effective RFI power is greater than that of the signal. 2) When the effective RFI power is not sufficiently large to capture the loop, there still will exist static phase error biases which cause the loop to lock at a phase between that of the desired signal and the composite RFI. 3) There is an increase in the rms phase jitter over the nominal design. These latter two effects degrade the bit error probability performance of the receiver. More significantly, if the nominal loop bandwidth is not designed properly to account for the increase in jitter phenomenon, frequency cycle slipping may result.

Simon, M. K.↗
NASA NTRS1977

Multiple rate digital command detection system with range clean-up capability

A multi-rate digital command system is disclosed which uses the composite signal of a mu-type ranging system as a subcarrier to transmit range codes and data from a station to a receiver where the range codes are sequentially phase modulated on a subcarrier of frequency by one of its own subharmonics and data is phase modulated on a selected ranging component. A range cleanup loop in a spacecraft locks the phase of a locally generated reference component to a received ranging component and retransmits the component to a ground station. When the inverse phase of a ranging component is received and detected, the cleanup loop is modified to demodulate phase modulated command symbols while continuing tracking the same ranging component. The command symbol rate is coherently related to the ranging signal component bit rate.

Lesh, J. R.↗
NASA NTRS2010

Closed-Loop Control of Vortex Formation in Separated Flows

In order to phase lock the flow at the desired shedding cycle, particularly at Phi,best, We designed a feedback compensator. (Even though the open-loop forcing at Wf below Wn can lead to phase-locked limit cycles with a high average lift,) This feedback controller resulted in the phase-locked limit cycles that the open-loop control could not achieve for alpha=30 and 40 Particularly for alpha=40, the feedback was able to stabilize the limit cycle that was not stable with any of the open-loop periodic forcing. This results in stable phase-locked limit cycles for a larger range of forcing frequencies than the open-loop control. Also, it was shown that the feedback achieved the high-lift unsteady flow states that open-loop control could not sustain even after the states have been achieved for a long period of time.

Colonius, Tim↗
NASA NTRS1972

Improvements in deep-space tracking by use of third-order loops.

Third-order phase-locked receivers have not yet found wide application in deep-space communications systems because the second-order systems now used have performed adequately on past spacecraft missions. However, a survey of the doppler profiles for future missions shows that an unaided second-order loop may be unable to perform within reasonable error bounds. This article discusses the characteristics of a simple third-order extension to present second-order systems that not only extends doppler-tracking capability, but widens the pull-in range and decreases pull-in time as well.

Tausworth, R. C.↗
NASA NTRS1973

Telecommunication systems engineering

Telecommunication network concepts are discussed together with carrier-tracking loops employing the phase-lock principle, phase and Doppler measurements in two-way phase-coherent tracking systems, range measurements by phase-coherent techniques, and questions of phase-coherent detection with perfect reference signals and with noisy reference signals. The design of one-way and two-way phase-coherent communication systems is considered, giving attention to the optimal design of single-channel systems, the design of two-channel systems, and the design of multichannel systems. Other topics explored include the design and the performance of phase-coherent systems preceded by band-pass limiters, symbol synchronization and its effects on data detection, noncoherent communication over the Gaussian channel, and tracking loops with improved performance.

Lindsey, W. C.↗
NASA NTRS1994

Pulsed acceleration in solar flares

We study the nonlinear dynamics of particle acceleration in solar flares by analyzing the time series of various quasi-periodic radio signatures during flares. In particular we present the radio and hard X-ray data of three flares which suppport the following tentative conclusions: (1) Particle acceleration and injection into magnetic structures occurs intrinsically in a pulsed mode (with a typical period of 1-2 s), produced by a single, spatially coherent, nonlinear system, rather than by a stochastic system with many spatially independent components ('statistical flare' produced by a fragmented primary energy release). (2) The nonlinear (quasi-periodic) mode of pulsed particle acceleration and injection into a coronal loop can be stabilized by phase locking with an MHD wave (oscillation) mode, if both periods are close to each other. (3) Pulsed injection of electron beams into a coronal loop may trigger nonlinear relaxational oscillations of wave-particle interactions. This is particularly likely when the limit cycles of both systems are similar.

Aschwanden, Markus J.↗
NASA NTRS1974

Pulse code modulated signal synchronizer

A bit synchronizer for a split phase PCM transmission is reported that includes three loop circuits which receive incoming phase coded PCM signals. In the first loop, called a Q-loop, a generated, phase coded, PCM signal is multiplied with the incoming signals, and the frequency and phase of the generated signal are nulled to that of the incoming subcarrier signal. In the second loop, called a B-loop, a circuit multiplies a generated signal with incoming signals to null the phase of the generated signal in a bit phase locked relationship to the incoming signal. In a third loop, called the I-loop, a phase coded PCM signal is multiplied with the incoming signals for decoding the bit information from the PCM signal. A counter means is used for timing of the generated signals and timing of sample intervals for each bit period.

Kobayashi, H. S.↗
NASA NTRS1992

Behavior Of Costas Loop In Reception Of Telemetry

Report presents study of behavior of Costas loop in reception of binary non-return-to-zero telemetry signal phase-shift-modulated onto sinusoidal or square-wave subcarrier signal in turn phase-modulated onto carrier signal. Equations describe degradation of performance of Costas loop in terms of phase jitter, conditions causing loss of lock, and other effects. Costas loop used to track telemetry signal with residual carrier in presence of Doppler shift. Performance optimized by suitable choice of bit rate, waveform and frequency of subcarrier, and noise bandwidth of filters in arms of Costas loop.

Nguyen, Tien Manh↗
NASA NTRS1992

Effects Of Phase Jitter On Lock Detection In QPSK

Report presents study of performances of digital carrier-signal-tracking loops used in reception of radio signals containing quadrature-phase-shift-keyed (QPSK) modulation representing binary non-return-to-zero data. Presents mathematical models of received signal and of noise. Describes lock-detector algorithm and presents equations characterizing lock-detection performance. Equations applied in analyses of three QPSK carrier-tracking loops: Costas crossover loop, maximum a posteriori estimation loop, and generalized Costas loop. Results of computer simulations agree with equations.

Mileant, Alexander↗
DOE OSTI2021

JLAB LLRF 3.0 Development and Tests

The Jefferson Lab LLRF 3.0 system is being developed to replace legacy LLRF systems in the CEBAF accelerator. The new design builds upon 25 years of design and operational RF control experience, and our recent collaboration in the design of the LCLSII LLRF system. The new cavity control algorithm is a fully functional phase and amplitude locked Self Exciting Loop (SEL). This paper discusses the progress of the LLRF 3.0 hardware design, FPGA firmware development, User Datagram Protocol (UDP) operation, and recent LLRF 3.0 system tests on the CEBAF Booster cryomodule in the Upgrade Injector Test Facility (UITF).

Plawski, T. E.↗
DOE OSTI2022

First SELAP Algorithm Operational Experience of the New LLRF 3.0 RF Control System

The JLAB LLRF 3.0 system has been developed and is replacing the 30-year-old LLRF systems in the CEBAF accelerator. The LLRF system builds upon 25 years of design and operational RF control experience (digital and analog), and our recent collaboration in the design of the LCLSII LLRF system. The new system also incorporates a cavity control algorithm using a fully functional phase and amplitude locked Self Exciting Loop (SELAP). The first system (controlling 8 cavities) was installed and commissioned in August of 2021. Since then the new LLRF system has been operating with cavity gradients up to 20 MV/m, and electron beam currents up to 350 uA. This paper discusses the operational experience of the LLRF 3.0 SELAP algorithm along with other software and firmware tools like cavity and klystron characterization and quench detection.

Plawski, T. E.↗
NASA NTRS1972

Phase-lock demodulation of a PM signal contaminated with incidental AM

Signals from phase-modulated satellite transmitters usually exhibit some degree of incidental amplitude modulation. The effects of incidental AM are analyzed when this type of signal is demodulated by a phase-lock receiver which does not employ a limiter preceding the loop phase detector. The presence of incidental AM causes a reduction in the receiver output signal-to-noise ratio. The tolerable level of AM decreases in proportion to the phase modulation index Beta. For a square-wave modulating signal, a 1 db reduction results at the receiver PM channel output when Beta = 1 radian and the percentage of AM = 23, Beta = 1.2 radians and the percentage of AM = 16, or Beta = 1.5 radians and the percentage of AM = 4. Although only the PM channel of the receiver is used ordinarily, utilizing both the AM and PM channel by summing offers an improvement in S/N relative to the S/N ratio of the PM channel if the percentage of incidental AM is greater than fifteen.

Robinson, G. B.↗
NASA NTRS1985

The Statistical Loop Analyzer (SLA)

The statistical loop analyzer (SLA) is designed to automatically measure the acquisition, tracking and frequency stability performance characteristics of symbol synchronizers, code synchronizers, carrier tracking loops, and coherent transponders. Automated phase lock and system level tests can also be made using the SLA. Standard baseband, carrier and spread spectrum modulation techniques can be accomodated. Through the SLA's phase error jitter and cycle slip measurements the acquisition and tracking thresholds of the unit under test are determined; any false phase and frequency lock events are statistically analyzed and reported in the SLA output in probabilistic terms. Automated signal drop out tests can be performed in order to trouble shoot algorithms and evaluate the reacquisition statistics of the unit under test. Cycle slip rates and cycle slip probabilities can be measured using the SLA. These measurements, combined with bit error probability measurements, are all that are needed to fully characterize the acquisition and tracking performance of a digital communication system.

Lindsey, W. C.↗
NASA NTRS1990

A review of fade detection techniques

Several proposed propagation fade detection techniques are reviewed in light of general requirements presented for beacon fade characterization. The discussion includes an analysis of phase lock versus frequency lock beacon tracking loops and of excess noise injection type radiometers. The Advanced Communications Technology Satellite (ACTS) beacon fade detection schemes proposed by the Communications Satellite Corporation and the Jet Propulsion Laboratory are examined along with the fade detection technique used by Harris in the Advanced Communications Technology Satellite (ACTS) low burst rate (LBR) terminal.

Pergal, F. J.↗