Single-walled carbon nanotubes (SWCNTs) have been shown to exhibit excellent electrical properties, such as ballistic transport over several hundred nanometers at room temperature. Field-effect ...transistors (FETs) made from individual tubes show dc performance specifications rivaling those of state-of-the-art silicon devices. An important next step is the fabrication of integrated circuits on SWCNTs to study the high-frequency ac capabilities of SWCNTs. We built a five-stage ring oscillator that comprises, in total, 12 FETs side by side along the length of an individual carbon nanotube. A complementary metal-oxide semiconductor-type architecture was achieved by adjusting the gate work functions of the individual p-type and n-type FETs used.
This paper explores the relationship that exists between nonlinear normal modes (NNMs) defined as invariant manifolds in phase space and the spectral expansion of the Koopman operator. Specifically, ...we demonstrate that NNMs correspond to zero level sets of specific eigenfunctions of the Koopman operator. Thanks to this direct connection, a new, global parametrization of the invariant manifolds is established. Unlike the classical parametrization using a pair of state-space variables, this parametrization remains valid whenever the invariant manifold undergoes folding, which extends the computation of NNMs to regimes of greater energy. The proposed ideas are illustrated using a two-degree-of-freedom system with cubic nonlinearity.
We demonstrate the addition of time gating to a standard optical spectrum analyzer (OSA) operating in the spectral region ~1.06μm . This is accomplished by opening for 7 ns the optical input to the ...OSA with an electrically driven poled fiber in a Sagnac loop. The sequential interrogation with nanosecond resolution of the reflection from three fiber Bragg gratings along a piece of fiber allows distinguishing the spectral peaks created with a minimum separation of 85 cm. The passive extinction ratio of this device is >40 dB and returns to >40 dB from >23 dB on a 35-ns time scale directly after time gating.
Optical fiber sensors (OFS) based on long-period fiber gratings (LPFG) or on surface plasmon resonance (SPR) represent attractive solutions for detection systems in remote areas. An interrogation ...system consisting on wavelength modulation of fiber coupled distributed feedback (DFB) lasers was implemented and tested. The system uses a single photodetector to individually acquire the intensity of each DFB laser modulated by the OFS and the real transmission spectrum is reconstructed through curve fitting. Testing was accomplished by measuring the spectral features of an LPFG when changing the surrounding refractive index and errors lower than 1.8 nm in the 1530 to 1570 nm wavelength region were obtained.
Due to its advantages over other configurations, sideband-separating receivers are usually preferred for radio astronomy, particularly in the presence of high atmospheric noise. However, even with ...all the advances that have been made in recent years in the field of receiver technology, one of the most important figures of merit for this kind of receiver, the sideband rejection ratio, is still relatively low and typically around 10 to 20 dB. This is because keeping low amplitude and phase imbalances over large RF and IF bands is extremely difficult. In order to overcome this problem, it has been suggested that by introducing a digital back-end that mimics the performance of an IF-hybrid, such imbalances can be calibrated out. Until now, this has been demonstrated only at very low RF frequencies (below 4 GHz). Here, for the first time, we demonstrate that this technique can be applied at higher frequencies. We have implemented a sideband-separating receiver with a calibrated digital IF-hybrid spectrometer for the 3 mm band, and have demonstrated that, even in the presence of large imbalances of individual components, sideband ratios above 35 dB can be obtained in the entire RF band.
A method is proposed for determining the correlated and uncorrelated parts of phase noise spectra (PNS) of two continuous wave radio signals of different frequencies, ω1 and ω2. The PNS of the two ...signals and of mixed signals are measured. The PNS are modelled as having a correlated part that is the same for both signals, except for a multiplicative factor, and uncorrelated parts, that are different for the two signals. A property of the model that the PNS of some mixing products are linear combinations of the PNS of the signals at ω1, ω2, and ω1 − ω2 is experimentally verified. The difference of the PNS at ω1 + ω2 and ω1 − ω2 is proportional to the correlated part of the PNS and is a part of auxiliary functions that are used for finding the multiplicative factor and the correlated, partly correlated, and uncorrelated phase noise at different offset frequencies. A conventional spectrum analyser was used to characterise two signal generators, a phase-coherent and a non-phase-coherent one. For the phase-coherent generator the phase noise of two signals was found to be correlated for offset frequencies below 10 Hz, partly correlated for 10 Hz–1 kHz and uncorrelated above 1 kHz.
The main directions in the development of the spectral systems for scintillation atomic emission analysis of powdered geological samples are aimed at increasing the sensitivity, spectral resolution, ...and speed of the spectrum analysis to reduce the detection limits. The goal of this study is development of high-speed multichannel emission spectrum analyzers with hybrid assemblies of arrays consisting of 14 photodetectors of two types, high-sensitivity BLPP-2000 array and high spatial resolution BLPP-4000 array, and theoretical estimation of the signal-to-noise ratio for scintillation and integral spectra recorded with the aforementioned photodetectors. The design of the high-speed MAES analyzers with BLPP-2000 and BLPP-4000 photodetector arrays provides parallel readout of arrays in hybrid assemblies. The Grand spectrometer equipped with such arrays consisting of 14 BLPP-2000 or BLPP-4000 photodetectors can record spectra in the 190–350 nm wavelength range. The minimum exposure time is 0.9 and 1.7 ms, respectively. It is shown that, when intensities of spectral lines are measured using the BLPP-2000 arrays, the signal-to-noise ratio is 3 times higher in integral analysis and 5 times higher in scintillation analysis as compared to measurements using the BLPP-4000 arrays.
Purpose: X‐rays produced by an x‐ray tube comprise a polychromatic spectrum. The American Association of Physicists in Medicine Task Group 116 (AAPM TG116) defined a method for producing a beam of ...standard quality for the purpose of calibrating digital x‐ray image receptors. The standard beam calibration condition is based on simulations of the International Electrotechnical Commission (IEC) RQA5. Our project was to measure the spectrum produced under TG 116 conditions and compare to the RQA5 spectrum predicted by simulation. Methods: X‐ray spectra were acquired using a CdTe solid state detector with a spectrum analyzer for three different x‐ray beams conforming to TG116 conditions. Acquired spectra were corrected for Cd and Te escape events using XRF‐XP software and for CdTe energy dependent efficiency. Spectra were simulated using the Tung1 computer program. Inherent filtration and tube potential were varied to obtain the best simulation of the RQA5 half‐value layer (HVL). Results: HVLs calculated from the acquired spectra are lower than HVLs measured. Under same kVp and inherent filtration, Al spectra and Cu spectra are very similar. Comparison of direct measurements with simulations reveals the strong influence of kVp on the resulting spectra. Acquired spectra are more similar to each other than to any of the simulated RQA5 spectra. Conclusion: Direct measurements of x‐ray spectra confirm the similarity of beams produced under TG116 conditions with either Cu or Al filtration. However, contrary to TG 116, the selection of kVp makes a significant difference in the spectra. Further work is needed to acquire spectra at the highest kVp allowed by TG 116 to fully assess the effect of kVp and HVL tolerances on the x‐ray spectrum.
A new series of piperidine derivatives namely 3,5-dimethyl-2,6-diaryl-1-propionyl-piperidin-4-ones were synthesized. The structure of the compound, 3, ...5-dimethyl-2,6-phenyl-1-propionyl-piperidin-4-one was determined unambiguously using NMR, IR and single crystal X-ray diffraction techniques. DFT and its time dependent version based calculations have been carried out to analyze its ground state electronic structure and to interpret the experimental spectroscopic data. The current study also explores into the spectroscopic property of these piperidine-4-one compounds and proposes these molecules as potential sunscreens and UV filters.
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•Synthesis of 3,5-dimethyl-2,6-diaryl-1-propionyl-piperidin-4-ones.•Characterized by UV, IR, 1H NMR, 13C NMR, HSQC, DEPT-135.•X-ray diffraction of study on compound 3,5-dimethyl-2,6-diphenyl-1-propionyl-piperidin-4-one 3a.•DFT calculation to investigate their electronic structure and UV spectral data.•DFT analysis for the validation of 2, 6 diaryl -piperidin-4-ones as potential sunscreens and UV filters.
Purpose: To evaluate a new rotating anode X‐ray tube from the resonant frequency point of view for stable and safe operation, and to validate a finite element model for insight into X‐ray tube rotor ...dynamics and vibration. Methods: The 3‐dimensional FEM model of the X‐ray tube motor has been developed using ANSYS and COMSOL. The resultant resonant frequency from the FEM simulation is substantiated by experiments. During deceleration of the X‐ray tube, an accelerometer and a corresponding amplifier send the time domain vibration response to a spectrum analyzer which generates the power spectrum. In the frequency domain analysis, a peak signifies large vibrations at that frequency. To corroborate the FEM model, the resonant frequency of the motor assembly without the anode attached was also measured. Lastly, a rough estimate of the resonant frequency can also be observed in angular speed curves which are obtained utilizing a quadrature position sensor. Results: The first mode resonance is expected at 20.3 Hz from the FEM simulation. This result matches closely with the peak at 22.2 Hz in the power spectrum and the location of the abrupt decreasing acceleration (slope) in the speed curve at 22 Hz. Without the anode, the FEM simulation result of 35.1 Hz is equal to the first peak at 35.1 Hz, and the angular acceleration is suddenly reduced at 34.4 Hz. Conclusions: For image‐guided interventional procedures using a hybrid system, the X‐ray tube should create flux at various times requiring repeatedacceleration and deceleration of the motor. Hence it is ideal that the resonant frequency is higher than operational speed, although alternatively the motor could accelerate through the resonant frequency quickly. Design improvements to modify the location of resonance of our motor assemblyare underway using the verified FEM model. NIH R01 EB007626, Richard M. Lucas Foundation
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