Several Hundreds Of MHz Research Articles

Two Equivalent Internal Rotations in the Microwave Spectrum of 2,6-Dimethylfluorobenzene.

Large amplitude motion of methyl groups in isolated molecules is a fundamental phenomenon in molecular physics. The methyl torsional barrier is sensitive to the steric and electronic environment in the surrounding of the methyl group, making the methyl group a detector of the molecular structure. To probe this effect, the microwave spectrum of 2,6-dimethylfluorobenzene, one of the six isomers of dimethylfluorobenzene, was measured using two pulsed molecular jet Fourier transform microwave spectrometers operating in the frequency range from 2 to 40 GHz. Due to internal rotations of two equivalent methyl groups with relatively low torsional barriers, all rotational transitions split into quartets with separations of up to several hundreds of MHz. The splittings were analyzed and modeled to deduce a torsional barrier of 236.7922 (21) cm-1 . The results are compared to those obtained from quantum chemical calculations and with other fluorine substituted toluene derivatives of the current literature where the methyl group is adjacent to a fluorine atom.

Amplitude and frequency sensing of microwave fields with a superconducting transmon qudit

Experiments with superconducting circuits require careful calibration of the applied pulses and fields over a large frequency range. This remains an ongoing challenge as commercial semiconductor electronics are not able to probe signals arriving at the chip due to its cryogenic environment. Here, we demonstrate how the on-chip amplitude and frequency of a microwave signal can be inferred from the ac Stark shifts of higher transmon levels. In our time-resolved measurements we employ Ramsey fringes, allowing us to detect the amplitude of the systems transfer function over a range of several hundreds of MHz with an energy sensitivity on the order of 10−4. Combined with similar measurements for the phase of the transfer function, our sensing method can facilitate pulse correction for high fidelity quantum gates in superconducting circuits. Additionally, the potential to characterize arbitrary microwave fields promotes applications in related areas of research, such as quantum optics or hybrid microwave systems including photonic, mechanical or magnonic subsystems.

Doppler Shift Time Expansion Resolution and Spectral Performance in Wideband Real-Time RF Channel Emulators

The possibility to test a radio frequency transceiver through the use of appropriate channel emulators allows evaluating their performance under various operating conditions. Many systems are able to operate with a relatively limited instantaneous bandwidth by applying known statistical models. Sometimes, it is necessary to evaluate the performance of a RF transceiver installed on high or very high speed platforms over predictable trajectories using optionally DEM (Digital Elevation Model) data of the terrain to estimate the number of stimulated paths and their contributions during the flight. When the instantaneous bandwidth of the signal becomes high (over several hundreds of MHz), one of the most important phenomena to consider is the Doppler spread induced by the channel and traditional narrowband models become useless. This paper presents some results when a time expansion is adopted to emulate the transceiver dynamic and the consequent Doppler spread with the aim of controlling the spectral purity of the emulated propagation channel.

Radio-frequency analysis of self-mode-locked quantum dot laser

We numerically study and confirm experimentally a transition from unlocked multi-modal emission to self-mode-locked emission of a single-section InAs/InGaAs quantum dot laser in dependence on the laser injection current. At this transition, the radio-frequency beat note linewidth as well as the modal power fluctuations quantified by an integrated relative intensity noise of all modes are found to be significantly reduced. A linewidth reduction from several hundreds of MHz to below 1 MHz is observed, with the lowest value observed being as small as 20 kHz. Experiment and simulation results are in good qualitative and quantitative agreement

Metamaterials-inspired resonator for wireless power transfer systems

Metamaterial-inspired resonator for a wireless power transfer system is proposed and studied numerically. The resonator consists of two orthogonal wire layers separated by a short distance. To reduce the resonant frequency from several hundreds of MHz it is placed in a background dielectric material with high permittivity. By tuning the permittivity of the background material, the radiation losses can be significantly reduced resulting in a high Q-factor of 900. The wireless power transfer efficiency greater than 80% from the wire resonator to a small loop receiver has been numerically demonstrated at the frequency of 19 MHz. The large area of the proposed resonator (50 cm × 50 cm) offers a possibility to charge multiple receivers simultaneously.

On peak current in atmospheric pulse-modulated microwave discharges by the PIC-MCC model

Pulse modulation provides a new way to tailor the electron density, electron energy and gas temperature in atmospheric radio-frequency (rf) discharges. In this paper, by increasing the rf frequency to several hundreds of MHz, or even much higher to the range of GHz, a very strong peak current in the first period (PCFP) with much larger electron energy can be formed during the power-on phase, which is not observed in the common pulse modulation discharges at a rf frequency of 13.56 MHz. The PIC-MCC model is explored to unveil the generation mechanism of PCFP, and based on the simulation data a larger voltage increasing rate over a quarter of a period and the distribution of electron density just before the power-on phase are believed to play key roles; the PCFP is usually produced in the microplasma regime driven by the pulsed power supply. The effects of duty cycle and pulse modulation frequency on the evolution of PCFP are also discussed from the computational data. Therefore, the duty cycle and pulse modulation frequency can be used to optimize the generation of PCFP and high-energy electrons.

An electronically tuned wideband probehead for NQR spectroscopy in the VHF range

Nuclear quadrupole resonance spectroscopy is an analytical method which allows to characterize materials which contain quadrupolar nuclei, i.e. nuclei with spin ⩾1. The measurement technology is similar to that of NMR except that no static magnetic field is necessary. In contrast to NMR, however, it is frequently necessary to scan spectra with a very large bandwidth with a span of several tens of % of the central frequency so as to localize unknown peaks. Standard NMR probeheads which are typically constructed as resonators must be tuned and matched to comparatively narrow bands and must thus be re-tuned and re-matched very frequently when scanning over a whole NQR spectrum. At low frequencies up to few MHz dedicated circuits without the need for tuning and matching have been developed, but many quadrupole nuclei have transitions in the VHF range between several tens of MHz up to several hundreds of MHz. Currently available commercial NQR probeheads employ stepper motors for setting mechanically tuneable capacitors in standard NMR resonators. These yield high quality factors (Q) and thus high SNR but are relatively large and clumsy and do not allow for fast frequency sweeps. This article presents a new concept for a NQR probehead which combines a previously published no-tune no-match wideband concept for the transmit (TX) pulse with an electronically tuneable receive (RX) part employing varactor diodes. The prototype coil provides a TX frequency range of 57MHz with a center frequency of 97.5MHz with a return loss of ⩽−15dB. During RX the resonator is tuned and matched automatically to the right frequency via control voltages which are read out from a previously generated lookup table, thus providing high SNR. The control voltages which bias the varactors settle very fast and allow for hopping to the next frequency point in the spectrum within less than 100μs. Experiments with a test sample of ZnBr2 proved the feasibility of the method.

Investigation into the electromagnetic impulses from long-pulse laser illuminating solid targets inside a laser facility

Abstract Emission of the electromagnetic pulses (EMP) due to laser-target interaction in laser facility had been evaluated using a cone antenna in this work. The microwave in frequencies ranging from several hundreds of MHz to 2 GHz was recorded when long-pulse lasers with several thousands of joules illuminated the solid targets, meanwhile the voltage signals from 1 V to 4 V were captured as functions of laser energy and backlight laser, where the corresponding electric field strengths were obtained by simulating the cone antenna in combination with conducting a mathematical process (Tiknohov Regularization with L curve). All the typical coupled voltage oscillations displayed multiple peaks and had duration of up to 80 ns before decaying into noise and mechanisms of the EMP generation was schematically interpreted in basis of the practical measuring environments. The resultant data were expected to offer basic know-how to achieve inertial confinement fusion.

SPIDYAN, a MATLAB library for simulating pulse EPR experiments with arbitrary waveform excitation

Frequency-swept chirp pulses, created with arbitrary waveform generators (AWGs), can achieve inversion over a range of several hundreds of MHz. Such passage pulses provide defined flip angles and increase sensitivity. The fact that spectra are not excited at once, but single transitions are passed one after another, can cause new effects in established pulse EPR sequences. We developed a MATLAB library for simulation of pulse EPR, which is especially suited for modeling spin dynamics in ultra-wideband (UWB) EPR experiments, but can also be used for other experiments and NMR. At present the command line controlled SPin DYnamics ANalysis (SPIDYAN) package supports one-spin and two-spin systems with arbitrary spin quantum numbers. By providing the program with appropriate spin operators and Hamiltonian matrices any spin system is accessible, with limits set only by available memory and computation time. Any pulse sequence using rectangular and linearly or variable-rate frequency-swept chirp pulses, including phase cycling can be quickly created. To keep track of spin evolution the user can choose from a vast variety of detection operators, including transition selective operators. If relaxation effects can be neglected, the program solves the Liouville-von Neumann equation and propagates spin density matrices. In the other cases SPIDYAN uses the quantum mechanical master equation and Liouvillians for propagation. In order to consider the resonator response function, which on the scale of UWB excitation limits bandwidth, the program includes a simple RLC circuit model. Another subroutine can compute waveforms that, for a given resonator, maintain a constant critical adiabaticity factor over the excitation band. Computational efficiency is enhanced by precomputing propagator lookup tables for the whole set of AWG output levels. The features of the software library are discussed and demonstrated with spin-echo and population transfer simulations.

Characteristics Simulation of Ultra-High Frequency Electromagnetic Wave Excited by Equivalent Partical Discharge Mathematical Model in GIS

UHF electromagnetic waves radiated by PD in GIS can range from several hundreds of MHz to several GHz. FDTD method is usually used to simulate the characteristics of the UHF electromagnetic waves. Compared with the traditional ideal Gaussian pulses in simulation, one equivalent PD mathematical model based on the actual measure PD sources was considered to simulate the propagation characteristics. Time domain and frequency domain spectrum of signals excited by the two PD sources were studied to analyze the propagation characteristics. It can draw the conclusions that each insulator detect leads to each electromagnetic and its time domain and frequency domain spectrum. So it is recommended to use equivalent PD mathematical model to simulator the propagation characteristics.

Detecting response of microelectromechanical resonators by microwave reflectometry

Microwave reflectometry is proposed as an effective technique to detect the vibration of capacitively transduced microelectromechanical resonators. The transducer capacitor is probed by an incident wave, which is reflected being modulated by the time variations of the resonator displacement. Calculations demonstrate that the sensitivity of the technique is maximum for a given microwave frequency depending on the transducer total capacitance. Experimental data show that capacitance variations as low as 3 zF/√Hz are measurable at 4 GHz for the studied devices. Such a performance corresponds to a sub-picometer resolution in vibration amplitude of the microelectromechanical resonator. The measurement technique is particularly appropriate for resonant sensors when high signal-to-noise ratio and fully electrical detection are required. It can be used for device resonance frequency up to several hundreds of MHz.

Adiabatic and fast passage ultra-wideband inversion in pulsed EPR

We demonstrate that adiabatic and fast passage ultra-wideband (UWB) pulses can achieve inversion over several hundreds of MHz and thus enhance the measurement sensitivity, as shown by two selected experiments. Technically, frequency-swept pulses are generated by a 12 GS/s arbitrary waveform generator and upconverted to X-band frequencies. This pulsed UWB source is utilized as an incoherent channel in an ordinary pulsed EPR spectrometer. We discuss experimental methodologies and modeling techniques to account for the response of the resonator, which can strongly limit the excitation bandwidth of the entire non-linear excitation chain. Aided by these procedures, pulses compensated for bandwidth or variations in group delay reveal enhanced inversion efficiency. The degree of bandwidth compensation is shown to depend critically on the time available for excitation. As a result, we demonstrate optimized inversion recovery and double electron electron resonance (DEER) experiments. First, virtually complete inversion of the nitroxide spectrum with an adiabatic pulse of 128ns length is achieved. Consequently, spectral diffusion between inverted and non-inverted spins is largely suppressed and the observation bandwidth can be increased to increase measurement sensitivity. Second, DEER is performed on a terpyridine-based copper (II) complex with a nitroxide-copper distance of 2.5nm. As previously demonstrated on this complex, when pumping copper spins and observing nitroxide spins, the modulation depth is severely limited by the excitation bandwidth of the pump pulse. By using fast passage UWB pulses with a maximum length of 64ns, we achieve up to threefold enhancement of the modulation depth. Associated artifacts in distance distributions when increasing the bandwidth of the pump pulse are shown to be small.

Inverse Compton scattering of coherent synchrotron radiation in an energy recovery linac

We propose inverse Compton scattering (ICS) of coherent synchrotron radiation (CSR) from a relativistic short electron bunch in energy recovery linacs (ERL) as a high-flux subpicosecond x-ray and $\ensuremath{\gamma}$-ray source. An advantage of the CSR scheme over a conventional ICS source is that no external laser is required, and synchronization between CSR pulses and electron bunches is obtained automatically. Moreover, higher-flux x rays can be generated from the ICS of CSR in an ERL operated at a high repetition rate, 100 MHz to 1.3 GHz. Using parameters of the Compact ERL at KEK, $1\ifmmode\times\else\texttimes\fi{}{10}^{13--14}\text{ }\text{ }\mathrm{phs}/\mathrm{s}$ b.w. 10% (the number of photons ${\mathrm{pulse}}^{\ensuremath{-}1}$ bandwidth ${\mathrm{unit}}^{\ensuremath{-}1}$) x ray with a 100 fs--1 ps pulse duration can be obtained, for an energy range from 0.04 to 4 keV. In the case of a 5-GeV ERL, $\ensuremath{\gamma}$ rays with energy around tens of MeV are generated with $1\ifmmode\times\else\texttimes\fi{}{10}^{8}\text{ }\text{ }\mathrm{phs}/\mathrm{pulse}$ b.w. 10% at a repetition rate of several hundreds of MHz.

Electromagnetic pulses at short-pulse laser facilities

Electromagnetic Pulse (EMP) is a known issue for short-pulse laser facilities, and will also be an issue for experiments using the advanced radiographic capability (ARC) at the National Ignition Facility (NIF). The ARC diagnostic uses four NIF beams that are compressed to picosecond durations for backlighting ignition capsules and other applications. Consequently, we are working to understand the EMP due to high-energy (MeV) electrons escaping from targets heated by short-pulse lasers. Our approach is to measure EMP in the Titan short-pulse laser at Lawrence Livermore National Laboratory (LLNL) and to employ that data to establish analysis and simulation capabilities. We have installed a wide variety of probes inside and outside the Titan laser chamber. We have high-frequency B-dot and D-dot probes, a photodiode, and fast current-viewing and integrating current transformers. The probe outputs are digitized by 10 and 20 Gsample/s oscilloscopes. The cables and oscilloscopes are well shielded to reduce noise. Our initial measurement campaign has yielded data useful mainly from several hundreds of MHz to several GHz. We currently are supplementing our high-frequency probes with lower-frequency ones to obtain better low-frequency data. In order to establish analysis and simulation capabilities we are modeling the Titan facility using various commercial and LLNL numerical electromagnetics codes. We have simulated EMP generation by having a specified number of electrons leave the target and strike the chamber wall and other components in the chamber. This short impulse of electrons has a correspondingly broad spectrum, exciting high-frequency structure in the resulting EMP. In this paper, we present results of our initial measurement campaign and comparisons between the measurements and simulations.

Laser frequency offset locking using a side of filter technique

We report on the demonstration of a novel scheme for laser frequency offset locking. Following standard techniques, a beat signal is generated between the laser to be frequency stabilized and a second laser with known frequency. A frequency dependent error signal is derived from this beat note by using the amplitude response of an electronic high pass filter. The steep slope of the error signal near the zero-crossing allows for precise frequency locking by a servo loop, while simultaneously a broad capture range of several hundreds of MHz is obtained.

High-speed data converters for communication systems

Discusses the ever increasing demands on IC specifications, namely speed and resolution, We will see that often more than 14 bits and several hundreds of MHz are required, In addition, many applications impose continuous reductions in power consumption. As a result, market challenges favor research on high speed data converters, In turn, the results achieved lead to new architectural solutions which create new needs, This paper analyses this process and discusses recent circuit solutions suitable for meeting key system specifications.

High-speed photonic/electronic conversion

We have successfully demonstrated the integration of an ultrasensitive III-V detector, based on a differential pair of optical thyristors, with a dedicated novel 1 GHz CMOS comparator amplifier followed by an R-S flip flop. The maximum frequency achieved so far for the receiver is 50 MHz. Further work is underway to improve this limit to at least the frequency achievable with the thyristor pair, i.e., several hundreds of MHz. Because of its small area and its high sensitivity to optical input signals, our receiver is a promising building block for CMOS based optoelectronic circuits.

Silicon micro/nanomechanical device fabrication based on focused ion beam surface modification and KOH etching

Selective Ga+ ion implantation and milling by focused ion beam exposure and subsequent wet chemical etching is used to fabricate micro/nanomechanical elements in Si. Freestanding elements with a ≈ 30 nm membrane thickness are made by controlled selective underetching between unexposed and exposed areas. Ultrahigh-frequency cantilever beams have been made with resonances in the tens of MHz range. Using a U-shaped beam cross section, mechanical stiffness could be increased 100-fold, which in turn increased the beam resonance frequency to several hundreds of MHz. The direct-write patterning/milling technique was used to fabricate various arbitrary shapes with vertical sidewalls such as submicrometer-sized containers, cups, and other nanomechanical devices.

Examination to eliminate undesirable phase delay of an avalanche photodiode (APD) for intensity-modulated light

Undesirable phase delay in avalanche photodiode (APD) between input light and output current is noticeable when modulation frequency of the incident light is over several hundreds of MHz. The degree of the delay increases with a decrease of the incident intensity. To clarify this phenomenon, relationships among the phase delay, output current, and the multiplication factor (current gain) of APD have been investigated. From this investigation, we found that the multiplication factor of APD increased with a decrease of the incident intensity, resulting in a decrease of the cutoff frequency of the APD. Supposing that the cutoff frequency defines the signal delay, the amount of the phase delay has been quantified with respect to the multiplication factor. Such a phase delay can be eliminated by superimposing a bias light on the modulation light so that the total intensity remains constant. To confirm this, a servo control system using an intensity modulated laser diode and unmodulated laser diodes (LDs) was constructed. The drive current of the unmodulated LD was changed automatically with respect to the modulated LD intensity, thus keeping the APD output at a constant value. The resulting APD phase delay was within 0.05°, which is only 0.05% of that of the uncompensated system.

5309104 Asymmetric radio frequency coil for magnetic resonance imaging

Nuclear quadrupole resonance spectroscopy is an analytical method which allows to characterize materials which contain quadrupolar nuclei, i.e. nuclei with spin ⩾1. The measurement technology is similar to that of NMR except that no static magnetic field is necessary. In contrast to NMR, however, it is frequently necessary to scan spectra with a very large bandwidth with a span of several tens of % of the central frequency so as to localize unknown peaks. Standard NMR probeheads which are typically constructed as resonators must be tuned and matched to comparatively narrow bands and must thus be re-tuned and re-matched very frequently when scanning over a whole NQR spectrum. At low frequencies up to few MHz dedicated circuits without the need for tuning and matching have been developed, but many quadrupole nuclei have transitions in the VHF range between several tens of MHz up to several hundreds of MHz. Currently available commercial NQR probeheads employ stepper motors for setting mechanically tuneable capacitors in standard NMR resonators. These yield high quality factors (Q) and thus high SNR but are relatively large and clumsy and do not allow for fast frequency sweeps. This article presents a new concept for a NQR probehead which combines a previously published no-tune no-match wideband concept for the transmit (TX) pulse with an electronically tuneable receive (RX) part employing varactor diodes. The prototype coil provides a TX frequency range of 57 MHz with a center frequency of 97.5 MHz with a return loss of ⩽−15 dB. During RX the resonator is tuned and matched automatically to the right frequency via control voltages which are read out from a previously generated lookup table, thus providing high SNR. The control voltages which bias the varactors settle very fast and allow for hopping to the next frequency point in the spectrum within less than 100 μs. Experiments with a test sample of ZnBr2 proved the feasibility of the method.