Direct EPR irradiation of a sample using a quartz oscillator operating at 250 MHz for EPR measurements

Direct irradiation of a sample using a quartz oscillator operating at 250 MHz was performed for EPR measurements. Because a quartz oscillator is a frequency fixed oscillator, the operating frequency of an EPR resonator (loop-gap type) was tuned to that of the quartz oscillator by using a single-turn coil with a varactor diode attached (frequency shift coil). Because the frequency shift coil was mobile, the distance between the EPR resonator and the coil could be changed. Coarse control of the resonant frequency was achieved by changing this distance mechanically, while fine frequency control was implemented by changing the capacitance of the varactor electrically. In this condition, EPR measurements of a phantom (comprised of agar with a nitroxide radical and physiological saline solution) were made. To compare the presented method with a conventional method, the EPR measurements were also done by using a synthesizer at the same EPR frequency. In the conventional method, the noise level increased at high irradiation power. Because such an increase in the noise was not observed in the presented method, high sensitivity was obtained at high irradiation power.     

Twin LC Resonator System for EPR Measurements Operating at Radio-Frequency

Two LC resonant circuits resonating at the same frequency (245 MHz) faced each other with reverse polarity (twin electron paramagnetic resonance (EPR) resonator). For the inductor of the LC circuit, a square single-turn one-loop coil (width, 38 mm) was fabricated. Each LC circuit was independently tuned using mechanical variable capacitors. A cylindrical phantom (diameter, 25 mm) including a 1 mM nitroxide radical physiological saline solution was located at the center of two coils (distance between these coils, 50 mm). Two resonant frequencies (the lower and the higher ones) were observed at each LC circuit of the twin EPR resonator with termination of the other LC circuit. The lower resonant frequency alone was observed when the powers from two LC circuits of the twin EPR resonator were combined by a 180° combiner. On the other hand, the higher frequency alone was observed when they were combined by a 0° combiner. EPR signals could be obtained using the 180° combiner (lower frequency) but not the 0° combiner (higher frequency).     

Microwave frequency modulation in CW EPR at W-band using a loop-gap resonator

Loop-gap resonator (LGR) technology has been extended to W-band (94GHz). One output of a multiarm Q-band (35GHz) EPR bridge was translated to W-band for sample irradiation by mixing with 59 GHz; similarly, the EPR signal was translated back to Q-band for detection. A cavity resonant in the cylindrical TE011 mode suitable for use with 100 kHz field modulation has also been developed. Results using microwave frequency modulation (FM) at 50 kHz as an alternative to magnetic field modulation are described. FM was accomplished by modulating a varactor coupled to the 59 GHz oscillator. A spin-label study of sensitivity was performed under conditions of overmodulation and gamma2H1(2)T1T2<1. EPR spectra were obtained, both absorption and dispersion, by lock-in detection at the fundamental modulation frequency (50 kHz), and also at the second and third harmonics (100 and 150 kHz). Source noise was deleterious in first harmonic spectra, but was very low in second and third harmonic spectra. First harmonic microwave FM was transferred to microwave modulation at second and third harmonics by the spins, thus satisfying the "transfer of modulation" principle. The loaded Q-value of the LGR with sample was 90 (i.e., a bandwidth between 3 dB points of about 1 GHz), the resonator efficiency parameter was calculated to be 9.3 G at one W incident power, and the frequency deviation was 11.3 MHz p-p, which is equivalent to a field modulation amplitude of 4 G. W-band EPR using an LGR is a favorable configuration for microwave FM experiments.     

Automatic tuned MRI RF coil for multinuclear imaging of small animals at 3T

We have developed an MRI RF coil whose tuning can be adjusted automatically between 120 and 128 MHz for sequential spectroscopic imaging of hydrogen and fluorine nuclei at field strength 3 T. Variable capacitance (varactor) diodes were placed on each rung of an eight-leg low-pass birdcage coil to change the tuning frequency of the coil. The diode junction capacitance can be controlled by the amount of applied reverse bias voltage. Impedance matching was also done automatically by another pair of varactor diodes to obtain the maximum SNR at each frequency. The same bias voltage was applied to the tuning varactors on all rungs to avoid perturbations in the coil. A network analyzer was used to monitor matching and tuning of the coil. A Pentium PC controlled the analyzer through the GPIB bus. A code written in LABVIEW was used to communicate with the network analyzer and adjust the bias voltages of the varactors via D/A converters. Serially programmed D/A converter devices were used to apply the bias voltages to the varactors. Isolation amplifiers were used together with RF choke inductors to provide isolation between the RF coil and the DC bias lines. We acquired proton and fluorine images sequentially from a multicompartment phantom using the designed coil. Good matching and tuning were obtained at both resonance frequencies. The tuning and matching of the coil were changed from one resonance frequency to the other within 60 s.     

Negative corona discharge from a water droplet under the pulsating DC field

Corona discharge from a fine water droplet always involves deformation of the droplet shape or Taylor-cone formation, emission of fine water jets or disruption of droplet. Therefore, corona discharge from a water droplet always manifests complicated aspects. In addition, disruption of Taylor cone simultaneously affects not only discharge current but also motion of water droplet. To confirm corona discharge phenomena from a water droplet protruded from a tip of a metal capillary tube with a diameter of 1 mm, negative corona discharge was investigated by using a water droplet located at a tip of grounded rod electrode facing a ring electrode with positive dc voltage superimposed by ac one. Since the droplet has inherent resonant vibrating frequency defined by the size or volume, the volume of water droplet was adjusted at 20 nL where the corresponding resonant frequency was 500 Hz. The period of the event of successive corona discharge is exactly consistent with resonant frequency defined by the size of the water droplet. As a result, corona pulse trains with a definite duration appeared intermittently corresponding to its resonant vibration. When dc voltage superimposed by ac voltage with resonant frequency of 500 Hz was applied to the water droplet, corona pulse trains appeared at the period corresponding to the frequency. The maximum value of corona current reasonably increased with the applied voltage. Even when the frequency of ac field superimposed on dc field was varied from the resonant frequency, corona pulse trains occur corresponding to not only the superimposed field frequency but also resonant frequency.     

基于STM32F103RCT6无线充电电路的设计与研究

摘要：随着手机的普及、电动汽车和无线电技术的不断发展,电子产品也越来越丰富。对各种电池的充电速度和工作时间提出越来越高的要求。由于目前电池的电量工作时间较短、快速充电、远距离充电等问题不能解决,从而给人们的生活带来了极大的不便,特别限制了电动汽车工业的发展。所以针对电池的充电问题,本文设计了一款电磁谐振式无线充电器。该充电器采用STM32超低功耗单片机作为无线充电电路的主控制器,通过对发射信号频率的控制,使处于异地的输出电路工作在谐振状态,实现最大功率无线传送,接收端线圈输出再经过整流、稳压、滤波后输出+5V电压,最终经过智能控制电路给便携式设备中的锂电池充电。并通过指示灯显示电池是否正在充电、充满、电池反接和电源欠压等状态。该设计的充电设备具有使用灵活、方便、工作可靠,电能利用率高,无线充电距离远等特点,同时具有过压、过流保护,具有广泛的应用价值。     

Varactor Controlled Gunn Oscillator at W-Band

This paper describes the experimental circuit and measured performance of varactor tuned Gunn oscillator at W-band. The power output of 12.5 dBm has been achieved when packaged GaAs Gunn diode is used. Linear frequency excursion of 150 MHz with power variation of 1 dB has been observed when varactor was given reverse bias from 0 to 20 volts. GaAs hyperabrupt varactor is used in parallel to gunn diode at a distance of odd multiple of g/2 in waveguide channel.     

Electronically tunable surface-coil-type resonator for L-band EPR spectroscopy

The automatic frequency control (AFC) circuit in conventional electron paramagnetic resonance (EPR) spectrometers automatically tunes the microwave source to the resonance frequency of the resonator. The circuit works satisfactorily for samples stable enough that the geometric relations in the resonance structure do not change in a significant way. When EPR signals are measured during in vivo experiments with small rodents, however, the distance between the signal source and the surface-coil detector can change rapidly. When a conventional AFC circuit keeps the oscillator tuned to the resonator under those conditions, the resultant frequency change may exceed +/-5 MHz and markedly shift the position of the EPR signal. Such a shift results in unacceptable effects on the spectra, especially when the experimenter is dealing with narrow EPR lines. The animal movement also causes a mismatching of the resonator and the 50-ohm transmission line. Direct results of this mismatching are increased noise; shifts in the position of the baseline; and a high probability of overdriving the signal preamplifier with consequent loss of the EPR signal. We therefore designed, built, and tested a new surface-coil resonator using varactor diodes for tuning the resonance frequency to the fixed frequency oscillator and for capacitive matching of the resonator to the 50-ohm transmission line. The performance of the automatic matching system was tested in vivo by measuring EPR spectra of lithium phthalocyanine implanted in rats. Stability and sensitivity of the spectrometer were evaluated by measuring EPR spectra with and without the use of the automatic matching system. The overall experimental performance of the spectrometer was found to significantly improve during in vivo experiments using the automatic matching system. Excellent matching between the 50-ohm transmission line and the resonator was maintained under all experimental circumstances that were tested. This should allow us now to carry out experiments that previously were not possible.     

Tunable feedback loop for suppression of relaxation oscillations in a diode-pumped Nd: laser

An optoelectronic noise suppression circuit connected to the RF input of a commercial diode laser mount is presented. Adjustable phase shift and amplitude control enable efficient suppression of the relaxation oscillations of a diode laser-pumped Nd:YVO₄ laser at variable frequencies between a few hundred kHz and 1.8 MHz. At least 25 dB reduction of the relaxation oscillations at 500 kHz and 1.5 MHz noise peaks is demonstrated.     

An active resonator system for CW-ESR measurement operating at 700 MHz

An active resonator system operating at 700 MHz, which can attain a high Q for CW-ESR measurements of a high loss sample, was developed. This system consisted of a loop-gap resonator (LGR), a receiver coil, an excitation coil, and a phase tunable amplifier. A part of the RF power at the LGR was picked up by the receiver coil, amplified, and irradiated to the LGR again by the excitation coil, which made up a feedback circuit. Because the feedback circuit provided the energy that canceled the loss in the resonator, the Q of the active resonator system increased. When a sample tube (inner diameter, 20 mm; axial length, 31 mm) containing a nitroxide radical and physiological saline solution was placed in the resonator, the Q could be varied from 55 to 4000. It was possible to obtain a Q of the active resonator system with sample that was higher than that of the value of the LGR without a sample in a no-feedback condition. The ESR signal intensity increased up to 7 times with the increase in Q. The sensitivity increased up to 50%, which was a much smaller advance than that of the Q, because the noise level also increased with the increase in signal intensity.     

偏置电压对磁致伸缩/压电层合换能结构磁电性能影响

分析和测试了偏置电压调整时PZT5/Terfenol-D/PZT8层合换能结构磁电性能. 提出了一种磁致伸缩/压电层合磁电换能结构的一阶谐振频率控制方法. 通过改变压电驱动层的直流电压对磁电层合结构的预应变进行改变, 从而调整谐振频率. 分析偏置电压、 应变、 弹性模量、 谐振频率和谐振磁电电压系数之间关系. 分析表明: 在较小应变情况下, 控制电压几乎可以线性调节谐振频率, 而层合结构谐振磁电电压系数几乎与偏置电压无关. 实验研究验证: 理论与实验结果较好吻合. 在-170 V-+170 V的偏置电压时, 谐振频率可以几乎线性调整. 最大频率调整量达到1 kHz, 偏置电压对一阶纵振频率的控制率达到: 2.94 Hz/V. 在偏置磁场为0-225 Oe时, 谐振频率调整量与偏置磁场无关. 偏置磁场会改变谐振磁电电压系数, 在大于178 Oe静态磁场偏置时, 磁电电压系数最大, 达到1.65 V/Oe.     

Electroreflectance spectroscopy of compressively strained InGaN/GaN multi-quantum well structures

The built-in piezoelectric field induced by compressive stress in InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) was investigated using the electric field dependent electroreflectance (ER) spectroscopic method. InGaN/GaN MQW structures were prepared on sapphire substrates of different thicknesses. Thinning the sapphire substrate enables control of the compressive stress by changing the curvature of the wafer bowing. The wafer bowing-induced mechanical stress alters the piezoelectric field in the InGaN/GaN MQW. The flat band voltage, estimated by measuring the applied reverse bias voltage that induces a 180° phase shift in the ER spectra, was decreased from −11.21 V to −10.51 V by thinning the sapphire substrate thickness from 200 to 60 μm. To calculate the piezoelectric field (F_pz) from the compensation voltage, the depletion width was obtained from the capacitance–voltage measurement. The F_pz estimated from the energy shift in ER peak in a bias range from 0 to −12 V was changed by 110 kV/cm.     

High-speed atomic force microscopy with phase-detection

In order to improve the scanning speed of tapping mode AFM, we have studied the phase-detection mode AFM with a high frequency (1.5 MHz) cantilever. The phase shifts versus tip-sample distance with different types of samples including polymer, semiconductor, and graphite were measured and the interaction forces were analyzed. It was found that the phase shift in repulsive region is nearly linear as a function of distance, which can be used for feedback control in general, except that some blunt tips cause reversed polarity of phase shift due to excessive energy dissipation. High-speed image with scan rate of 100 Hz was obtained which were controlled with phase shift as a feedback signal.     

Simulation Investigation of Millimeter Wave Varactor-tuned Transmission Cavity-stabilized Oscillator

A new circuit configuration for millimeter wave varactor-tuned transmission cavity-stabilized oscillator has been proposed in this paper. Compared to conventional varactor-tuned reflection cavity-stabilized oscillator, in this configuration, a high quality factor transmission cavity directly coupled to varactor diode is employed to improve the performances of the oscillator. The operation frequency of this oscillator can be tuned by varying the resonant frequency of the transmission cavity through changing bias voltage of the varactor diode. An equivalent circuit model for the oscillator has been presented in order to theoretically investigate the performance characteristics of the oscillator. On the basis of this model, electrical tuning characteristics have been studied. Mode jumping phenomena during electrical tuning process have been analyzed for obtaining stable operations of the oscillator. The analytical formulae of quality factor and efficiency have been derived in terms of relevant circuit parameters. Particular emphasis has been paid on several circuit parameters which have a substantial impact on circuit performance. Some design considerations have been pointed out according to the simulation results, which are useful to the design and fabrication of this type of oscillators.     

Al/CdSe/GaSe/C resonant tunneling thin film transistors

An Al/CdSe/GaSe/C thin film transistor device was prepared by the physical vapor deposition technique at a vacuum pressure of 10⁻⁵ mbar. The x-ray diffraction measurements demonstrated the polycrystalline nature of the surface of the device. The dc current-voltage characteristics recorded for the Al/CdSe/C and Al/CdSe/GaSe/C channels displayed a resonant tunneling diode features during the forward and reverse voltage biasing, respectively. In addition, the switching current ratio of the Al/CdSe/C increased from 18.6 to 9.62×10³ as a result of the GaSe deposition on the CdSe surface. Moreover, the alternating electrical signal analyses in the frequency range of 1.0 MHz to 1.8 GHz, showed some remarkable properties of negative resistance and negative capacitance spectra of the Al/CdSe/GaSe/C thin film transistors. Two distinct resonance-antiresonance phenomena in the resistance spectra and one in the capacitance spectra were observed at 0.53, 1.04 and 1.40 GHz for the Al/CdSe/C channel, respectively. The respective resonating peak positions of the resistance spectra shift to 0.38 and 0.95 GHz when GaSe is interfaced with CdSe. These features of the thin film transistors are promising for use in high quality microwave filtering circuits and also for use as ultrafast switches.     

Low-voltage blue light emission from n-ZnO/p-GaN heterojunction formed by RF magnetron sputtering method

High quality n-ZnO/p-GaN heterojunction was fabricated by growing highly crystalline ZnO epitaxial films on commercial p-type GaN substrates via radio frequency (RF) magnetron sputtering. Low-voltage blue light emitting diode with a turn-on voltage of ∼2.5 V from the n-ZnO/p-GaN heterojunction was demonstrated. The diode gives a bright blue light emission located at ∼460 nm and a low threshold voltage of 2.7 V for emission. Based on the results of the photoluminescence (PL) and electroluminescence (EL) spectra, the origins of the EL emissions were studied in the light of energy band diagrams of ZnO–GaN heterojunction, and may attribute to the radiative recombination of the holes in p-GaN and the electrons injected from n-ZnO, which almost happened on the side of p-GaN layer. These results may have important implications for developing short wavelength optoelectronic devices.     

Coupling of waveguide and resonator by inductive and capacitive irises for EPR spectroscopy

An analytic circuit model for slot coupling from a waveguide to a loop-gap resonator (LGR) in a context of electron paramagnetic resonance (EPR) spectroscopy is presented. The physical dimensions of the waveguide, iris, LGR, and aqueous sample are transformed into circuit values of inductance, capacitance, and resistance. These values are used in a solution of circuit equations that results in a prediction of the radio frequency (rf) currents, magnitude and phase, frequency, and magnetic and electric stored energies near the critical coupling. The circuit geometry reflects magnetic flux conservation between the iris and LGR as well as modification of the outer loop LGR currents by the iris. Unlike conventional models, coupling is not explicitly based on a mutual inductance between the iris and LGR. Instead, the conducting wall high-frequency rf boundary condition is used to define surface currents, regions, and circuit topology with lumped-circuit values of self-inductance, capacitance, and resistance. Match is produced by a combination of self-inductive and capacitive circuit coupling. Two conditions must be met to achieve match. First, the equivalent resistance of the LGR as seen by the iris must be transformed into the waveguide characteristic impedance. This transformation is met at a particular frequency relative to the natural LGR resonance frequency. The frequency shift magnitude is largely determined by the LGR properties, weakly dependent on the iris length and placement, and independent of other iris dimensions. The second condition for match is that the iris reactance at this frequency shift must cancel the residual reactance of the LGR. This second condition is sensitive to the iris dimensions. If both conditions are not simultaneously satisfied, overcoupling or undercoupling results. A slotted iris with a length equal to the size of the large dimension of the waveguide is found to have many properties opposite to a conventional iris with shorter length. Notably, the magnetic field near the iris tends to reinforce rather than oppose the magnetic field in the resonator. The long iris improves the LGR EPR performance by providing increased rf magnetic field homogeneity at the sample, higher signal, and reduced total frequency shift since the shifts due to sample and iris tend to cancel. Investigations reveal that the first match condition can be adjusted by LGR dimensional changes and such adjustment can eliminate the frequency shift. Results are consistent with Ansoft High Frequency Structure Simulator (Version 10.1, Ansoft Corporation, Pittsburgh, PA) simulations and can be extended to cavity resonators.     

Combining a Magnetic Field Modulation Coil with a Surface-Coil-Type EPR Resonator

It is thought that the design of magnetic field modulation coils is one of the factors limiting enlargement of the sample size in electron paramagnetic resonance (EPR) measurements. In this study, we miniaturized the magnetic field modulation coil and combined it with a surface-coil-type resonator (SCR). The inductor of the SCR was a circular single-turn one-loop coil (diameter, 1 mm), and the magnetic field modulation coil was a twin-loop coil consisting of two solenoid coils each made of 15 turns of copper wire on a cylindrical bobbin with an axial length of 3 mm and an elliptical cross section (major axis, 7 mm; minor axis, 3 mm). The former was located on the latter via a spacer (thickness, 3 mm) in such a way that the directions of their axes coincided. Their combined size was about 10 mm wide, 10 mm deep, and 6 mm high. The transmission lines of the SCR were set on resonance at about 700 MHz. EPR measurements of a phantom (comprising agar that included a nitroxide radical and physiological saline solution), made with a miniaturized modulation coil combined with the SCR, exhibited a sensitivity similar to that for the conventional method. Authors' address: Hidekatsu Yokoyama, Department of Pharmaceutical Sciences, International University of Health and Welfare, 2600-1 Kitakanemaru, Ohtawara 324-8501, Japan     

Study of broad bandwidth vibrational energy harvesting system with optimum thickness of PET substrate

In this paper, we present the development of a flexible PET-based (polyethylene terephthalate; PET) vibrational energy harvesting system with broad bandwidth. This broad bandwidth harvesting system comprises of four units of individual ZnO (zinc oxide) piezoelectric harvester in the form of a cantilever structure connected in parallel, and rectifying circuit with storage module. This system has ability to convert mechanical energy into electrical energy from the varying ambient vibration. The design and simulation of a piezoelectric cantilever plate was described by using commercial software ANSYS FEA (Finite Element Analysis) to determine the optimum thickness of PET substrate, internal stress distribution, operation frequency and electric potential. With the optimum thickness predicted by developed accurate analytical formula analysis, the one-way mechanical strain that is efficient to enhance the induced electric potential can be controlled within the piezoelectric ZnO layer. In addition, the relationship among the model solution of piezoelectric cantilever plate equation, vibration-induced electric potential and electric power was realized. An individual piezoelectric harvester consists of flexible PET substrate, piezoelectric ZnO thin film with (002) c-axis preferred orientation, and selectively deposited UV-curable resin lump structure which is used to change the resonant frequency of the harvester. In combination with multi-harvesters and rectifying with storage module together, an energy harvesting system with broad bandwidth can be fabricated. One individual harvester achieves a maximum OCV (open-circuit voltage) up to 4 V with power density of 1.247 μW/cm³. So far, we succeeded in accomplishing a broad bandwidth system with operating frequency range within 100 Hz-450 Hz to enhance powering efficiency. When the DC voltage (direct current voltage) across a storage module is charged up to 1.55 V after rectification, a flash LED (light emitting diode) is driven.     

Efficiency and stability enhancement of a virtual cathode oscillator

A virtual cathode oscillator(VCO) with a resonant cavity is presented and investigated numerically and theoretically,and its efficiency and stability are enhanced. An equivalent circuit method is introduced to analyze the resonant cavity composed of anode foil and feedback annulus, and a theoretical expression for the fundamental mode frequency of the resonant cavity is given. The VCO is investigated in detail with a particle-in-cell method. We obtain the microwave frequencies from simulation, theoretical expression, and relative references, and draw three important conclusions. First, the microwave frequency is a constant when the diode voltage is changed from 588 kV to 717 kV. Second, the fluctuation of the microwave frequency is very small when the AK gap is changed from 1.2 cm to 1.6 cm. Third, the microwave frequency agrees with the theoretical result. The relative error, which is calculated according to the theoretical and simulation frequencies, is only1.7%.