Design of a tunable frequency selective surface absorber as a loaded receiving antenna array

An effective approach to designing a tunable electromagnetic absorber is presented and experimentally verified; it is based on an idea that an existing frequency selective surface （FSS） absorber is regarded as a loaded receiving antenna array. The existing absorber is effectively simplified by withdrawing half of the loaded resistors; a more compact one is obtained when lumped capacitors are introduced. Building on this, a varactor-tunable absorber with a proper bias network is proposed. Numerical simulations of one tunable absorber with 1.6 mm in thickness show that a wide tuning range from 3.05 GHz to 1.96 GHz is achieved by changing the capacitance of the loaded varactor from 0.5 pF to 5.0 pF. An experiment is carried out using a rectangular waveguide measurement setup and excellent agreement between the simulated and measured results is demonstrated.     

Integrated ferroelectric millimeter-wave phase shifters with a periodic structure

Integrated phase shifters with a periodic structure that includes BaSrTiO₃ ferroelectric varactors parallel-and series-connected to the transmission line are investigated theoretically and experimentally. The phase shifters are designed for the frequency range 26–32 GHz. The dispersion characteristics and insertion losses in the transmission line of the phase shifters are analyzed with regard to the dependence of the capacitance and loss tangent of the varactors on control voltage and temperature. It is shown that parasitic amplitude modulation in the phase shifters can be suppressed if the connection scheme of the varactors takes into account the voltage dependence of the their loss tangent. In the phase shifters with series-connected varactors, the temperature dependence of the phase shift is much weaker than in those with parallel-connected varactors.     

超导量子器件片上皮法电容的制备及测量

片上集成电容是超导量子芯片上的核心器件，其数值一般在百飞法(fF)至皮法(pF)范围.采用常规微纳加工技术在蓝宝石基片上制备了铌-氧化硅-铝(Nb/SiO₂/Al)平行板电容.利用刻蚀工艺制备了平行板电容器的下极板Nb,利用剥离工艺制备平行板电容器的上极板Al和介电层SiO₂.室温下利用锁相放大原理和桥式电路原理测定电容大小，两种方法测定电容值基本一致，表明锁相放大原理测试pF级电容的可靠性.利用该电容与铝基约瑟夫森结组成谐振器，制备了中心频率位于4.35 GHz的约瑟夫森参量放大器.在稀释制冷机中10 mK温度下测定直流偏置谐振器的磁通-频率相位图，拟合数据获得的电容值与室温测定电容值接近，表明在mK、GHz条件下工作的片上集成电容可在室温、kHz条件下测定其数值大小.     

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.     

A Gunn-Diode Active Leaky-Wave Frequency Scanning Antenna

A frequency tunable active leaky-wave scanning antenna using Gunn-diode voltage control oscillator (VCO) as source is developed. The frequency tuning controlled by changing either the varactor diode dc bias or the Gunn diode dc bias is demonstrated. The measured scanning angle of active antenna is close to 15 degree as the Gunn VCO frequency tuned from 12.58GHz to 12.98GHz. To excite the first higher order mode of the microstrip leaky-wave antenna is fed asymmetrically. The dominant mode excitation has been successfully suppressed by adding a sequence of covered wire in the middle line of the microstrip leaky wave antenna. This is a prototype of frequency scanning antenna using two terminal device, which can be easily scaled up to millimeter wave frequency region.     

基于减反膜外腔反馈半导体激光器拉曼光的产生

通过直接对减反膜外腔反馈半导体激光器进行电流调制的方法,得到了两束位相锁定且频率差在6.0-9.3GHz范围内连续可调的激光,其中6.835和9.192GHz分别对应Rb⁸⁷和Cs¹³³原子基态超精细能级之间的频率差,激光功率分别可以达到6.87mW和5.09mW. 根据减反膜外腔反馈半导体激光器的特点,通过调整外腔腔长、激光器工作温度、电流以及所加射频调制信号的功率和频率,在调制频率小于等于4.0GHz时可以将载波完全压制. 调制频率大于4.0GHz时,虽不能将载波完全压制,但由于外腔与调制频率共振时对调制的增强也得到了调制深度很高的激光,并对其中的物理机理作了分析. 通过后续滤波等方法处理以后,该拉曼光源可以广泛应用到原子的量子操控中. 关键词： 受激拉曼光 高频调制 调制增强     

Frequency-tunable transmon in a three-dimensional copper cavity

We have realized a frequency-tunable transmon in a three-dimensional cooper cavity using a direct current superconducting quantum interference device. Both the transition frequency of the transmon and the frequency of the dressed cavity can be varied with the applied external flux bias, which are well consistent with the theoretical model. The range of the variable transition frequency is from 5.188 GHz to 7.756 GHz. The energy relaxation time of the transmon is hundreds of nanoseconds.     

可调频高功率宽谱微波的产生和辐射

设计了一种可调谐频率的高功率宽谱微波辐射装置,装置由可调谐长度的1/4波长低阻同轴谐振器、环形开关、电容耦合器和宽谱辐射天线组成,中心频率调谐为200～400 MHz。低阻传输线与环形开关构成1/4波长短路谐振器,它产生的宽谱微波振荡通过耦合器耦合到宽谱辐射天线上辐射,而耦合器由集中电容与分布电感组成,实现宽谱微波在频率调谐范围内以较为一致的耦合度提取微波能量。通过转动螺杆滑动安装在同轴谐振器内芯上的环形开关,达到改变谐振频率的目的。最后,将可调频宽谱辐射装置与输出电压为500 kV的Tesla变压器脉冲功率源联试,得到200～400 MHz宽谱微波辐射,辐射因子为95～130 kV,频谱百分比带宽为10%～30%。     

Highly efficient magnetically tunable high frequency spur-line notch filter

We report a cobalt ferrite nanorods (CFO NRs) based magnetically tunable spur-line notch filter where vertically aligned CFO NRs has been grown on silver nanoparticles coated silicon substrate. The CFO NRs are coupled with high frequency spur-line bandstop filter in flip-chip configuration and the device showed excellent tunable microwave properties in the presence of a low bias magnetic field. The center frequency of the tunable filter is ~16.4 GHz which is shifted to ~14.9 GHz with ~8.7% tunability by applying bias magnetic field ~320 Oe. The magnetic field tuning of the center frequency is explained on the basis of the change in permeability value of the CFO NRs with bias magnetic field as the NRs are used in the partially magnetized state. For validation, permeability value is also calculated by using numerical equations. The experimental reflection of the device has been supported with a simulation using CST microwave studio software.     

一种高选择性的可调0.83~2.15 GHz带通滤波器

设计了一种宽频率调节范围的高选择性可调带通滤波器。宽调节范围由一对简单的加载变容管的平行耦合线谐振器设计实现。由于谐振器之间存在电磁混合耦合以及通过加载变容管引入的源与负载频变耦合,该滤波器最终引入了三个自适应性传输零点。而且,三个传输零点在整个调节范围内能够保持相同的相对位置,因此,设计的滤波器实现了在整个宽频率调节范围内的高选择性和良好的带外抑制。同时,在中心频率调节的过程中,通过选择合适的耦合系数,该滤波器可以实现恒定的相对带宽。最终设计出的可调滤波器的调节范围为0.83~2.15 GHz, 可调范围可达88.6%,并保持9%±0.3%的相对带宽不变。测试结果表明该滤波器具备了高选择性和良好的带外抑制能力。     

Experimental generation of frequency-tunable entangled optical beams with continuous variables

Frequency tunable continuous variable (CV) entangled optical beams are experimentally demonstrated from a non-degenerate optical parametric oscillator working above the threshold.The measured correlation variances of amplitude and phase quadratures are 3.2 and 1.5 dB,respectively,below the corresponding shot noise level (SNL) in the tuning range of 580 GHz (2.25 nm).The frequency tuning is realized by simply controlling the temperature of the nonlinear crystal.     

基于有源超表面的导电板雷达截面增强捷变设计

提出了一种使用散射方向图的动态可重构以实现雷达散射截面增强的捷变设计方法。结合变容二极管加载,使用具有嵌入式偏置回路的物理单元,所提出的有源超表面可以在具有梯度电压的外加直流偏置下,对平面电磁波的正入射或斜入射产生可电调的反射系数相位分布,以达到对反射波角度的灵活重定向,进而有助于单站或双站雷达散射截面增强的捷变效果。以导电平板为例,对三种不同的入射反射场景进行了计算与全波仿真,在设计频率10 GHz处,所提出的设计产生了可重构的散射方向图,表明了该设计对反射波角度的实时控制,并结合实验测量验证了单站与双站雷达散射截面的有效增强结果。     

铁电薄膜可调带通滤波器

铁电薄膜的介电常数随外加电场强度的增加而减小.依据铁电薄膜的这一特性,提出了一种新颖的基于共面传输线结构的铁电薄膜可调带通滤波器.为了减小传输损耗,滤波器的导体部分由超导薄膜构成.滤波器的输入输出采用抽头线的方式分别与谐振器相接,外加电压通过输入输出端口直接施加到共面谐振器缝隙处的铁电薄膜上,用以改变铁电薄膜的介电常数,从而改变谐振器的谐振频率,实现带通滤波器通带频率的移动.这种新型可调带通滤波器具有结构紧凑、尺寸小及施加外加偏压容易等优点.仿真结果表明:铁电薄膜的介电常数在外加偏压下从250减小到150时,带通滤波器的传输特性曲线的形状基本保持不变,通带的中心频率从10.283GHz增加到10.518GHz,其3dB带宽保持在0.150GHz左右,反射损耗始终小于-17dB.     

Excess capacitance of ZnO-Au varactors

Doped ZnO single crystals were deposited with gold and indium in 1×10^–8 Torr vacuum. The lithium-doped ZnO single crystals and the gold interface revealed not only a Schottky diode but also varactor characteristics. TheI-V andC-V characteristics of ZnO:Li-Au devices were determined in the 0–140 mV and 0–1.5 V ranges.The frequency dependence of ZnO:Li-Au varactors was investigated in the 6–550 kHz range and the value of the most efficient varactor frequency was found to be 50 kHz for the lithium-doped samples prepared.To bring further insight into the matter the concept of excess capacitance was introduced and 1/C ²=f(–V) curves were rearranged between 0–150 mV where Schottky characteristics are non-linear. The excess capacitance values of lithium-doped varactors were determined at four different frequencies and ranged from 26 pF at 50 kHz to 70 pF at 6kHz.Finally, the bulk donor concentrations of the single crystals were calculated from the modifiedC-V curves to beN _D= 3×10²⁰ m^–3. On the other hand, the bulk donor concentration determined from the non-modifiedC-V curves wasN _D=1.02×10²² m^–3.     

Application of nanodimensional barium-strontium titanate films in tunable microwave devices

In this work, we pioneer application of nanodimensional films (≈20 nm thick) in electronically tunable microstrip microwave devices. It is shown experimentally that, when the control bias voltage varies in the range 0–15 V, the intrinsic Q factor of the microstrip resonator is higher than 100 and remains almost constant when the resonant frequency 2.6 GHz is varied within 0.3 GHz.     

A novel polarization converter based on the band-stop frequency selective surface

A dual-passband single-polarized converter based on the band-stop frequency selective surface(FSS)with a low radar cross-section(RCS)is designed in this article.The unit cell of the proposed converter is formed by a polarization layer attached to the band-stop frequency selective surface.The simulation results reveal that the co-polarization reflection coefficients below-10 d B are achieved in 3.82–13.64 GHz with a 112.4%fractional bandwidth(the ratio of the signal bandwidth to the central frequency).Meanwhile,a polarization conversion band is realized from 8.14 GHz to 9.27 GHz with a polarization conversion ratio which is over 80%.Moreover,the 1 d B transmission window is obtained in two nonadjacent bands of 3.42–7.02 GHz and 10.04–13.91 GHz corresponding to the relative bandwidths of 68.9%and 32.3%,respectively.Furthermore,the radar cross-section of the designed structure can be reduced in the wideband from 2.28 GHz to 14 GHz,and the 10 d B RCS reduction in the range of 4.10–13.35 GHz is achieved.In addition,the equivalent circuit model of this converter is established,and the simulation results of the Advanced Design System(ADS)match well with those of CST Microwave Studio(CST).The archetype of the designed converter is manufactured and measured.The experiment results match the simulation results well,which proves the reliability of the simulation results.     

Brillouin-assisted microwave frequency measurement with adjustable measurement range and resolution

We present a reconfigurable microwave frequency measurement technique with adjustable measurement range and resolution. The key novelty of the technique is the employment of stimulated Brillouin scattering, which results in a tunable amplitude comparison function, leading to an adjustable measurement range and resolution. The proposed technique is switchable between a wideband tunable narrow measurement range (～2 GHz) with high resolution (±0.05 GHz) and a fixed wide measurement range (12 GHz) with moderate resolution (±0.25 GHz).     

Temperature-dependent capacitance–voltage biasing of the highly tunable TlGaTe2 crystals

The temperature effects on the capacitance–voltage characteristics as well as the room temperature capacitance–frequency characteristics of TlGaTe₂ crystals are investigated. A very wide range of linearly varying tunable capacitance from 6.0 μF to 60 pF was recorded. The capacitance–voltage characteristics, being recorded in the temperature range of 290–380 K, revealed a linear increase in the build in voltage associated with exponential decrease in the density of non-compensated ionized carriers with increasing temperature. The high temperature (up to 380 K) biasing ability, the linear tunability and the high dielectric constant values (∼10³) make the TlGaTe₂ crystals applicable in microelectronic components.     

The Study on the Performance of the Wavelength Conversion Based on a Tunable External Cavity Semiconductor Laser

In this paper, the performance of the wavelength conversion based on a tunable external cavity semiconductor laser (ECL) was studied. The static wavelength conversion was achieved in the tunable wavelength range of the ECL. The dynamic wavelength conversions of 155 Mb/s and 622 Mb/s nonreturn to zero (NRZ) signals were realized. The simulation of the ECL output characteristics using modified compound cavity rate equation shows that the higher bit rate wavelength conversion can be realized by increasing the relaxation oscillation frequency of the ECL. When the modulated bit rate is much lower than the relaxation oscillation frequency, under a fixed input signal power, there is an optimal bias current for the wavelength converter based on ECL.     

The Study on the Performance of the Wavelength Conversion Based on a Tunable External Cavity Semiconductor Laser

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