一种用于高场MRI的多源射频发射机

介绍了高场磁共振成像（MRI）多源发射技术的原理,提出了一种用于高场MRI系统的多源射频信号发射机.它能并行输出多路频率、相位、幅度,可快速独立调节的射频脉冲信号.该射频发射机的实现基于单片现场可编程门阵列（FPGA）和多通道数模转换器（DAC）芯片,FPGA读取预存于双端口随机存取存储器（RAM）中的射频信号参数,并利用读取的参数分别实现每路信号的直接数字频率合成（DDS）和信号调制等核心功能,获得多路数字射频信号;FPGA输出的数字信号经过高性能DAC转化为模拟信号,即所需要的射频信号.该射频发射机在设计中大量采用软件无线电技术,即利用Xilinx提供的IP核实现DDS和信号调制等主要功能,具有集成度高、体积小、灵活度高的优点,同时,该设计可以大大缩短开发时间,有效降低实现的难度和成本,为高场MRI谱仪的多源射频发射机的设计研制提供了一种低成本、高效、高性价比的方案.     

基于中频相位锁定技术的宽带调谐光电振荡器稳定性研究

提出了一种基于中频相位锁定技术提升宽带调谐光电振荡器长期稳定性的方法.该方法通过外部可调本振源对光电振荡器射频信号进行下变频操作,产生中频信号,该中频信号与光电振荡器的起振频率无关.将该中频信号锁相在稳定的参考源上,能够提升宽带调谐光电振荡器的长期稳定性,同时分析了锁相环对光电振荡器相位噪声性能的影响.实验结果显示,光电振荡器的频率调谐范围可达5~15GHz,10kHz频偏处的相位噪声为-121.2dBc/Hz,频率稳定性达到6.9×10-11/103 s,表明该方法能够显著提升宽带调谐光电振荡器的稳定性.     

基于FPGA的核磁共振数字化发射机

提出了一种基于FPGA的核磁共振数字化正交发射机的设计技术.利用FPGA内嵌块RAM以及丰富的逻辑资源,并基于ARM7等嵌入式通讯系统完成发射机的高速调制,可以灵活地实现脉冲的频率、相位以及幅度等的调制,精确地控制脉冲的间隔时间.同时FPGA对两片DDS芯片的同步控制还可以实现正交射频信号的发射.     

基于电吸收调制激光器的双功能微波光子系统

提出了一种基于电吸收调制激光器的双功能系统,可以同时实现微波信号的产生和其相位噪声的测量。该系统由基于电吸收调制激光器的光电振荡器模块和基于光延时线技术的相位噪声测量模块构成。通过使用单个电吸收调制激光器代替激光源和强度调制器,所提出的双功能系统不仅成本低廉、结构简单,而且性能表现优异;有利于在基于OEO的射频系统,特别是信号产生系统的研制、优化与工作过程中,及时评估信号源的质量并作出相应的参数调整以优化其性能,为光电振荡器的相位噪声测试提供了简单的解决方案。实验结果表明,由光电振荡器生成的9.952 GHz信号的边模抑制比为66dB,相位噪声为-116.53dBc/Hz@10kHz。此外,相位噪声测量系统的相位噪声基底达-133.71 dBc/Hz@10 kHz,其测量灵敏度优于商用信号分析仪R&S FSV40。     

基于级联MZMs倍频系数可调的相位编码信号光产生方法

提出了一种基于级联马赫-曾德尔调制器(MZMs)倍频系数可调的相位编码信号光产生方法。该方法的系统核心器件为级联MZMs,通过合理选择两个MZMs的直流偏置电压和调制指数等参数,可任意选取±k(k=1,2,…,5)阶边带中的一组边带。通过可编程光滤波器滤除5阶以上的高阶边带,再结合相位调制,可产生载波2、4、6、8和10倍频的相位编码信号。该方法是通过改变施加在级联MZMs上的射频驱动信号幅度实现倍频系数调节,方案简洁灵活。通过仿真实验验证了利用4GHz的微波信号源分别产生载频32GHz和40GHz的相位编码信号,从而实现了8和10倍频信号输出,且所生成的相位编码信号具有很好的脉冲压缩性能。     

双中频超外差式信号补偿闭环反馈系统的设计

为补偿搜索雷达发射机探测信号或系统的不稳定性,提出了一种雷达发射机的双通道信号补偿闭环反馈系统;该反馈系统采用双中频超外差式结构,将发射机的高频发射信号两次下变频,得到的中频信号反馈到雷达发射机前端,与发射机一起构成闭环控制系统;中频信号对发射机输入端的中频信号实时检测,以补偿发射信号的幅度和相位不稳定性,从而提高了搜索雷达对探测目标的高精度测量;系统应用结果证明,该系统方案稳定可靠,具有一定的实用价值和参考意义。     

采用级联相位调制器和强度调制器产生四倍频光载毫米波的光纤无线通信系统

理论和实验研究了基于载波抑制原理采用级联的相位调制器和强度调制器产生四倍频光载毫米波的原理。基于此原理,在中心站利用10 GHz的本振射频信号,通过调节两个调制器的相位差和强度调制器的偏置电压产生40 GHz光载毫米波信号,2.5 Gb/s的基带数据信号直接调制在光载毫米波上,经过20 km标准单模光纤传输至基站。实验研究表明,下行链路信号通过20 km标准单模光纤传输后,传输功率代价小于0.8 dBm,基带信号眼图依然清晰张开,简化了系统配置,信源稳定可靠。     

基于锁模光学频率梳的高速数据传输

大数据时代网络数据流量的爆炸式增长给通信系统的容量和数据传输速率带来极大的挑战.本文基于锁模光学频率梳的宽光谱范围和高相位相干性提出了一种高频正交幅度调制信号生成方法,通过电光调制器对光学频率梳进行幅度相位整形并下变频至射频域,生成携带编码信息的高速、高阶、低相位噪声的调制信号,再结合锁模光学频率梳窄线宽、多波长的特性,仅使用单个激光器即可实现基于波分复用技术的大规模并行高速通信.仿真验证了该方案的可行性,随后在100 m的自由空间光链路中使用光子微波信号进行16元正交幅度调制通信实验,实现了误码率低于10^-6的14 Gbit/s数据传输.     

面向光载无线系统的混合频相调制信号产生

为提高光载无线系统传输容量,提出一种混合频相调制信号光学产生技术。利用保偏光纤布拉格光栅对不同输入偏振态响应不同的特性,将加载到信号偏振态的基带信息转换到输出波长上,并通过在接收端的外插拍频实现频率调制。同时通过光学相位调制将信息映射到拍频产生的微波信号上,最终产生混合频相调制信号。搭建基带传输式光载无线系统,验证了该信号产生方法的有效性。产生了码率为1Gbit/s、频率为12.5GHz/9.5GHz的混合频相调制信号,该信号经过5km光纤传输后由接收端解调。信号的时域波形及解调后信号眼图与星座图等结果证明了所产生的混合频相调制信号的可靠性。     

基于级联马赫-曾德尔调制器的载频连续可调相位编码波形光产生

提出一种基于级联马赫-曾德尔调制器(MZM)的载频连续可调相位编码波形光产生方法。该方法通过设置级联MZM的直流偏置点和调制指数,实现对±4阶光边带的选取,通过控制施加在级联MZM上的射频驱动信号相位,可在非平衡时间脉冲整形(UTPS)系统的输出端产生倍频系数为16的相位编码信号。通过调整UTPS系统中两个线性调频布拉格光栅的色散值,可实现对相位编码信号载频的连续调谐。仿真中利用2GHz的射频信号分别生成了载频为32GHz和19.2GHz的相位编码信号。仿真验证了所提方法的有效性,也验证了生成的信号具有较好的压缩性。     

基于数字调制技术的核磁共振射频脉冲发生器

在核磁共振（NMR）领域,射频脉冲信号的质量、形状对NMR性能及应用有着重要影响.本文基于现场可编程门阵列（FPGA）和直接数字频率合成（DDS）芯片AD9910设计了一种硬件结构更为简单的NMR射频脉冲发生器,实现了射频脉冲各项参数的数字化调制.其频率、相位、振幅的控制精度分别达到了32位、16位和14位,脉冲调制的时间精度为0.01 μs,可灵活生成持续时间不小于0.1 μs、载波频率不高于400 MHz的各类软脉冲和硬脉冲.同时,针对脉冲序列的特点建立了"脉冲+延时"的基础模型,提出了一种通用性更强的列表式脉冲序列控制方案,精简了对上级控制单元的控制需求.此外,对射频脉冲信号的频谱特性进行了理论分析,并采用Hanning窗对软脉冲的包络波形进行了优化处理,仿真和实验结果表明,Hanning窗可以有效抑制软脉冲的频谱泄漏问题.     

Photonic vector signal generation employing a novel optical direct-detection in-phase/quadrature-phase upconversion

This work demonstrates the feasibility of the generation of an RF direct-detection vector signal using optical in-phase/quadrature-phase (I/Q) upconversion. The advantage of the proposed transmitter is that no electrical mixer is needed to generate the RF signal. Therefore, I/Q data of RF signals are processed at baseband at the transmitter, which is independent of the carrier frequency of the generated RF signal. A 10 Gb/s 16 quadrature amplitude modulation signal is experimentally demonstrated. Following transmission over a 50 km single-mode fiber, the power penalty is negligible. Moreover, I/Q imbalance of the proposed transmitter is studied and compensated by digital signal processing, which is both numerically and experimentally verified.     

An all-optical phase detector by amplitude modulation of the local field in a Rydberg atom-based mixer

Recently, a Rydberg atom-based mixer was developed to measure the phase of a radio frequency (RF) field. The phase of the signal RF (SIG RF) field is down-converted directly to the phase of a beat signal created by the presence of a local RF (LO RF) field. In this study, we propose that the Rydberg atom-based mixer can be converted to an all-optical phase detector by amplitude modulation (AM) of the LO RF field; that is, the phase of the SIG RF field is related to both the amplitude and phase of the beat signal. When the AM frequency of the LO RF field is the same as the frequency of the beat signal, the beat signal will further interfere with the AM of the LO RF field inside the atom, and then the amplitude of the beat signal is related to the phase of the SIG RF field. The amplitude of the beat signal and the phase of the SIG RF field show a linear relationship within the range of 0 to π/2 when the phase of the AM is set with a difference π/4 from the phase of the LO RF field. The minimum phase resolution can be as small as 0.6° by optimizing the experimental conditions according to a simple theoretical model. This study will expand and contribute to the development of RF measurement devices based on Rydberg atoms.     

Design and evaluation of a downconverter based on MicroTCA.4

Modern low-level RF(LLRF) control systems of particle accelerators are designed to achieve extremely precise field amplitude and phase regulation inside the accelerating cavities. The RF field signal is usually converted to an intermediate frequency(IF) before being sampled by ADC. As the down-conversion is an important procedure of digital signal processing in LLRF system, designing a high performance and broad band downconverter compatible with various accelerators is important. In this paper, the design of a downconverter based on Micro TCA and its performance evaluation on different frequency points are presented. The major design objective of this module is a wider operating frequency range and more flexibility in application.     

新型四倍频光生毫米波矢量信号调制技术

提出一种基于双并联马赫-曾德尔调制器(MZM)的新型四倍频光生毫米波技术,并用于矢量信号调制。传统的四倍频调制技术,由于数据信号同时调制到+2,-2阶边带上,拍频检测后两个边带上数据信号会产生相位叠加,只适用于不归零码(NRZ)等强度调制格式。提出的矢量信号调制技术将数据信号调制在一个-1阶边带上,另一个+3阶边带不携带数据,在拍频检测后幅度和相位信息被正确保留。同时,四倍频模块降低了传输过程中对电和光器件的带宽需求。理论分析和仿真结果表明,通过此方法产生的携带在60GHz载波上的6.25×108 symbol/s的四相相移键控(QPSK)信号,经过20km单模光纤传输后,误差向量幅度(EVM)损耗可以忽略。     

基于幅度相位联合校正算法的8-QAM自由空间相干光通信

提出了基于幅度相位联合校正算法的8阶正交幅度调制(8-QAM)自由空间相干光通信方案。与传统的自由空间光通信方案相比,本文提出的方案不仅无需信道状态信息(CSI)就可以实现对原信号的高精度还原,同时还具有较高的光谱效率。仿真实验表明,当接收端与发射端的频偏f 0=20 MHz、激光器联合线宽Δf=10 kHz、信噪比为20 dB时,该方案在归一化大气闪烁标准差σ=0.25、归一化相位噪声方差σΦ^2=0.07的对数正态湍流信道条件下,其误码率(BER)与未采用联合校正算法的8-QAM系统相比低4个量级。     

FPGA-based amplitude and phase detection in DLLRF

The new generation particle accelerator requires a highly stable radio frequency (RF) system. The stability of the RF system is realized by the Low Level RF (LLRF) subsystem which controls the amplitude and phase of the RF signal. The detection of the RF signal's amplitude and phase is fundamental to LLRF controls. High-speed ADC (Analog to Digital Converter), DAC (Digital to Analog Converter) and FPGA (Field Programmable Gate Array) play very important roles in digital LLRF control systems. This paper describes the implementation of real-time amplitude and phase detection based of the FPGA with an analysis of the main factors that affect the detection accuracy such as jitter, algorithm's defects and non-linearity of devices, which is helpful for future work on high precision detection and control.     

FPGA-based amplitude and phase detection in DLLRF

The new generation particle accelerator requires a highly stable radio frequency（RF） system. The stability of the RF system is realized by the Low Level RF（LLRF） subsystem which controls the amplitude and phase of the RF signal. The detection of the RF signal＇s amplitude and phase is fundamental to LLRF controls. High-speed ADC（Analog to Digital Converter） ,DAC（Digital to Analog Converter） and FPGA（Field Programmable Gate Array） play very important roles in digital LLRF control systems. This paper describes the implementation of real-time amplitude and phase detection based of the FPGA with an analysis of the main factors that affect the detection accuracy such as jitter,algorithm＇s defects and non-linearity of devices,which is helpful for future work on high precision detection and control.     

Carrier generation and IF signal up-conversion using optical injection locking and stimulated Brillouin scattering

We theoretically investigate the intermediate frequency (IF) signal up-conversion with dual modes modulation (DMM) and single-mode modulation (SMM) of two single sideband (SSB) modulation modes for radio-over-fiber (RoF) systems. The simulation results indicate that the SMM can overcome the dispersion-induced power penalties of the generated carrier and up-converted IF signals. Then, we experimentally demonstrate an approach for RF carrier generation with SSB modulation and IF signal up-conversion with SMM using a combination of direct modulation in an injection-locked distributed feedback (DFB) laser and stimulated Brillouin scattering (SBS). The experimental results are compared with the predictions of the numerical simulation in SMM condition, and both are in good agreement with each other. The advantages and limits of the proposed method are discussed.