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设计了一种由单片机PIC18LF4520控制DDS芯片AD9911的频率源电路。阐述了单片机控制DDS的软硬件实现方法,以及AD9911内部寄存器的配置要点。系统设计外围电路简单,可方便地实现对频率源电路输出频率、相位和工作模式的控制,输出信号频率范围为25~75 MHz。实验结果表明,该频率源具有输出频率精确、频率分辨率高和相位噪声低等特点,符合通信系统对频率源的设计要求。 相似文献
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AD8310是AD公司生产的一种高速电压输出型对数放大器。它可对DC到440MHz的频率范围进行解调。在15ns内对接地负载的驱动电流可达25mA,且功耗极低。AD8310采用先进的压缩技术,其动态范围高达95dB,而误差仅为±3dB。该器件性能稳定且易于使用,外部不需其它重要元件。文中介绍了该芯片的内部结构、原理以及应用设计方法。 相似文献
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软件无线电是一种基于宽带A/D器件、高速DSP芯片,以软件为核心(Software-Oriented)的崭新的体系结构,A/D转换和数字下变频是其中非常重要的两个关键。对AD6652进行了配置,因其集成了DDC,使得ADC的数据输出在内部直接连接到接收器的DDC输入矩阵,从而简化了设计,减少了连接产生的寄生信号。 相似文献
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Acid/base mobile phase modifiers affect enantioseparations in ways that are not yet understood for the lack of systematic studies, which makes the scale-up of preparative separations difficult to predict. Shifts of the selectivity of certain pairs of enantiomers upon exposure of the column to these modifiers is amply documented. Furthermore, once the modifier has been removed from the mobile phase, the improved selectivity remains, this phenomenon has been named the memory effect. We selected four enantiomeric pairs for a systematic study of this memory effect. The selectivity of 4-chlorophenylalanine ethyl ester (4CPEE) improves after a solution of ethanesulfonic acid (ESA) is percolated through the column. The selectivity of propranolol HCl and Tröger's base increases after a solution of diiospropylethylamine is percolated through the column. The selectivity of these three pairs of enantiomers is inversely affected by percolation of the opposite acid/base solution. Each of these four compounds reached an equilibrium concentration that maintained the separation of the enantiomeric pairs. In contrast, the selectivity of trans-stilbene oxide (TSO) is not affected by either acid/base modifier. Preparative separations can be used to detect changes in the active surface of the chiral polymer stationary phase by measuring the change in selectivity and resolution when modifiers are used. Preparative method development was carried out on analytical columns and scale-up to 1 cm ID columns were performed in this study. 相似文献
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Mary J. Gresser 《Tetrahedron》2010,66(34):6965-9228
The attempted first stereoselective synthesis of 2,2′-biindoline using a metathesis-Sharpless asymmetric dihydroxylation strategy results in the synthesis of the heterocycle in poor to modest stereoselectivity. Attempts to improve the ee by varying the heteroatom protecting groups in key intermediates did not enhance the outcome of the Sharpless AD reaction. Therefore a limitation of this AD reaction is the use of 1,4-substituted but-2-enes where these substituents are ortho-substituted aromatics. 相似文献
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Baogen Su Zongbi BaoHuabin Xing Yiwen YangQilong Ren 《Journal of chromatography. A》2009,1216(26):5140-5146
The enantioseparation of trans-3-ethoxycarbonyl-4-(4′-fluorophenyl)-1-methyl piperidine-2,6-dione (3), which is one of the important racemic precursors of trans-(−)-paroxetine, has been investigated using supercritical fluid chromatography on a Daicel Chiralpak AD column. Supercritical CO2 modified with methanol, ethanol and 2-propanol were used as mobile phase. The influence of type and concentration of alcohol modifier on retention factor, enantioselectivity and resolution were studied. Among methanol, ethanol and 2-propanol, 2-propanol was proved to be the most favorable modifier, and 9.5% (v/v) of 2-propanol was the preferred concentration at which racemate 3 could be separated with resolution of 15.86 and retention factor of 6.323. The effects of pressure and temperature were investigated at 9.5% (v/v) of 2-propanol in the pressure range of 12–24 MPa and temperature range of 303.15–318.15 K. It was found that the lower pressure and temperature were favorable to the enantioseparation. Using van’t Hoff plot, the isoenantioselective temperature was calculated to be 410 K. The enantioseparation process was “enthalpically driven” under experimental conditions. Finally, the retention factors were satisfactorily correlated by a simplified lattice–fluid model with average absolute relative deviation (AARD%) of both enantiomers smaller than 1.76%. 相似文献
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