如何加强信号路径的性能

信号路径的设计为系统设计工程师提供不少可供他们发挥的机会.以设有模拟/数字转换器的信号路径为例来说,工程师进行设计时不但要为传感器提供缓冲,而且还要解决模拟/数字转换器开关电容的输入充电问题,以及要尽量减少系统的噪音源,只要构思的设计能解决这些问题,便可大幅提升系统的性能.今期的信号路径设计专辑将会深入讨论这方面的问题.     

Intrinsic echolocation capability of Hector's dolphin, Cephalorhynchus hectori.

A sonar system's echolocation capabilities can be inferred from the ambiguity distribution (defined here in terms of the conventional signal response function) of each of its transmitted signals. Several records of sounds emitted by Hector's dolphin are analyzed. The computed ambiguity distributions indicate that the sonar clicks of Hector's dolphins should be capable of resolving the ranges of targets as close together as 2 cm apart, but that target velocities cannot be resolved to any useful degree from a single echo.     

差分测量的重要性

许多实际应用都要求放大强噪声环境中的徵弱信号。通常，信号传感器离放大器有一定的距离，所以经常引入大量的噪声和杂音。有效的信号恢复常常依赖于为具体的应用精心地选择最佳的放大器。有三种常见的应用系统类型：单端输入和单端输出(基于运算放大器)系统；差分输入和单端输出(基于仪表放大器)系统；以及差分输入和差分辅出(基于差分放大器)系统。有些设计工程师可能要使用单端输人、屏蔽电缆系统，类似图A所示。这里，首先在屏蔽电缆和公共端(或者“地”)之间施加输入信号，然后它通过电缆传送到运算放大器。像这样的单端系统很容易引…     

A collaborative study: high-pressure liquid chromatographic determination of carbadox and pyrantel tartrate in animal feeds

Carbadox (CBX), an antibacterial agent, and pyrantel tartrate (PT), an anthelmintic, are formulated either separately or together in swine feeds. The official Association of Official Analytical Chemists (AOAC) spectrophotometric methods for both drugs are long, nonspecific, and require standard addition techniques. Results by this technique are positively biased. A simple, direct, specific, high-pressure liquid chromatographic (HPLC) method to determine either one or both drugs simultaneously with apparent accuracy and precision is developed. Drugs are released from feed matrices by water, extracted with dimethylformamide (DMF), cleaned up on alumina, and quantitated by direct comparison to standards using a Whatman Partisil 10 ODS-3 column and a mobile solvent containing 23.5 +/- 1.5% DMF in phosphate buffer (pH 2.0). Fourteen laboratories participated in a collaborative study of this method for determination of CBX and PT in animal feeds.     

Synthesis of some heterocyclic systems from nucleophilic substitution reactions with η6-o-dichlorobenzene-η5-cyclopentadienyliron hexafluorophosphate

A method for the synthesis of heterocylic systems related to 9,10-dihydroanthracene with two hetero-atoms at the 9,10-positions is described. It involves the nucleophilic substitution reaction of η⁶-o-dichlorobenzene-η⁵-cyclopentadienyliron hexafluorophosphate with two nucleophilic groups (OH, SH and/or NH₂) located in the 1,2-positions of a benzene ring to give a cyclopentadienyliron complexed heterocycle. Upon pyrolytic sublimation of the complex, the free heterocyclic compound is then obtained.     

Conformational substates modulate hydride transfer in dihydrofolate reductase

In earlier studies of the hydride-transfer reaction catalyzed by dihydrofolate reductase (DHFR) we identified features of the protein correlated with variations in the reaction barrier. We extend the scope of those studies by carrying out potential of mean force (PMF) simulations to determine the hydride-transfer barrier in the wild-type protein as well as the G121V and G121S mutants. While our prior studies focused on the reactant state, our current work addresses the full reaction pathway and directly probes the reactive event. The free energy barriers and structural ensembles resulting from these PMF calculations exhibit the same trends reported in our previous work. Fluctuations present in these simulations also exhibit trends associated with differences in the hydride-transfer barrier height. Moreover, vibrational modes anticipated to promote hydride transfer exhibit larger amplitudes in simulations that generate lowered barriers. The results of our study indicate that discrete basins (substates) on a potential energy landscape of the enzyme give rise to distinct hydride-transfer barriers. We suggest that the long-range effects of mutations at position 121 within DHFR are mediated by differentially preorganized protein environments in the context of distinct substate distributions, with concomitant changes to the dynamic properties of the enzyme.     

Determination of silicon in fats and oils by electrothermal atomic absorption spectrometry with in-furnace air oxidation

A graphite-furnace atomic absorption spectrometric emthod is reported for determination of dimethylpolysiloxane (silicone) in edible fats and oils. It incorporates an air-oxidation step in the furnace program to reduce matrix interferences. The detection limit is 0.3 mg kg^?1 in soils, and short-term precision is about 6% at silicon concentrations of 1.7–2.0 mg kg^?1. The procedure was successfully applied to a variety of commercial frying fats and oils, to quantify silicone loss during polish filtration during oil manufacture, to quantify sorption of antifoam onto fried food, and to detect fugitive sources of silicone encountered in deep-fat frying operations.