双电桥灵敏度的探讨

一般的大学物理实验教科书中,都安排了“用双电桥测低电阻”这个实验。但对双电桥灵敏度未作进一步的讨论,本文试图对此作些探讨。一、双电桥电路图1给出了它的原理性电路。其中R_x是被测电阻,     

用单电桥测电阻应注意的问题

本文着重讨论用单电桥测电阻时,在电桥不平衡的特殊情况下,如何计算R_x值。电桥的原理线路如图所示。当调节R_s使检流计指针指零,电桥达到平衡时,有式中K为比例臂,R_s为比较臂。当用电桥测电阻时,电桥达到平衡后可由(1)式算出R_x的值。     

电桥实验的改革尝试

随着数字仪表的飞速发展和大量普及，以及非平衡电桥在传感技术中的广泛应用，有必要对电桥实验内容进行改革，我们把传统的平衡电桥实验改为非平衡电桥实验，从理论和实验上研究了非平衡电桥的工作特性和应用．     

西林电桥测电容的方法研究

本文介绍了西林电桥测量电容的实验过程与方法,西林电桥是根据实验需要选取适合实验仪器的自组电桥。本文用自组西林电桥验证了当交流电源频率为1kHz,待测电容数量级为10^-6～10^-7F时,交流阻抗应在10^2～10^3Ω之间,相邻桥臂的阻抗应该近似相等,即倍率为1,其灵敏度最高,测量效果最好。     

谈双“单电桥”测低电阻的两类方法

谈双“单电桥”测低电阻的两类方法李龙海（北京理工大学应用物理系，１０００８１）物理实验中，人们常称惠斯登电桥为“单电桥”，认为其只适于测１０Ω一１０８Ω之间的“中电阻”，至于欲测１Ω以下的“低电阻”，只能应用“双电桥”——开尔文电桥．其实不然，我们利...     

惠斯登电桥的灵敏度

见到某院校近期的物理实验讲义，对其惠斯登电桥灵敏度的推导及结论持异议．先摘录原文如下：电桥的灵敏度S：单位桥臂（如凡）相对不平衡所引起的偏格数称为电桥的灵敏度．＿凸oS一十（l）一AN．／R．因为在平衡电桥内，若比较臂电阻R。（或任何一个桥臂电阻）变动3R。，电桥就失去平一。3V。；＾、。、，衡，产生电位差LI’和电流I。一于，使检流计””“『“““一’””“”’““’R”“““’“”＿．。＿、．＿＿、，，、＿凸V＿，G偏转，上式中西0为检流计G通电流于时的””””’“”’一“““”“”’—““““R－“”“…     

基于双臂电桥的组合式测微方法

利用普通物理实验室的常见器材,如千分尺、变阻箱、检流计、电阻丝、导线等,根据螺旋放大、电桥平衡原理设计了螺旋电桥式测微仪.介绍了该装置的原理、使用方法和测量效果.     

交流电桥的灵敏度和抗干扰问题的探讨

通过理论和实验,阐述了提高交流电桥灵敏度的方法和测量电路的选择。     

电桥法测电阻综述

本文对测量电阻的多种电桥法进行了介绍，并分析了各种方法的使用范围及改进方法。     

交流电桥实验的计算机辅助分析

交流电桥实验的计算机辅助分析倪新蕾，潘元胜（南京大学物理系．２１０００８）交流电桥是测量交流元件的常用仪器，图１是测量电路，Ｚｘ为待测电抗元件的复阻抗，其平衡条件为：图１交流电桥众所周知，在交流电桥实验中，要使电桥完全调平衡是十分困难的，一般在平衡指...     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.