双电桥测量的应用

双电桥作为一种测量电路迄今主要用于测量小电阻,然而它的测量作用远不止此,本文将介绍使用双电桥完成的一系列重要测量,如电压比例的比较与高值电阻负载系数的测量;低值电阻负载系数的测量与分流器在标称电流下的检定;用双电桥测量高值电阻等。     

交流量子电阻传递电桥的研制

国际单位制变革后,阻抗参数需溯源至量子化霍尔电阻,其中将交流量子电阻传递到被测交流标准电阻是主要难点之一,需要研制10^–8量级的阻抗电桥,而常规精密阻抗电桥的准确度在10^–4至10^–5量级,仅有四端对同轴阻抗电桥在理论上能实现10^–8量级的准确度,但结构十分复杂,需要多次平衡,其过程存在相互影响,使平衡收敛十分缓慢,测量效率很低,同时存在频点单一的问题。针对以上不足,提出微差补偿网络隔离供电和可调虚部补偿输入比例的措施,大幅提高电桥收敛速度,并实现用于交流量子电阻传递的多频点四端对同轴阻抗电桥。同时给出四端对同轴阻抗电桥的校验方法和整体验证的标准器,验证结果表明：研制的交流量子电阻传递电桥10∶1的测量不确定度达到10^–8量级。     

数字微差补偿器及其在电流比较仪电桥中的应用

设计一种能够在四象限内对输出信号进行调节的数字微差补偿器,核心器件为16 位串行输入乘法型数模转换器,以正弦电压信号作为参考,可根据接收的数字编码分别调节输出补偿信号的同相分量和正交分量。提出一款计算机控制的电流比较仪电桥线路,应用数字微差补偿器和牛顿迭代算法实现电流比较仪磁势的自动平衡。利用研制的电桥对双螺线型计算电阻进行测量,结果表明,该电桥在1592 Hz频率范围内的标准测量不确定度为2×10^-7,能够满足交流电阻量值传递要求。     

基于比率法的高精度电桥电路及其应用

提出一种基于比率法的高精度电桥电路,用于测量电阻传感器中的高精度电阻。该电桥电路是基于模数转换器的数字量输出,采用模拟输入电压与参考电压比率的方法,将电桥的不平衡输出经放大后作为模数转换器的模拟量输入,将电阻式传感器相同一侧另一桥臂两端的电压差经放大后作为模数转换器的参考电压,经过模数转换后两者比率的数字量作为系统的输出。经过该电路处理后,电阻信号与系统的输出之间成线性关系,并克服了供电电压波动对电阻测量结果的影响,消除了系统的零点漂移和温度漂移。该电桥电路具有高准确度的电阻测量性能。     

低温电流比较仪用于小电流测量的比例误差分析

小电流准确测量是电磁测量领域的热点和难点,低温电流比较仪作为目前最准确的电流比例量具,其电流分辨力高,在白噪声频带范围可达到甚至小于1fA/Hz1/2,电流增益准确性高.本文通过对基于SQUID的低温电流比较仪进行分析,低温电流比较仪比例误差小于10-11,可应用于nA量级电流的放大及准确测量.     

交叉调制对电桥平衡的影响及其抑制方法的研究

详细讨论了由于电桥指零仪系统的非线性而产生的谐波交叉调制效应对电桥平衡的影响,设计并制作了一种高Q值的谐振滤波器以抑制此效应的影响。谐振滤波器的电感线圈采用高导磁率的瓷罐型铁芯绕制而成,通过调节两个瓷罐的间隙的大小以获得最大的Q值,从而降低由于铁芯损耗产生的热噪声。实验结果表明,该谐振滤波器的等效输入噪声小于1nV/Hz ,采用该滤波电路后,交叉调制效应造成的影响被有效降低了两个数量级以上。     

基于改进型两次平衡对检法的感应分压器自校准方法

介绍了参考电势变压器和指零仪变压器设计原理,分析了屏蔽间泄漏对测量结果的影响;对常规两次平衡参考电势对检法进行了改进,使零平衡和段平衡测量过程中,参考电势和测差电路均可实现等电位保护;对自校准方法进行了推导,校准结果仅与段平衡和零平衡时锁相放大器测量的电压差值相关,与参考电势变压器、指零仪变压器、辅助变压器等的误差无关,屏蔽间泄漏的影响也得到消除。对1kV感应分压器进行了校准实验,并对校准结果进行了测量不确定度分析,其相对扩展不确定度的评估结果为5.4×10^-8(k=2)。     

热电阻引线电阻补偿电路

一、桥式电路图1中,热电阻Rt与电桥是用两条线连接的,引线电阻与热电阻阻值之和构成一个桥臂的总电阻,引起桥路的不平衡以及测量误差。图2中,热电阻Rt用三条线与电桥相连,设三条引线电阻相等,记为r0,其中一条线置于电源回路,r上产生的微小压降并不影响桥路的平衡,另两条线分别置于桥的两臂,若r相等也不影响桥路的平衡,这样一来,零点时的输出V0仍等于零。图1桥式电阻-电压变换器摇摇摇摇摇摇图2三线制桥式电路二、双电流源线电阻补偿电路集成运算放大器与电流源搭配起来,可以组成引线电阻补偿电路。电流源可以由半导体三极管、场效应管、恒…     

用ER系列电阻输入型仪表测量温差

用ＥＲ系列电阻输入型仪表测量温差宋炉舒，叶宗琅（江油市长城特钢公司第四钢厂计控科，６２１７０２）ＥＲ系列热电阻输入型仪表的功能类似于ＸＯ系列仪表，但ＸＱ系列的输入电路是由平衡电桥测量原理构成，而ＥＲ系列电阻输入型仪表的输入电路是由稳流源、运算放大器等...     

一种新型低值宽带无感分流器时间常数测量方法

高压脉冲或电力系统中的高频谐波通常采用宽带无感分流器记录瞬态电流波形。分流器的带宽取决于其时间常数, 进而可用于评定其引入的测量不确定度。利用高精度数字多用表测量得到分流器的直流电阻, 通过同步采样方波输入电压以及被测分流器两端的输出电压, 利用最小二乘拟合算法直接计算得到被测分流器的残余电感, 据此确定分流器的时间常数。同步采样采用2根等长电缆与信号发生器的输出进行连接, 避免了信号的传输时延。研究了阻抗匹配对电缆电阻所造成的方波电压和分流器输入端电压差异的影响。结果表明该测量方法可以直接测量阻值1Ω以下、带宽10MHz以下低值无感分流器的电感与时间常数, 成功地解决了宽频电流传感器及宽带无感分流器量值溯源难题。     

Characterization of the Input Noise Sources of a dc SQUID

From the theory of the intrinsic noise in a dc SQUID by Tesche and Clarke, we derive the expressions of the current and voltage input noise spectral densities in a dc SQUID current amplifier operated in a flux locked mode. The expected current and voltage noises are compared, at audio frequencies, with the experimental results obtained with a low noise dc SQUID in which the input load (resonant and not) and the operating temperature (1–4 K) are changed. In order to evaluate the input voltage noise, which is directly related to the current noise around the SQUID loop and is usually neglected, we have used as the input circuit a LC resonator with a very high quality factor (10⁶). Both the voltage and current input noises exceed the expected values by the same factor of about 8. This means that the modulus the optimum source impedance of the SQUID amplifier is still in agreement with the value expected from the theory, which is approximately given by the product of the input coil inductance and the angular frequency. To explain the excess noise results, we propose a model in which the voltage and current input noises are due to a thermal magnetic noise source which is present near the SQUID.     

Effects on DC SQUID characteristics of damping of input coil resonances

The possibility of improving dc SQUID performance by damping the input circuit resonances caused by parasitic capacitances is studied experimentally. A high-quality dc SQUID was coupled to a first-order axial gradiometer built for neuromagnetic research, and a resistor-capacitor shunt was connected in parallel with the input coil of the SQUID. Ten differentRC shunts were studied with the SQUID operating in a flux-locked loop, carefully shielded against external disturbances. It was found that increasing the shunt resistance resulted in smoother flux-voltage characteristics and smaller noise. At best, the minimum obtainable equivalent flux noise level was one-fourth that for the unshunted SQUID. The noise level is a function of the shunt resistanceR _s only, except for shunt capacitance values bringing the low-frequency resonance of the input coil close to the flux modulation frequency. At a constant bias current level, where the amplitude of the flux-voltage characteristics is at maximum, the equivalent flux noise varies asR _s ^/–0.7 . The results agree reasonably well with recently published predictions based on numerical simulations where the whole input circuit with parasitic capacitances was taken into account.     

Flux-locked current source reference

The quantization of flux in a closed superconducting circuit is used to provide a stable reference current. A 10-mA current source is coupled through a toroidal transformer to a DC superconducting quantum interference device (SQUID) input, and the resulting signal is fed back as an error current. The result is a net flux linkage that exhibits short-term stability of 1 part in 10⁹/h. The net current is quantized with a step size of 59.4 nA, and it will exhibit the same stability as the flux provided the mutual inductance of the transformer remains constant. This current is passed through a precise 100-Ω resistor and compared against Zener diode references. The observed temperature coefficient for the flux transformer is 28.5±3 ppm/K at 4.2 K. Possible sources for the temperature dependence are discussed     

The Renormalization Effects in the Microstrip-SQUID Amplifier

The peculiarities of the microstrip-DC SQUID amplifier caused by the resonant structure of the input circuit are analyzed. It is shown that the mutual inductance, that couples the input circuit and the SQUID loop, depends on the frequency of electromagnetic field. The renormalization of the SQUID parameters due to the screening effect of the input circuit vanishes when the Josephson frequency is much greater than the signal frequency.     

能量天平研究进展

能量天平法已在中国计量科学研究院开展了几年的研究。研究的目的与功率天平法是一致的,即通过测量普朗克常数来建立质量量子基准,但具体方法不同。2010年以来该项研究取得一定进展。例如对于直流互感量测量这一关键问题,已研究出一种新的方波补偿方法,测量不确定度达到10^-7量级（k=1）。还研发了一种涡流传感器,可更准确地对可动线圈定位。线圈系统的准直问题也得到进展,研制了一种油阻尼器,大大减小了由于天平基础振动和线圈加热上升气流引起的线圈抖动。这些措施改善了线圈位置及互感测量的不确定度。测量普朗克常数h的最新结果为6626104(59)×10^-34J·s,综合的相对不确定度为8.9×10^-6（k=1）。对所遇到的困难和进一步的改进措施也在文中进行了讨论。     

Performance of a 2-hole Y-Ba-Cu-O rf SQUID

A two-hole rf SQUID has been fabricated out of bulk YBCO by drilling two holes and careful erosion of the wall between the holes. Commercial SQUID electronics is inductively coupled to the SQUID through a copper coil glued in one of the holes. Periodic oscillations in V-B characteristic of the SQUID are observed at 77 K. The spectral density of the flux noise in the white noise region is 5·5 × 10⁻⁴Φ₀ / √Hz. The flux noise is frequency-dependent below 200 Hz.     

Proposal of rectifier type superconducting fault current limiter with non-inductive reactor (SFCL)

A rectifier type superconducting fault current limiter (SFCL) with non-inductive reactor has been proposed. The concept behind this SFCL is the appearance of high impedance during non-superconducting state of the coil. In a hybrid bridge circuit, two superconducting coils connected in anti-parallel: a trigger coil and a limiting coil. Both the coils are magnetically coupled with each other and have same number of turns. There is almost zero flux inside the core and therefore the total inductance is small during normal operation. At fault time when the trigger coil current reaches to a certain level, the trigger coil changes from superconducting state to normal state. This super-to-normal transition of the trigger coil changes the current ratio of the coils and therefore the flux inside the reactor is no longer zero. So, the equivalent impedance of both the coils increased thus limits the fault current. We have carried out computer simulation using EMTDC and observed the results. A preliminary experiment has already been performed using copper wired reactor with simulated super-to-normal transition resistance and magnetic switches. Both the simulation and preliminary experiment shows good results. The advantage of using hybrid bridge circuit is that the SFCL can also be used as circuit breaker. Two separate bridge circuit can be used for both trigger coil and the limiter coil. In such a case, the trigger coil can be shutdown immediately after the fault to reduce heat and thus reduce the recovery time. Again, at the end of fault when the SFCL needs to re-enter to the grid, turning off the trigger circuit in the two-bridge configuration the inrush current can be reduced. This is because the current only flows through the limiting coil. Another advantage of this type of SFCL is that no voltage sag will appear during load increasing time as long as the load current stays below the trigger current level.     

Low temperature permeability and current noise of ferromagnetic pickup coils

For a non-destructive measurement of intensities of charged particle beams a Cryogenic Current Comparator is used which captures the azimuthal magnetic field of the beam by a superconducting pickup coil at 4.2 K and transforms it into a current which is detected by a SQUID based current sensor. The current noise of the pickup coil and the bandwidth of this transformer depend on the frequency response curve of the complex permeability of the ferromagnetic core material embedded in the pickup coil. A measurement of the series inductance L_S and series resistance R_S of such a coil allows an indirect evaluation of the current noise contribution of the core using the Fluctuation–Dissipation-Theorem. These measurements were done with a commercial LCR-Meter in a frequency range from 20 Hz to 2 MHz. The current noise density was also directly measured using a SQUID-sensor. A comparison with between the direct and indirect measurement showed a good coincidence. Due to the critical temperature of the LTS-SQUID, noise measurements above 4.2 K are not possible apart from using an anti-cryostat. The measurement of the series inductance L_S and series resistance R_S with an LCR-Meter works in the whole temperature range and provides a comfortable access to the magnetic properties of core materials. Compared to direct measurements, the indirect measurement thus allows a technologically simpler and broader determination of the core noise.