Coaxial Electrical Circuits for InterferenceFree Measurements
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The authors have between them more than 60 years of experience in making electrical measurements in National Measurement Laboratories. These laboratories are the source of measurement standards and techniques for science and engineering and are dedicated to maintaining the international system of units (SI) by establishing and disseminating the values of measurement standards with the lowest possible uncertainty. Careful attention to detail is required in designing measurement systems that eliminate electrical interference and are as simple and as close to first principles as possible. This book draws on their experience by offering guidance and best practice for designing sensitive electrical measurement circuits. In particular the book describes examples that demonstrate the elegance, flexibility and utility of balancedcurrent coaxial networks in obtaining the ultimate in noisematching and interference elimination for precise and accurate voltage, current and power measurements. It also updates an earlier book on coaxial AC bridges by including recent AC measurements of quantum Hall resistance to establish a primary quantum standard of impedance and by extending impedance measurements in general to higher frequencies.
Inspec keywords: voltage measurement; quantum Hall effect; circuit noise; power measurement; electric impedance measurement; coaxial cables; bridge circuits; interference suppression; electric current measurement
Other keywords: voltage measurement; interference elimination; power measurement; quantum Hall resistance; impedance measurement; noise matching; balanced current coaxial networks; interferencefree measurement; coaxial AC bridges; AC measurement; coaxial electrical circuit; current measurement
Subjects: General electrical engineering topics
 Book DOI: 10.1049/PBEL013E
 Chapter DOI: 10.1049/PBEL013E
 ISBN: 9781849190695
 eISBN: 9781849190701
 Page count: 350
 Format: PDF

Front Matter
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1 Introduction
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This chapter serves as an introduction to the book “Coaxial Electrical Circuits for InterferenceFree Measurements”. It provides a discussion on interactions between circuits and eliminating electrical interference.
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2 Sources, detectors, cables and connectors
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In this chapter the miscellaneous but nonetheless important properties of sources and detectors and of coaxial cables and connectors are discussed so that the practical examples of network design can be successfully implemented.
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3 The concept of a lowfrequency coaxial network
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This chapter presents the application of coaxial network for interference free measurements. Interferencefree balancedcurrent circuitry evolved from the need to develop bridge measurements of standard impedances to obtain ever higher accuracies to satisfy demands for better characterisation of passive electronic and electrical circuit components. In this book, we are concerned with conductorpair networks where one network of the pair has a low impedance compared to the other, and where the current in any one conductor is matched by an equal and opposite current in the other conductor of the pair. The potential differences between corresponding nodes of the pair of networks are quantities of interest because, for example, detectors or sources are to be connected between a pair of corresponding nodes.
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4 Impedance measurement
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The purpose of this book to encourage coaxial techniques within the formalism of coaxial networks, to show that these techniques have the great advantage of obtaining accurate and certain results, and to show how to implement each concept with practical, constructable apparatus. The bridge networks with which we are concerned will therefore consist of impedances provided with, and defined in terms of, terminalpair coaxial terminals and will be connected with conductorpair coaxial cables. Equalising the current in the inner conductors and screening outer conductors of cables so that the currents are equal and opposite will ensure that the cables have negligible external fields (see section 1.1.1), and therefore, the cables do not interact with each other so that their routing is immaterial. The network will also have little response to external interference.
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5 General principles of accurate impedance measurement
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We now begin our discussion of balanced current coaxial techniques as applied to impedance measurement with AC bridges. As mentioned in the introduction, this provides a good example of how these techniques can be applied in general to sensitive electrical measurements of other parameters voltage, current, power and so forth. Before moving on, it is as well to note the advantage of a substitution measurement in comparing two nearly equal impedances, and indeed in metrology in general. We can make a formal statement of the philosophy of a substitution measurement as follows.
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6 Impedance standards
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This chapter details the key SI primary and secondary standards of capacitance, resis tance and inductance for applications covering the frequency range DC to 100 MHz. These standards include the ThompsonLampard calculable crosscapacitor, Gibbings quadrifilar resistor, Campbell mutual inductance standard as well as the latest standard of impedance derived from the quantum Hall effect (QHE). Impedance measurements span one of the widest ranges of any physical quantity, from nanoohms to teraohms (i.e. a ratio of 10). This chapter also discusses the limitations of the standards and their evolution to meet the contemporary scientific and industrial needs.
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7 Transformers
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In this chapter, we are concerned with the other category of transformer that may deliver some power, but the design objective is rather to approach an ideal transformer. In an ideal transformer the EMF induced in each and every turn is equal and the voltage developed by a winding is very close to the sum of these induced EMFs, i.e. to one part in a million or better. Isolation may also be required. In the case of current transformers, the ratio of the currents in the windings is the important quantity, and the accompanying voltages are only of incidental interest.
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8 General considerations about impedance comparison networks
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In this chapter some guidance on the design and use of impedancecomparing networks is discussed.
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9 Bridges to measure impedance ratios
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This chapter discussed all the concepts and devices described so far to show how they may be combined to make practical highaccuracy bridges. The number of networks that may be devised is endless, and therefore, we have chosen just those examples that either are of considerable practical importance to national standards laboratories or illustrate the use of some device or concept. In principle, we show how to relate like impedances of different values and then how to generate the impedance standards of R, L and M from the unit of capacitance. The reader should feel encouraged to devise other networks to serve other special needs. We will often present a network as an admittance bridge; of course the distinction between this and an impedance bridge is purely one of algebraic convenience.
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10 Application of interferencefree circuitry to other measurements
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This chapter discussed the other measurements with the application of interferencefree circuitry. Resistance thermometry is the most precise method of measuring temperature in the range from 30 to 1000 K. The sensor is usually a compact resistor of platinum (or platinumrhodium alloy for higher temperatures), whose value at ambient temperatures is about 25 or 100 Ω. The change of resistance with temperature is about 0.4% per Kelvin, so that measurement of the resistance to 1 ppm yields temperature with a resolution of about 0.25 mK. A given thermometer can be calibrated at the various 'fixed points', which are the melting or triple points of water and various metals, and a temperature scale derived from interpolation between these. It also shows the superconducting cryogenic current comparator, Josephson voltage source and accurate voltage measurement. There are many new and exciting possibilities for applying the higherfrequency techniques described in the previous chapters, and we now indicate only a few examples of the key future directions of this technology.
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Back Matter
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