This book presents a wide-ranging survey of these subjects and the techniques which are used for calibrating and comparing them.
Inspec keywords: power meters; electric current measurement; thermoelectric devices; reflectometers; microwave measurement; diodes; calorimetry; calibration; calorimeters; power measurement; voltage measurement
Other keywords: power measurement; frequency 1 MHz to 200 GHz; noncalorimetric power meter calibration method; voltage measurement; current measurement; reflectometer; radio frequency power measurement; thermistor; calorimetry; microwave power meter; pulsed power measurement; radiofrequency power meter; force operated instrument; microwave power measurement; thermoelectric device
Subjects: Measurement standards and calibration; Power and energy measurement; Electrical instruments and techniques; Microwave measurement techniques; Current measurement; Thermal variables measurement; Measurement standards and calibration; Calorimetry; Voltage measurement
- Book DOI: 10.1049/PBEL007E
- Chapter DOI: 10.1049/PBEL007E
- ISBN : 9780863411205
- e-ISBN: 9781849193665
- Page count: 292
- Format: PDF
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Front Matter
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1 Background and fundamentals
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This book deals with the measurement of power from 1 MHz to 200 GHz, a frequency range which covers roughly the radio-frequency and microwave regions of the electromagnetic spectrum. Radio-frequency and microwave techniques are sometimes thought of as two separate subjects occupying two distinct frequency ranges. Microwaves, according to their currently recom mended definition [1], lie between 300 MHz and 300 GHz, with wavelengths from 1 m to 1 mm. They are often termed decimetre, centimetre and millimetre waves. Their exploitation dates from around 1940 and at that time the techniques adopted stood in sharp contrast to conventional lower frequency radio engineering methods. The use of waveguides, the need to take into account propagation delays and to think nearly always in terms of travelling waves, all served to make microwaves a distinct subject. However, since then the distinction has become less significant. For example, coaxial lines are used widely at microwave frequencies [2,3], whilst for precise measurements propagation delays must be taken into account at frequencies well below 300 MHz typically from 30 MHz upwards. Moreover, the frequency range covered by many power meters and other instruments lies partly in the rf region and partly in the microwave region. Thus in reality one is dealing with a continuous spectrum. Nevertheless, for some purposes it is convenient to divide this spectrum into sub-ranges. Often a break is made at 1 GHz, which coincides with the highest frequency at which one normally attempts to measure voltage and current directly. Another significant frequency is 40 GHz, which for many years was very roughly the highest frequency at which coaxial lines were in regular use.
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Part A: Calorimeters
2 Introduction to calorimeters
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Calorimeters are heat-measuring instruments. They form the basis of the vast majority of primary standards for rf and microwave power and, in addition, some types also find use as secondary standards. One problem which all have in common is the conflict between accuracy on the one hand and convenience factors such as adequate sensitivity and rapid response on the other hand. The main reason for this conflict is the limited range of thermal resistances of available materials, which forces compromises on the designer. The result is that calorimeters intended for use as primary standards tend to have long time constants, typically in the range 1 to 10 minutes, and are often very bulky, whilst commercially made secondary instruments have unknown errors which are difficult to evaluate without calibration against a more accurate instrument. Some calorimeter designs have succeeded in narrowing the gap between the two types, but this gap is still considerable. In the past it was common to find calorimetric power meters used as everyday instruments on the bench, but for most routine purposes the preference today is for the speed and sensitivity obtainable from thermistors, thermoelectric and diode power meters. Consequently for non-standards work the calorimeter has tended to be displaced by these instruments, in spite of its potentially high accuracy and stability of calibration.
3 Dry load calorimeters
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These chapters describe a set of rugged broad-band dual load calorimetric power meters, both coaxial and waveguide, which were intended for operation at frequencies from zero to 75 GHz. In comparison with later instruments the accuracy was very modest, with an uncertainty of 2 percent for the coaxial version and 1-2.5 percent for the waveguide versions. Nevertheless, these designs established the general direction in which subsequent instruments followed. New precision loads and connectors developed in the 1960s led to a new generation of coaxial calorimeters with better performance and with uncertainties below 0.5 percent for frequencies up to 8 GHz. These instruments, which were fitted with 14 mm precision connectors and were developments of an earlier version fitted with an N type connector were amongst the first to make use of the tractorial load. This calorimeter, of which a 7 mm version was later produced has frequently been quoted in the literature and it will be used to illustrate the design principles and methods of evaluating the effective efficiency.
4 The microcalorimeter
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The term 'microcalorimeter', although sometimes used as a general description for any calorimeter capable of measuring milliwatt power levels, is most commonly associated with the type of instrument shown in Fig. 4.1. The load is a bolometer mount and the purpose of the measurement is to determine the effective efficiency of this mount. After the measurement, the mount is removed from the calorimeter and used as a secondary standard. The technique can give excellent results, with uncertainties for waveguide bolometers as low as 0.1 percent at 10 GHz, 0.3 percent at 35 GHz and 0.5 percent up to 100 GHz. Coaxial mounts may also be calibrated by the microcalorimeter technique, but they pose an additional problem because of the difficulty of estimating uncertainties arising from heat losses through the inner conductor.
5 Flow calorimeters
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In a flow calorimeter it is the rise in temperature of the fluid on passing through the load which is measured. The rf power is proportional to the product of this temperature rise, the flow rate, and the specific heat of the fluid. Instruments have been built operating up to 40 GHz for standards use and up to higher frequencies for commercial use. However, interest in the flow calorimeter for precise measurements began to decline in the 1960s and subsequent efforts were devoted largely to refinements of the dry load and microcalorimeter types. Consequently only a small number of precision flow calorimeters with uncertainties below 1 percent have been constructed.
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Part B: Non-calorimetric power meters
6 Bolometers
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In this chapter, bolometric power meters make use of a temperature-sensitive resistor, referred to as the bolometer element, which is housed in a mount. The rise in temperature caused by the absorption of RF or microwave power produces a change in resistance, which is detected by a bridge circuit. Although the principle of the bolometer goes back to 1880, present forms date from the 1940s, when the need first arose for reliable power measurements at frequencies above 1 GHz. They are of three main types: barretters, thermistors, and film bolometers. Almost all are directed heated, that is the bolometer element functions both as the absorber of the power and as the temperature sensor. Barretters consists essentially of a thin metal wire and have a positive temperature coefficient of resistance. They have been used extensively in standards laboratories but less frequently elsewhere. Thermistors consist of small beads of semiconducting material and are the natural choice for commercial instruments. They are less fragile than barretters and possess a relatively large negative temperature coefficient of resistance. Film bolometers, which employ thin metal films as temperature-sensitive resistors, are used in some standards laboratories.
7 Thermoelectric power meters
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Thermoelectric power meters take several forms. At frequencies below 1 GHz instruments based on the traditional thermoelement may be used. Thermoelements are indirectly heated evacuated wire thermocouples which form the basis of many RF/DC substitution instruments. Their RF/DC substitution error is usually small below 30 MHz but becomes appreciable above this frequency, so that it is necessary to apply correction factors. Above 300 MHz these corrections become large as a consequence of resonances. Directly heated wire thermocouples are potentially capable of operation up to higher frequencies than thermoelements because of their comparative simplicity, but their development was cut short by the relative success of wire barretters, which are even simpler and can be made with smaller diameters if the Wollaston wire technique is used. Consequently thermoelectric power meters did not become common for frequencies above 1 GHz until the development and application of thin film technology, which resulted in the appearance of commercial thin film instruments in the 1960s. A further advance was made in the 1970s, when instruments employing a combination of thin films and microelectronics techniques first appeared. Since that time thin film thermoelectric power meters have shown a steady increase in popularity, largely at the expense of the thermistor. Nevertheless they possess a number of disadvantages which have only partly been overcome. The main problems are that the thermocouple output voltage is not exactly proportional to the rf power level, which can lead to errors of several percent if not corrected for, and the sensitivity is dependent on ambient temperature. In addition, the variation of sensitivity with frequency is larger for thermoelectric devices than for thermistors. The difficulties are reduced by the use of microprocessor based instruments which store and automatically apply correction factors, but such corrections are usually only approximate and, although they add greatly to the convenience of the instruments, the necessity for them makes precision measurements more difficult. Both indirectly heated and directly heated thin film thermoelectric power meters exist, but most instruments in common use are of the directly heated type. These cannot be used for RF/DC substitution measurements, as they incorporate a DC blocking capacitor which separates the input signal from the thermocouple output voltage. Instead, they often incorporate a reference oscillator which supplies a known rf power at a fixed frequency and which enables changes in sensitivity with time to be detected and corrected for.
8 Diode power meters
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Diode power meters are essentially semiconductor diode detectors operating in their square law region. They are capable of measuring small powers less than a nanowatt and possess low zero drift coupled with an intrinsically fast response. In many respects, however, they compare unfavorably with other common types of power meter. Historically the main problems have been poor stability and reliability, temperature-dependence of the diode parameters, and difficulties in obtaining a flat frequency response. Until the 1960s the behaviour of diodes which could function in the microwave range was considered too poor for serious use in power measurement applications, their main uses being mixing and detection. New manufacturing techniques resulted in devices of greater stability and ruggedness, while improved circuit construction enabled a flatter frequency response to be achieved. These advances made possible the use of semiconductor diodes for commercial coaxial power meters operating up to 18 GHz and beyond. Such instruments became common in the 1970s. Nevertheless, the detector diode is still far from an ideal device for power measurement. In particular the sensitivity is inherently temperature dependent, the square law response is followed only at low power levels, and the rf impedance of the diode varies with both temperature and signal level, which creates problems in rf matching. Theoretically the sensitivity (measured by the voltage or current responsivity and expressed in volts per watt or amps per watt respectively) is calculable, but in practice the performance does not agree well with theory and calibration is therefore necessary.
9 Force-operated instruments
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In this chapter, of the large number of experimental force-operated power meters which have been constructed, none has been developed to the stage where it could compete either with the best calorimeters in terms of accuracy or with everyday instruments such as thermistor, thermoelectric and diode power meters in terms of convenience. Consequently their practical application has been extremely limited and their main contribution to the measurements field lies in the fact that the comparisons which have been carried out with thermal instruments provide confirmation that the latter are not seriously in error. From a more general point of view such comparisons also provide added confidence that the formulas for calculating forces due to electromagnetic fields are correct, in particular those using the concepts of linear and angular momentum.
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Part C: Calibration and comparison techniques
10 Other types of power meter
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This chapter talks about the different types of power meter.
11 Basic techniques for calibrating power meters
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The simplest and most obvious method for comparing two power meters, or calibrating one against the other, is to connect each in turn to a stable source. If the power meters have identical reflection coefficients, then each will absorb exactly the same amount of power from the source. Unfortunately such an assumption is justified only for relatively crude measurements, since the actual reflection coefficients of the power being compared will usually differ significantly from one another. In this chapter, basic techniques for calibrating power meter were taken.
12 Power meter calibration using reflectometers
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In this chapter, reflectometers enable measurements of power and reflection coefficient to be carried out simultaneously. Scalar reflectometers give only amplitude information. Vector reflectometers, on the other hand, measure complex quantities and display the reflection coefficient either in terms of modulus and phase or in terms of real and imaginary parts. Six-port reflectometers also measure complex quantities, but in these the phase information is deduced from ratios of amplitude readings rather than by the use of a phase meter or phase-sensitive detector.
13 Connectors and adapters
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This chapter discusses connectors and adapters used for radio frequency and microwave power measurement.
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Part D: Other topics
14 Instruments and techniques for pulsed power measurements
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This chapter presents a description of the chief features of pulsed power meters and the main ways in which they can be calibrated in a traceable way from fundamental electrical measurement standards. The chief quantity of interest is usually the peak power, but one might also wish to determine other properties such as the shape of the pulse envelope or the energy per pulse.
15 Voltage and current measurements
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The difficulties associated with voltage and current measurements increase with frequency and most voltmeters and ammeters are restricted to the region below 1 GHz [1-4]. Above 1 GHz voltages and currents in transmission lines can be deduced from a knowledge of the power flowing in the line, the characteristic impedance, and the reflection coefficient of the load. This approach enables the concepts of voltage and current to be applied at frequencies well above those for which these quantities could be measured directly [5], provided that only one mode is present in the line. Because of this restriction, it is only in regions which are free from unwanted modes, including evanescent modes generated by discontinuities, that one can talk about voltages and currents in high frequency transmission lines.
16 International intercomparison of standards
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Apart from the earliest comparisons, which took place at a time when basic techniques were still being assessed, the main role of both power and voltage comparisons has been that of providing confidence in the implementation of the techniques and in ensuring international consistency. From the results which are available it appears that the claims of those national laboratories which have taken part have in the main been substantiated, although such comparisons cannot of course detect errors which are common to all laboratories.
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Appendices
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The properties of linear n-port electrical networks can be expressed in a number of different ways. At low frequencies the impedance matrix is often the most appropriate form. This enables the voltages at the ports to be calculated from the currents.
Back Matter
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