Research on high frequency and microwave power benchmarks and their applications - research background and terminology

1.1 Project Background

Electronic measurement exists in various fields of scientific research, industrial production and engineering applications. The research on electronic measurement methods has important theoretical significance and practical value. The rapid development of manufacturing technology and the growing social demand have made the application frequency band of electronic products wider and wider, and the amount of information transmitted has become larger and larger. The fields and objects of electronic measurement have also been continuously enriched and expanded. The division of time domain, frequency domain and modulation domain has emerged, and many new measurement objects such as vector modulation error and bit error rate have also appeared. However, the basis of electronic measurement is still the basic parameters. Only by improving the measurement accuracy of basic parameters such as power can the accuracy of other applications be guaranteed.

High-frequency and microwave power is a quantity that describes the signal size and the energy transmission characteristics when the signal passes through an electronic system or transmission line. It is one of the most important parameters in electronic measurement. In radio electronic technology, it is often necessary to measure the output power of the transmitting device and the sensitivity of the receiving device, which requires the measurement of various levels of power.

At present, various high-frequency and microwave power measurement equipment commonly used in China include various power meters, spectrum analyzers, measurement receivers, etc., which are widely used in electronic product production lines and R&D laboratories, defense and communication industries. Due to the actual conditions in China, the radio measurement system in China is relatively complex. The transmission of power values ​​needs to go through multiple links. From the national power standard, it may have to go through the first-level metering station, the second-level metering station, and the third-level metering station before it can be transmitted to the final power measurement instrument.

With the rapid development of high-tech, especially digital processing technology, massive storage technology, broadband transmission technology and other new technologies, the accuracy and frequency range of power measurement in scientific research, national defense and industry are increasingly demanding. "A small mistake will lead to a thousand miles of error." The original power standard and transmission system can no longer meet domestic needs. Therefore, the study of establishing a new high-frequency and microwave power standard and conducting research on the transmission system and its application are of great significance and practical value to better support the entire radio measurement system, ensure the unification of high-frequency and microwave power values ​​in China, and improve the accuracy of power measurement.

This paper strives to improve the level of high-frequency and microwave power measurement in China from four aspects: improving the accuracy of power standards, expanding the frequency band of power standards, expanding the range of power value transmission, and improving the uncertainty assessment of power measurement results.

The project was supported by the National World Bank Loan Technology Development Project Sub-project: Broadband (0.01~18GHz) Coaxial Power Benchmark Project and the 2003 Special Project on Value Transfer and Quality Safety of the General Administration of Quality Supervision, Inspection and Quarantine: High-frequency and Microwave Power, Attenuation Transfer Standard and Value Traceability System Project.

1.2 Development and Status of High-frequency and Microwave Power Measurement

1.2.1 Characteristics of high frequency and microwave power measurement

High-frequency and microwave power measurement refers to the measurement of signal power in high-frequency and microwave frequency bands, generally from 10MHz to 300GHz.

The unit of measurement for high-frequency and microwave power is watt (W) and its decimal multiples, such as picowatt (pW=10-12 ^W ), nanowatt (nW=10-9 W), microwatt (uW ^{= 10-6} W), milliwatt (mW=10-3 ^W ), kilowatt (kW=103 ^W ), megawatt (MW=106 ^W ), gigawatt (GW=109 ^W ), but the logarithmic units decibel watt (dBW) and decibel milliwatt (dBmW) are often used in applications. dBW is a logarithmic unit with a base of 10 that uses 1W as a reference power level to express the magnitude of power; dBmW is a logarithmic unit with a base of 10 that uses 1mW as a reference power level to express the magnitude of power. The logarithmic unit expression for high-frequency and microwave power is:

Where P is the power value in W or mW. _P0 is the reference power in W or mW. High-frequency and microwave power measurements can be divided into two categories according to the purpose of measurement: Power itself is the parameter being studied. For example, determining the transmission power of a radar device or measuring the power compression of an amplifier.

Power measurement is performed to measure other parameters. Power is a basic parameter for electronic measurement. Many other parameters can be derived from power and are called derived parameters, such as attenuation and impedance. The power ratio method in attenuation measurement and the six-port reflectometer method in impedance measurement belong to this type of power measurement.

Compared with DC and low-frequency power measurements, high-frequency and microwave power measurements have the following characteristics:

1) Wide measurement range: Conventional high-frequency and microwave power measurements range from nanowatts to megawatts. As for the actual power measurement range, it is even wider. The noise and signal power sent back to the ground from radio stars or spacecraft are mostly less than 10-13 ^W , while the pulse power emitted by long-range radars to space is as high as 10-10 ^W or more. As mentioned earlier, the frequency of high-frequency and microwave power measurement usually ranges from 10MHz to 300GHz, but by comparing with voltage, the measurement range of high-frequency and microwave power meters can be extended to DC.

2) Many types of transmission lines and connectors: As the frequency increases from low to high, the transmission lines in electronic systems include double-wire, cable, coaxial line, stripline, microstrip line, metal waveguide, dielectric waveguide and other types. Each type of transmission line has different models, sizes and specifications according to different frequency bands and different impedances. For example, for the commonly used 50 ohm impedance coaxial transmission line, there are different connector forms, including APC-14, N/APC-7, APC3.5, K/2.92 and 2.4 mm. In addition to the 50 ohm impedance system and connector, the coaxial transmission line also has a 75 ohm impedance system and connector. For the metal waveguide system that transmits non-TEM waves, it is further subdivided into many waveguide bands with different cross-sectional sizes. At present, the most widely used is the rectangular cross-sectional metal waveguide system, and the commonly used bands are 1mm, 3mm, 8mm, 1.25cm, 2cm, 3cm, 5cm and 10cm rectangular cross-sections.

Due to the variety of transmission lines and connector forms, in addition to the complexity of mechanical connection, it also causes changes in the electrical properties of electromagnetic wave transmission. The impact of changes in electrical properties on measurement is mainly manifested in the measurement uncertainty caused by electromagnetic leakage and impedance mismatch.

For such a wide range and frequency band, such complex transmission lines and connector forms, it is obviously necessary to adopt different measurement methods and measurement equipment, and to establish corresponding measurement standards and instruments for this purpose.

1.2.2 Measurement methods and instruments for high frequency and microwave power

Since high-frequency and microwave power measurements are based on converting high-frequency or microwave energy into energy forms such as heat, force, DC or low-frequency electricity and then measuring it, a power measuring instrument is always composed of a conversion part that senses, absorbs and realizes energy conversion and a corresponding indicator. Generally, a power measuring instrument is called a power meter, the energy conversion part is called a power socket, power probe or power sensor, and the corresponding indicator is called a power indicator. Its structure is shown in Figure 1-1.

The common power meters are as follows:

1) Calorimeter: A calorimeter is an instrument that measures the temperature change after a load absorbs power. The measured power value is obtained based on the difference in temperature change caused by the DC power and the measured power. According to the difference in the absorbed power load, it is divided into flow calorimeter[9] and dry calorimeter[10]. At present, except for flow calorimeters used for medium and high power measurement, calorimeters are rarely used as commercial power meters. Many national power standards use dry calorimeter designs with a power range of milliwatts.

2) Bolometer: The bolometer is named after the use of a bolometer element as a power sensor. There are three common types of bolometer elements: ballast resistor, thermal resistor and thermistor. The bolometer can be regarded as a simplified calorimeter. The bolometer element will change its resistance when it absorbs high-frequency or microwave power, which causes a temperature rise. This change can be detected by the bridge in the power indicator. If a balanced bridge such as a Wheatstone bridge and a four-wire resistance bridge is used, the high-frequency or microwave power can be calculated based on the change in DC power of the bolometer element before and after absorbing high-frequency or microwave power. The most common bolometer currently is the thermistor power meter, which is generally only used for microcalorimeter power reference and value transfer and is rarely used as a commercial power meter. The power measurement range is generally (1~10) mW.

3) Thermoelectric power meter: A power meter that uses the thermoelectric effect (Seebeck, Peltier and Thomson effect) to measure power is called a thermoelectric power meter or a thermocouple power meter. It can be divided into two types, self-heating type and external heating type, according to the position of the thermoelectric element (thermocouple). While measuring temperature changes, the self-heating thermocouple is also a load that absorbs high-frequency or microwave power. It has high sensitivity and short response time, but has a large reflection coefficient and poor overload capacity, and cannot measure signal power below 100 kHz. The external heating thermocouple measures the temperature rise of the load right behind the absorbing load. It has a small reflection coefficient and strong overload capacity. It can measure signal power up to DC, but its sensitivity is lower than that of the self-heating type and its response time is also long [19]. Thermoelectric power meter is the most commonly used commercial power meter, and its power measurement range is generally -30dBm to 20dBm.

4) Diode power meter: A power meter that uses diode detection to measure power is called a diode power meter. Most of the crystal diodes used in the early days were point contact diodes. Due to their fragile structure, poor consistency, and poor stability, they can only be used as an indication of relative level, but not for absolute power measurement.

With the improvement of semiconductor technology, a new type of crystal diode, the low-barrier Schottky diode, has been developed for use in diode power meters. This new type of surface-contact low-barrier Schottky diode has good square-law characteristics [20]. In order to suppress the influence of harmonics, the current diode power meters all use a dual diode structure. This structure, similar to a voltage doubler, can effectively suppress the second and higher even harmonics. In order to obtain better linearity and a wider power measurement range, the latest diode power meters use diode cascade, dynamic channel switching, and automatic correction technology. Diode power meters are also one of the most commonly used commercial power meters, and the power measurement range is generally -70dBm to 20dBm.

5) Other power meters: There are also some power meters that use other physical effects, such as mechanical effect power meters, Hall effect power meters, quantum interference effect power meters and electronic beam power meters. They have only been developed in a few laboratories for exploratory research on power measurement methods. Due to the harsh use conditions, low measurement accuracy or too narrow measurement range, they are no longer used. The

main technical indicators describing commercial power meters include: power range, frequency range, input reflection coefficient of power sensor, correction factor K, etc., where K is defined as

Where Pm _is the reading of the power indicator and Pi _is the incident power of the power meter.

In addition, the effective efficiency ηeff _and the calibration factor Kb _are quantities that specifically describe the characteristics of the bolometer and are defined as

Where Pb _is the DC replacement power of the bolometer, and PL _is the high-frequency and microwave power absorbed by the bolometer.

According to the basic measurement principle, this article divides high-frequency and microwave power meters into two categories:

1) Direct measurement type: that is, using the thermal effect of high frequency or microwave power to measure the temperature change after absorbing high frequency or microwave power, and derive the value of the measured power based on the temperature change. Calorimeters and bolometers belong to this category.

The principle of direct measurement type power meters is the first law of thermodynamics.

That is, the change in the temperature of an object is related to the total work applied to it. If the temperature of the object can be kept constant by reducing or increasing the DC or low-frequency power while increasing or decreasing the high-frequency or microwave power, then the change in high-frequency or microwave power is equal to the change in DC or low-frequency power. This is the isothermal method. The high-frequency or microwave power and the DC or low-frequency power can also be changed separately. If the power change brings about the same temperature change, it means that the power change is equal. This is the heating method. Both methods are alternative measurement methods. Through substitution, the values ​​of high-frequency and microwave power are traced back to DC power, and then traced back to basic physical quantities. Figure 1-2 shows the traceability chain from the values ​​of high-frequency and microwave power to basic physical quantities.

2) Indirect measurement: that is, measuring quantities related to power, such as voltage and field strength, and then using the relationship between them and power to calculate the power value. Thermoelectric and diode power meters, as well as the power meters using other physical effects mentioned above, belong to this category.

In theory, any physical quantity or principle that can be related to power can be used to measure power, such as some new physical phenomena discovered in recent years, including the relationship between electromagnetic field strength and the Raman oscillation frequency of cesium fountain atoms, the modulation of electromagnetic field strength on lasers [28], and high-frequency voltage and power measurement using MEMS capacitors. However, in actual applications, the following points must be considered:

1) It should have a linear relationship with power, or a small nonlinearity.

2) It should have a stable relationship with power, good repeatability, and be relatively less affected by the environment.

3) It should be easy to implement and be able to measure power in a wide frequency band.

The above phenomena and effects cannot meet such requirements for the time being, so currently only thermoelectric and diode power meters are used for actual power measurement.

1.2.3 High frequency and microwave power measurement system

In order to ensure the accuracy and consistency of power values, there is currently a complete value system in the world.

The power value system can be divided into three levels and two transmission processes.

1) Power benchmark and international comparison: First, each country establishes its own power benchmark based on physical principles to complete the reproduction of power values. Then, according to the international mutual recognition agreement, participate in the international comparison organized by the International Bureau of Weights and Measures (BIPM). Since high-frequency and microwave power are very important basic quantities, the first international comparison of radio parameters is power comparison. Since 1950, more than 10 international power comparisons have been carried out. Through international comparisons, countries can find the differences between each other's power benchmark values ​​and coordinate the international reference values ​​of power values.

2) Transmission of the power benchmark vector value transmission system: The middle layer is the power value transmission system, and the value transmission devices used by the calibration laboratories of measurement units and manufacturers belong to this level. There are also multiple levels within the transmission system. The highest level of value is transmitted by the national benchmark, and then transmitted downward step by step. Since the national benchmark only gives the value of continuous wave low power, the values ​​of medium power and pulse peak power are all realized at the power transmission system level.

3) Transfer from the value transfer system to the working measuring instrument: The bottom layer is the working measuring instrument, which is directly used for actual power measurement applications. After multiple levels of transfer within the value transfer system, the final power value is transferred to the working measuring instrument. As an example, Figure 1-3 shows the schematic diagram of my country's N-type coaxial power value transfer.

The value transfer of high frequency and microwave power is different from the value transfer of parameters such as noise and attenuation. Generally, what is transferred is the calibration factor, that is, the ability to transfer power measurement, or the characteristics of the power meter, rather than the value of the power source. The basic method of value transfer is the comparison method, that is, comparing the reading difference of the standard power meter and the calibrated power meter for the same signal, so as to obtain the characteristics of the calibrated power meter and realize value transfer.