Yozo Shimada (Division Head) and Kazuhiro Shimaoka (Senior Researcher), Electromagnetic Waves Division, the Metrology Institute of Japan (Director: Koichi Chiba) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Ryoji Chubachi), in collaboration with the National Institute of Information and Communications Technology (NICT; President: Masao Sakauchi), have developed a national measurement standard for calibration of ultrahigh-frequency power meters in the 110 - 170 GHz band.

An isothermal control technology for the measurement of radio-frequency (RF) power and direct-current (DC) power converted into heat and a technology for their equivalency evaluation have been newly developed. These technologies were used to achieve a national measurement standard for the calibration of RF power meters in the ultrahigh-frequency band of 110 to 170 GHz that was previously difficult, and have enabled the traceable calibration of RF power meters. In addition to improving the reliability of electromagnetic interference testing for millimeter-wave automotive radars, it is expected to contribute to the development of security systems in airports, evaluation technology for electromagnetic shielding materials, and the ultrafast semiconductor field.

Details of these results will be presented at the 2014 General Conference of the Institute of Electronics, Information and Communication Engineers to be held on March 19 to 21, 2014 at Niigata University (Niigata City, Niigata).

(Photo) : Developed national measurement standard for calibration of radio-frequency power meters

In recent years, application of terahertz waves, the electromagnetic waves in the frequency range from 100 GHz to several THz, is becoming popular in the field of communication technology, material analysis, security, study of artwork and other fields. For example, the use of the terahertz wave band, where there are still very few users, is being studied for wireless large capacity information transmission, which requires a wider bandwidth. However, as terahertz waves are easily absorbed by the atmosphere, there has been a delay in their use, as well as in the research into its precise measurement methods, which is known as the “terahertz gap.” Of the most basic measurement quantities related to terahertz waves, a technology known as the “optical frequency comb” is being used to develop the frequency measurement standard, but for power, development has been delayed in many countries, and there is a growing demand to establish a primary standard that will act as the national measurement standard. Furthermore, there is also a need to measure the spurious power in electromagnetic interference tests in the frequencies (121 GHz, 153 GHz, and 159 GHz) that are double the carrier frequencies used for millimeter-wave automotive radars (60.5 GHz, 76.5 GHz, and 79.5 GHz), and because of these situations, the demand for a traceable RF power meter is rising.

In an effort to establish measurement methods for a wireless high-capacity information transmission system using the 120 GHz frequency band, one of the study groups in the Technical Examination Services of the Ministry of Internal Affairs and Communications made an enquiry to AIST at the end of 2010 regarding the possibility of conducting RF power calibration in this frequency band using a national measurement standard. However, at the time, RF power calibration in this frequency band using a national measurement standard was unavailable not only in Japan but also at the national metrology institutes in Europe and the United States. Accordingly, a joint research was started between AIST and NICT to achieve an RF power calibration service ahead of other nations.

Generally, an RF power meter is used to measure RF power. However, with these kinds of power meter, calibration must be conducted by comparing the meter with a power meter of even higher accuracy to guarantee its accuracy. The measuring instrument with higher accuracy that acts as the standard for accurate calibration is called the measurement standard. In the present study, the researchers have developed a measurement standard that is able to measure RF power (measurement made in Watts (W)) in the 110 to 170 GHz frequency band with an accuracy of 4%.

With this measurement standard, the RF power to be measured will initially be absorbed into an electromagnetic wave absorber and converted into heat. The most accurate measurement of RF power can be made by comparing this heat to the equivalent DC power. Two new technologies were developed to achieve these thermal measurements.

One is a thermal measuring method using isothermal control technology, which has enabled thermal measurements to be made about ten times faster than conventional methods. With this method, highly accurate measurement results can be obtained during the relatively short time when the signal source output is stable. The other is an equivalency evaluation technology that compares the RF power and DC power that have been converted into heat. Generally, for RF power and DC power converted into heat, there is a difference in thermal distribution inside the electromagnetic wave absorber creating a difference in measured values. This time, by conducting a detailed analysis of thermal distribution difference within the electromagnetic wave absorber, a technology to accurately evaluate RF power has been developed.

Figure 1 is a photograph of the national measurement standard for the calibration of RF power meters in the 110 to 170 GHz frequency band (Isothermally-controlled waveguide-type twin-dry calorimeter) developed using these technologies. Figure 2 shows the principle of its operation.

Figure 1 : Isothermally-controlled waveguide-type twin-dry calorimeter,  the national measurement standard developed in the present study

Figure 2 : Operating principle of the isothermally-controlled waveguide-type twin-dry calorimeter

In the isothermally-controlled waveguide-type twin-dry calorimeter, a DC heater is installed around the electromagnetic wave absorber. When there is no input of electromagnetic waves, DC power of h₁ is consumed and converted into heat by the heater. There is a thermo-electric element and a temperature reference block behind the electromagnetic absorber. The temperature difference between the electromagnetic absorber and the temperature reference block is detected by the thermo-electric element and isothermal control is conducted so that the temperature difference is maintained at zero. When electromagnetic waves are generated by the electromagnetic wave generator, the electromagnetic waves pass through the adiabatic waveguide and are absorbed by the electromagnetic wave absorber and converted into heat. The temperature of the electromagnetic wave absorber will rise against the temperature of the reference block. When this occurs, DC current is reduced to h₂ so that the temperature difference will again return to zero. Electromagnetic power P_s can be measured as the amount of reduction of DC power (Fig. 2 right).

By using this measurement standard, the measurement of RF power in the 110 to 170 GHz frequency band, conventionally without a clear-cut standard and in which the variance was 30% or higher between the various measuring equipment manufacturers, can be determined to an accuracy of 4 %. In addition to its contribution in the improvement of reliability in electromagnetic interference tests for millimeter-wave automotive radars, whose use is rapidly increasing, there are expectations for its use in the development of security systems in airports, evaluation technology for electromagnetic wave shielding materials, and in the ultrafast semiconductor field.

Using the developed national measurement standard for calibration of RF power meters, NICT will launch an RF power meter calibration service for wireless communication equipment on March 25, 2014. AIST will develop application technology for even higher frequencies to realize a national measurement standard for frequency band in excess of 300 GHz within several years.