Among the critical quantities of radio frequency and microwave measurement, power is a fundamental one in the electromagnetic measurement standard. Power measurement is made by measuring the equivalent DC substituted power which has electromagnetic energy delivered into the power sensors. The calibration of RF and microwave thermistor mount is to transfer the effective efficiency or calibration factor from a national secondary standard to a measurement reference standard.
In recent years, with the advancement of technology, precision power meters based on the DC substituted technique have been developed for commercial use, making it possible to measure simpler and faster than the previously developed calibration system. Therefore, the purpose of this paper is to improve the calibration measurement capability of national calibration laboratory by designing a calibration system through direct comparison transfer based on the DC substituted technique for calibration of the thermistor mount. To this end, a calibration system was designed and a study was conducted to improve mismatch errors.
In the calibration system of the coaxial reference thermistor mount, a matching attenuator was connected between the signal source and the load for reducing mismatch errors and the minimum required specification of the attenuator was determined as a 10 dB attenuator with a VSWR of 1.02 or less. In order to estimate the reflection coefficient of the signal source, which occupies the largest proportion of the mismatch error, the reflection coefficient of the equivalent signal source was modeled and corrected through the analysis of the signal flow graph of the 2-port network. The asymmetry between the output ports of the RF switch can be eliminated through cross-measurement techniques between standard thermistor mounts and the unit under test.
The feedthrough thermistor mount which is combined with the terminating thermistor mount and the power splitter is calibrated as one assembly. Therefore, a method for correcting the mismatch factor modified in the parallel power ratio method and estimating the calibration factor when an attenuator or adapter is connected to the unit under test is proposed. The mismatch factor according to the measurement configuration was modeled by applying the signal flow graph analysis and non-contact loop rule of the 3-port network and 2-port network.
To evaluate the performance of the thermistor mount calibration system proposed in this paper, tolerance analysis and measurement uncertainty evaluation were performed. The coaxial thermistor mount calibration system from 10 MHz to 1 GHz was analyzed to have a maximum measurement error of less than 0.46 % and a measurement uncertainty of less than 0.5 %. It was confirmed that the minimum required performance of the designed system was satisfied with less than 0.5 %. In the frequency band of 10 MHz to 18 GHz, the measurement error of the feedthrough thermistor mount calibration system is less than 1.9 % at the maximum, and the measurement uncertainty is less than 1.8 %, which is verified to meet the minimum required performance of less than 2.5 %. In the measurement configuration with an attenuator connected, the proposed measurement method confirmed that the measurement error was 3.6 % and maintained below 4 %, which is the system performance. As it was confirmed that the measurement error of the measurement configuration to which the adapter is connected is less than 2.7 % at the maximum and is satisfied within 2.9 % of the minimum required performance, it was confirmed that the designed calibration system satisfies all the minimum required performance.
The calibration system proposed in this study is expected that the overall calibration measurement capability of the industry will be improved by securing its own calibration capability for the commercial primary power measurement standard in the national calibration laboratory.