计量科学与技术

Low-Frequency Electric Field Standard Device Based on Parallel Plates

LIU Zhipeng, LIN Haoyu, WU Yanli, LI Enguang, ZHANG Bolin

2024, 68(8): 3-9, 37. doi: 10.12338/j.issn.2096-9015.2024.0086

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Due to the varying quality of probes produced by different manufacturers, the development of electric field standard devices has become particularly important. To meet the calibration requirements of low-frequency, high-field-strength electric field probes, a low-frequency electric field standard device was developed using the parallel plate method. The device consists of two square aluminum plates with a side length of 1 m and a plate spacing of 0.5 m. The frequency range is DC to 10 kHz, with an electric field strength range of 0 to 3000 V/m. Through theoretical analysis, software simulation, and experimental measurements, the effects of edge effects, plate material, structure shape, operating frequency, and other factors on the standard electric field generated by parallel plates were studied, and an uncertainty evaluation was conducted. A new measurement model was proposed, and uncertainty components caused by seven factors were evaluated separately: plate voltage, plate spacing, field uniformity within the parallel plates, probe fixture, probe alignment, finite size of parallel plates, and probe influence. The resulting expanded uncertainty is U=6.8% (k=2). The results show that the developed electric field standard device can meet the calibration requirements for low-frequency electric field probes.

Research on a Monopole Antenna Calibrator Based on the Equivalent Capacitance Method

LIU Guanjun, CHEN Yisheng, CHANG Zhifang

2024, 68(8): 10-17. doi: 10.12338/j.issn.2096-9015.2024.0101

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The equivalent capacitance method is the primary calibration approach for monopole antennas outlined in CISPR 16-1-6. This method employs an equivalent capacitor to simulate the actual monopole element for calibrating the corresponding antenna factor, with the capacitance of this equivalent capacitor matching that of the monopole antenna itself. Based on the working principle of the equivalent capacitance method, a low-cost, miniaturized, and highly versatile antenna calibrator has been designed. Simulation and experimental results demonstrate that this calibrator is well-suited for calibrating monopole antennas in the frequency range of 9 kHz to 30 MHz. The evaluation process and results of the measurement uncertainty are presented, showing that when calibrating the antenna factor of monopole antennas using this device, the extended uncertainty of the measurement results is approximately 1.7 dB (k=2).

A Review of Electric Field Measurement and Microwave Sensing Based on Rydberg Atomic Apertures

ZHAO Danhua, CHEN Jiarui, BAO Luwei, ZHU Jiahui

2024, 68(8): 18-24, 70. doi: 10.12338/j.issn.2096-9015.2024.0148

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In recent years, the application of Rydberg atoms in microwave measurement has emerged as a research hotspot in quantum metrology. Compared to traditional microwave sensing technology, Rydberg atomic systems demonstrate higher sensitivity, stronger anti-interference capabilities, and unique quantum traceability. These advantages give Rydberg atoms great potential in microwave measurements. Despite these advantages, the complex energy level structure of Rydberg atomic systems and their diverse interactions with electromagnetic waves pose challenges for engineering applications. Currently, the application of “atomic aperture” sensing technology based on Rydberg atoms in traditional microwave sensing is still in its initial stages, with significant room for improvement. To fully leverage the advantages of Rydberg atoms in microwave measurement and address their limitations in practical applications, we discuss the prospects of Rydberg atomic systems in current electromagnetic wave transceiver technologies, building on previous research findings. With ongoing technological advancements, Rydberg atoms are expected to achieve precise measurements across a wider range of frequency bands, providing robust support for wireless communication, radar detection, and other related fields.

Comparison of Calibration Methods in Free-Space Monostatic Reflection Coefficient Measurement

YANG Ruonan, LIANG Weijun, XU Hao

2024, 68(8): 25-31. doi: 10.12338/j.issn.2096-9015.2024.0080

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Microwave blackbodies provide high-precision brightness temperature signals for microwave radiometers to accurately calibrate observed target radiation signals. The emissivity of a microwave blackbody is a crucial parameter affecting its radiative characteristics. Therefore, accurately measuring blackbody emissivity is significant for enhancing radiometer calibration precision and ensuring measurement value traceability and effective transfer. Currently, blackbody emissivity is mainly obtained indirectly by measuring reflectivity. This study implements two calibration methods in free-space monostatic reflection coefficient measurement: the offset-short calibration method and the sliding-load calibration method. Time-domain gating techniques are utilized to address multipath reflection signals during small reflection measurements. A reflectivity measurement system was established, and the reflectivity of the same blackbody target was measured within the 75-110 GHz frequency band, with results analyzed and compared. The error terms solved by the two calibration methods exhibit high consistency, with measured emissivity reaching levels of 0.999-0.9999. When the measurement target satisfies approximation conditions, the sliding-load calibration method proves more efficient. Finally, using the offset-short method as an example, the Monte Carlo method was employed to evaluate the uncertainty of the solved blackbody target reflection coefficient.

Analysis of the Equivalent Source Reflection Coefficient and Limitations of K. Shimaoka’s Measurement Method

JIA Chao, ZHANG Yihang, CHEN Shuo, LI Yong, CUI Xiaohai

2024, 68(8): 32-37. doi: 10.12338/j.issn.2096-9015.2024.0090

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The equivalent source reflection coefficient plays a crucial role in microwave power measurement and uncertainty evaluation. This paper first discusses the relationship between the source reflection coefficient in amplitude-stabilized signal source systems and the equivalent source reflection coefficient when measuring power ratios using three-port devices. The concept of "equivalent" in the equivalent source reflection coefficient is explained from the perspective of the mismatch factor. Next, the working principle and limitations of K. Shimaoka's method for measuring equivalent source reflection coefficient using a network analyzer and three-port devices are briefly introduced. Finally, experiments and comparisons are conducted using N-type power dividers and directional couplers as measurement objects, based on K. Shimaoka's method and the traditional formula method in the 1-18 GHz frequency range. Results indicate that K. Shimaoka's method for measuring equivalent source reflection coefficient has certain limitations. Due to the small difference in transmission coefficients obtained in this calculation method, it is sensitive to minor changes and thus unsuitable for measuring equivalent source reflection coefficients when using three-port devices with good directionality (such as directional couplers).

Comparison of Direct Comparison Method in VNA and Power Transfer Standard Measurements

CAI Chengxin, ZHANG Yihang, YUAN Wenze, LI Yong, CUI Xiaohai

2024, 68(8): 38-43, 70. doi: 10.12338/j.issn.2096-9015.2024.0141

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The calibration factor is a parameter used to characterize the reading accuracy of RF power sensors, defined as the ratio of the power displayed by the sensor to the incident power. It is the object of power value transfer. This paper introduces the composition and measurement principles of the direct comparison method and the VNA-based direct comparison method. It describes the application of the equivalent source reflection coefficient in the direct calibration method. The calibration factor of the Rohde & Schwarz NRP50T power sensor was measured using both the direct calibration method and the VNA-based direct comparison method. Results show that the maximum difference between the corrected calibration factors measured by the two methods is 3.056%. The equivalent source reflection coefficient of the VNA-based direct comparison method is generally larger than that of the direct comparison method. The uncertainty evaluation method for the direct comparison method is well-established, making it currently the most prevalent RF power sensor calibration method.

Long-Term Performance Analysis of Spectrum Analyzers Based on Metrological Scientific Data

ZHANG Yue, DING Sheng, LIN Tao, ZHAO Kejia, ZHANG Aimin

2024, 68(8): 44-50, 63. doi: 10.12338/j.issn.2096-9015.2024.0133

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Spectrum analyzers are key instruments widely used in radio electronics. The long-term stability of their critical parameters has been a significant concern for users and manufacturers. Accurate tracking of technical specifications plays a crucial role in the research, development, improvement, usage, and maintenance of these instruments. The National Institute of Metrology has accumulated calibration data for spectrum analyzers spanning over a decade. These metrological scientific data are characterized by traceability and accuracy. Based on these data, this paper introduces the operational principles, key performance specifications, and measurement calibration systems and methods of spectrum analyzers. It focuses on analyzing variations in the displayed average noise level and input frequency response during long-term usage of typical spectrum analyzers among representative domestic users. This study aims to provide a more reliable basis for the development, production, testing, and maintenance of spectrum analyzers.

Experimental Study on the Influence of Input Power on Characteristic Parameters of Electromagnetic Reverberation Chambers

ZHANG Haomin, GUO Xiaotao, LIU Ke, LIU Tianxin

2024, 68(8): 51-57. doi: 10.12338/j.issn.2096-9015.2024.0063

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Electromagnetic reverberation chambers are typically calibrated for characteristic parameters such as field uniformity and normalized field strength under low input power conditions. However, in practical applications like electromagnetic compatibility immunity testing, these chambers operate under high input power conditions. Currently, relevant international and domestic standards ignore this difference, providing no specific theoretical or experimental evidence. This paper presents a designed experimental study to investigate the influence of input power on the characteristic parameters of electromagnetic reverberation chambers. A rapid calibration technique using 3D photoelectric field probes was employed for low input power calibration, while high input power parameter measurements were conducted under conditions consistent with actual automotive component immunity tests. Results show that the impact of different input power levels on field uniformity is generally within ±0.5 dB, and the effect on normalized field strength is within 2 dB. Based on these findings, recommendations are provided for calibrating field uniformity and normalized field strength during electromagnetic reverberation chamber calibration and testing processes.

Study on Calibration Results of Thermistor-Type Power Sensors

NIE Lu, LI Xiangjun, CUI Xiaohai, WANG Zihao, ZHAO Yan

2024, 68(8): 58-63. doi: 10.12338/j.issn.2096-9015.2024.0075

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Thermistor-type power meters use DC substitution to measure microwave and millimeter wave power. To compensate for environmental temperature changes, these power meters typically employ both measurement and compensation bridges, operating in a dual-bridge mode. When calibrating a coaxial thermistor-type power sensor using a microcalorimeter power standard, the measurement bridge alone cannot be used, even with stable environmental temperatures. To ensure valid calibration results, both environmental temperature compensation and ceramic core temperature variation compensation (due to dual-element errors) are necessary. Experiments show that at 18 GHz and 10 mW power level, the substituted power deviation for an N-type coaxial thermistor power sensor is 0.38% when measured with single versus dual-bridge power meters. For waveguide thermistor-type power sensors with single-element structures, calibrating effective efficiency does not require consideration of single versus dual-bridge measurement issues.

Full Waveform Calibration Technique for Millimeter-Wave Wideband FMCW Signals

HE Zhao, XU Qinghua, LIU Xuefei, ZHANG Yichi

2024, 68(8): 64-70. doi: 10.12338/j.issn.2096-9015.2024.0126

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To address the calibration needs of Frequency Modulated Continuous Wave (FMCW) signal testing equipment in millimeter-wave automotive radar applications and enhance measurement capabilities for complex analog-modulated signals in millimeter-wave broadband, we developed a hybrid measurement system. This system combines a frequency-domain full waveform metrology setup with FMCW testing equipment under calibration. By conducting in-situ, synchronous measurements of custom millimeter-wave FMCW signals, we achieved calibration and performance verification of commercial instruments. We thoroughly compared the performance of different measurement methods and equipment in the typical millimeter-wave radar frequency bands of 24 GHz and 60 GHz. Experimental results demonstrate that the frequency-domain full waveform metrology method outperforms relevant commercial instruments. The FMCW chirp deviation measurements under both 200 MHz narrowband and 2 GHz wideband conditions were an order of magnitude more precise, meeting the technical requirements for calibration and performance verification.

Nonuniform Frequency Sampling Technique for Near-Field Scattering Measurements

SI Weikang, WANG Weilong

2024, 68(8): 71-77, 17. doi: 10.12338/j.issn.2096-9015.2024.0066

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Traditional stepped frequency (SF) systems synthesize a large measurement bandwidth using a series of equidistant discrete frequency signals. However, due to sampling theorem limitations, time-domain signals corresponding to fixed frequency intervals exhibit periodicity. Consequently, background and multipath interference in the measurement environment may alias into the target area, adversely affecting imaging and scattering measurements. This paper explores rapid measurement methods for frequency-stepped systems and proposes a near-field scattering measurement technique based on nonuniform sampling. We analyze the impact of range ambiguity on scattering measurements and design optimization principles for the sampling function based on practical application requirements. The signal envelope is shaped using Poisson's formula and the principle of stationary phase (POSP). To address scattering image degradation caused by traditional nonuniform sampling reconstruction methods, we propose a weighted method compatible with the nonuniform sampling strategy. This method effectively suppresses aliasing interference, achieves high-resolution imaging, and improves the accuracy of near-field to far-field (NF-FF) transformations.

Data Processing Methods for Quiet Zone Reflectivity Level in Microwave Anechoic Chambers

ZHAO Xing, BAN Hao, HONG Li, LIU Xiao

2024, 68(8): 78-82, 37. doi: 10.12338/j.issn.2096-9015.2024.0087

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Microwave anechoic chambers are enclosed spaces specifically designed for measurements in the microwave frequency range, used as experimental sites for antenna and target scattering characteristics measurements to simulate free space. Compared to outdoor test sites, these chambers offer advantages of all-weather operation and low reflection. However, due to the imperfect absorption of electromagnetic waves by internal materials in specific frequency bands, interfering reflected signals still exist in the test area. These signals enter from various directions and overlap with direct waves between transceiver antennas, forming spatial standing wave distributions over certain distances. The interference reflection index in the test area is characterized by the quiet zone reflectivity level. When using microwave anechoic chambers to measure parameters such as antenna radiation patterns, the reflectivity level at different angles should be evaluated. The free-space voltage standing wave ratio (VSWR) method is commonly employed to measure this reflectivity level. In the test area, the vector sum of direct and reflected signals forms spatial standing waves along specific paths. The data processing method for these standing waves determines the calculated reflectivity level of the quiet zone, with the selection of peak-to-peak values in actual test curves directly influencing measurement results. This article discusses in detail the collection and processing methods of reflection level data in microwave anechoic chambers, aiming to optimize on-site measurement and testing schemes. Additionally, this research provides valuable insights for the optimal design of quiet zone scanning frames, potentially enhancing the overall accuracy and reliability of microwave measurements in anechoic environments.

2024 Vol. 68, No. 8

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