Reverberation Chamber Measurements Research Articles

Frequency Stirring Effects on Reference Antenna Efficiency Measurements in Reverberation Chambers

Frequency Stirring Effects on Reference Antenna Efficiency Measurements in Reverberation Chambers

Diffusion equation modeling in room-acoustic applications

The diffusion equation application in room acoustic simulations can be traced back to 1969. However, much attention from room-acoustic community to the diffusion equation was given in the late 1990s and the early 2000s. In the literature, the acoustic diffusion equation may be considered as an approximation of the acoustic transport equation. The diffusion equation modeling has been considered suitable for applications in enclosed spaces with proportionate dimensions. The most intriguing features of the diffusion equation are its computational efficiency due to a sparse meshing condition and its capability of energy flux solutions due to Fick’s law. Since the late 2000s, the diffusion equation modeling has evolved with boundary conditions formulated by the rigorous physical–mathematical theory [Jing and Xiang, JASA 123, 145–153 (2008]] and its validity has also been systematically investigated [Xiang et al., JASA 133, 3975–3985 (2013)]. This paper reviews the vibrant development in the room-acoustics during this era and discusses wide range of applications from coupled-volume systems, historically significant worship spaces to recent investigations on sound absorption measurements in reverberation chambers. This paper also discusses its link to the acoustic transport equation and compares the modeling results of both. Different numerical approaches to solving the diffusion equations will also be reviewed.

Analysis of Concurrent and Correlated Stirring Mechanisms in Reverberation Chambers

A new multivariate statistical analysis is proposed to derive the estimates of effective sample sizes of reverberation chamber (RC) measurement data. Concurrent correlation effects, related to different and simultaneously acting stirring mechanisms, are taken into account and individually quantified. The mean and variance of correlated measurement data are also estimated, as well as the variance of the grand mean. The analysis is independent on the parent probability distributions. Formulae are obtained through a step-by-step derivation, stemming from basic statistical principles and general assumptions, which are deemed to be applicable to the correlation structures of RC measurement data. Experiments and comparison with closed-form predictions confirm the validity and generality of the analysis.

A Hybrid EMC Testing Facility: Combining Transmission Line and Reverberation Chamber Measurement System

By combining a transmission line system (TLS) and a reverberation chamber (RC), a hybrid electromagnetic compatibility (EMC) testing facility is designed and constructed. Generally, the lowest usable frequency (LUF) of an RC is limited by its dimension, which limits the application of RCs for EMC testing at low frequencies. Therefore, to improve the field uniformity (FU) of an RC at frequencies lower than the LUF, a TLS is integrated into the RC. After optimizing the load resistance, length, and width of the TLS, the resonant frequency and electric field spikes of the hybrid system are eliminated. The FU of the E-field in the system is greatly improved in the frequency range of 0-30 MHz. Moreover, using an oscillating wall stirrer in the RC, the FU satisfies the standard (IEC 61000-4-21) above 80 MHz. Results show that combining the TLS and the RC testing system could be widely used for EMC testing in the frequency range of 0-30 MHz and 80 MHz-6 GHz.

Effects of User Proximity on Total Radiated Power and Total Isotropic Sensitivity Measurements in Reverberation Chambers

Effects of User Proximity on Total Radiated Power and Total Isotropic Sensitivity Measurements in Reverberation Chambers

Practical Correlation-Matrix Approaches for Standardized Testing of Wireless Devices in Reverberation Chambers

We extend the autocorrelation-based approaches currently used in standards to full correlation-matrix-based approaches in order to identify correlation between both spatially adjacent and non-adjacent samples in reverberation-chamber measurements. We employ a scalar metric that allows users to identify the number of effectively uncorrelated samples in new types of stirring sequences. To make these approaches practical and enhance their accuracy, we implement a thresholding technique that retains correlation related to important aspects of chamber configuration such as loading and undermoded conditions. We develop a method to propagate uncertainty in the complex correlation coefficients through to the number of effective samples for a given reverberation-chamber set-up by use of a bootstrap technique that is accurate even for highly skewed distributions of correlation coefficients. We further apply this method in a sensitivity study regarding the choice of threshold value. Agreement with existing approaches in determining the number of effectively uncorrelated samples is presented for a measurement example where spatially adjacent samples are utilized. Examples are then illustrated for non-spatially-adjacent correlated samples at microwave and millimeter-wave frequencies.

A Comparative Study of Error Vector Magnitude Measurement in a Reverberation Chamber

This letter shows a comparative analysis between the conventional and reverberation chamber (RC) measurements of error vector magnitude (EVM). The conventional methods, including direct link (DL) and anechoic chamber (AC), have been used to measure EVM for several years. However, the over-the-air testing of devices under test (DUT) is emphasized, and the EVM measurement in RC is a difficult question. For mass production testing, instead of using two chambers (RC and AC), it is expected to integrate EVM testing into the existing RC measurement system to give a pass/fail conclusion quickly. Hence, we measured EVM in 2 GHz (2412 MHz) and 5 GHz (5180 MHz) using RC, AC, and DL methods. From the measurement results, we find that the measured EVM is repeatable, stable, and insensitive to the location of DUT. Besides, the measured EVMs obtained from all scenarios deteriorate with the decrease of received power. Comparing the results from RC with that from the conventional methods, it could be concluded that with the reduction of multipath effect, the measured EVMs approach to the ideal values measured in AC. The differences among the methods could be used to calibrate the measured EVMs and give a pass/fail conclusion efficiently for mass production.

Absorption measurements—A never ending story?!

A brief historical review on measurement techniques of sound absorption or the acoustic surface impedance will be presented, starting from procedures in the 1930s. Many in situ and laboratory methods have been described over the years. Most of these methods are based on the assumption of plane wave propagation or diffuse sound field. Measurement examples will be shown for some of the methods presented and compared to each other. A commonly used measurement procedure of so-called diffuse sound absorption coefficients is described in the international standard ISO 354. The present revision of the standard only focuses on the conditioning of the sound field and further applying Sabine equation. A diffuse sound field is atheoretical approach. Geometrical acoustics as the basis of sound field simulation has been applied to measurements in a virtual reverberation chamber on absorber baffle set-ups [see Probst (2015)] and showed good agreement between measured and calculated values. This approach is used to investigate the effect of sample size and sample position. Correspondingly, non-standard measurements are compared with simulation results.

Diffusion equation modelling for energy flow analysis in reverberation chambers

Noise is a growing concern in the built environment. Sound absorbers are a viable option for noise treatment. However, the characterization of their absorption coefficient in standardized measurement chambers still show challenges for high accuracy as required in practice. In recent years, experimental analysis has shown that assumptions of diffuse sound fields made in well-known reverberation chambers are unfulfilled. Specifically, that sound intensities in chamber-based measurement methods are presumed to be isotropic or diffuse. Diffusion equation models have shown dramatic changes in energy flow in the presence of highly absorptive materials under test. This has been attributed to well-documented inconsistencies reported from reverberation chamber measurements across different laboratories. This work will demonstrate that the diffusion equation model is proving to be a computationally efficient and viable method for predicting sound energy flows, garnering an increasing amount of interest from the acoustical community.

On the Shielding Effectiveness Calculation of Enclosures Through Measurements in Reverberation Chambers

The IEC 61000-4-21 and IEEE 299.1 documents report similar but not equal procedures to measure the shielding effectiveness (SE) of enclosures through the use of reverberation chambers. In this article, it is shown that these two measurement procedures can be merged in a single, enhanced, one. In particular, it is shown that the evaluation of the SE of enclosure in Annex H of IEC 61000-4-21, which exploits only mechanical stirring, can be improved by using the hybrid stirring, which includes also the frequency stirring, thus increasing the number of uncorrelated samples. On the other hand, it is shown that at low frequencies the evaluation of the SE of enclosures based on the definition in IEEE 299.1 can be improved through the use of the maximum values of measured powers rather than the average ones. The single, enhanced, proposed procedure provides more prudent SE values than the two standards at the low frequencies, especially for standard IEEE 299.1. The presented measurement approach is supported by results obtained in three different laboratories.

Effects of Signal Bandwidth on Total Isotropic Sensitivity Measurements in Reverberation Chamber

Nowadays, wireless devices exhibit an explosive growth trend, which imposes great challenges on over-the-air (OTA) testing. Obviously, the reliability and efficiency of OTA testing have significant influences on the research and development of wireless devices. The reverberation chamber (RC) has become an efficient solution to OTA tests such as total isotropic sensitivity (TIS), especially for large-form-factor devices. In this work, the signal bandwidth (BW) effect on the TIS measurement of a long term evolution (LTE) device is analyzed in a comprehensive way. It is theoretically and experimentally verified that the measurement uncertainty is lower for the case of larger signal BW, and vice versa. This indicates the signal BW effects contribute to the measurement uncertainty and must be considered. The findings of this work allow more flexible and efficient TIS measurements.

The Effect of Zigzag Boundaries on the Reverberation Chamber Performance

The stirrer design is important in a reverberation chamber measurement system. Previous study shows that the rotating radius of the stirrer plays a key role for the stirrer performance. However, to identify the contribution from the structure, optimizing the stirrer structure while keeping the stirring volume unchanged is necessary. In this paper, when the stirring volume is kept invariant, we show that the detailed structure of stirrers can be optimized to improve the performance but the effect is not significant. A comparative study is given to confirm the effect of zigzag boundaries on the stirrers. Both simulations and measurements confirm the performance improvement, key performance indicators such as field uniformity and correlated angles are simulated and measured.

A Unified Approach for Uncertainty Analyses for Total Radiated Power and Total Isotropic Sensitivity Measurements in Reverberation Chamber

With the rapid development and explosive application of the internet of things (IoT) technology, IoT devices become ubiquitous and indispensable in people’s daily lives. IoT devices with various forms and massive production impose challenges on their over-the-air (OTA) testing. Nowadays, conducting OTA testing in an efficient, reliable, and cost-effective way has become an urgent demand for the IoT industry. Owing to the particular properties (e.g., low cost, high efficiency, and good repeatability) of the reverberation chamber (RC), it has become a popular OTA testing facility. This article analyzes the statistical uncertainties of two fundamental metrics for IoT devices, i.e., the total radiated power (TRP) and the total isotropic sensitivity (TIS), measured in the RC. Based on the derived probability density functions (PDFs) and associated statistics, a unified approach for uncertainty analyses of TRP and TIS has been proposed, which can greatly facilitate the uncertainty characterizations in the OTA testing.

Nondestructive Reverberation Chamber Measurement of Microwave Oven Total Radiated Power and Shielding Effectiveness

In this study, a reverberation chamber is used to perform measurements of shielding effectiveness (SE) and average total radiated power (ATRP) related to a domestic microwave oven for human exposure purposes. The ATRP is measured according to Annex E in the standard IEC 61000-4-21 procedure, and a new nondestructive procedure is proposed to measure SE that relies on the internal source of the microwave oven and on the reverberation chamber. Under mild hypothesis, the nondestructive procedure is both theoretically and experimentally equivalent to the IEEE 299.1 one. In addition, the measured ATRP values are within the limits defined in EN 62233.

Bayesian inference on the parameters of the truncated normal distribution and application to reverberation chamber measurement data

Assessing the performance of reverberation chambers (RCs) is a continuous problem which requires constant development and analysis. Several performance indicators exist, have been proposed and continue to be proposed in the literature. Each one of these indicators exhibits different levels of applicability, robustness or relevance towards a particular aspect of the field (statistical) behavior inside the RC. In this paper, we address this problem by introducing an alternative methodology able to estimate the distribution of the statistical parameters of interest. The methodology is based on Bayesian inference and it is applied to two different statistical models for RC measurement data and to two different loading conditions. The present paper is a significant expansion of a relevant abstract presented at the ‘Mathematical and Statistical Methods for Metrology’ Workshop, Torino, Italy, 30–31 May 2019. The corresponding abstract can be found at http://www.msmm2019.polito.it/programme.

Efficiency measurement of the flexible on-body antenna at varying levels of stretch in a reverberation chamber.

Flexible antennas have the potential to transform wearable and fabric-based wireless sensing technologies. The antenna discussed in this study is part of a sensing system that uses the back-scattered power level as the decision metric. For a good wireless sensor, it is necessary to offer a feasible read range and maintain good distinctions in the back-scattered power levels between the different states (i.e. level of stretch) of the antenna. Moreover, effects due to human body proximity should be minimised. For these reasons, the radiation efficiency is a crucial parameter to investigate. This study presents the radiation efficiency measurement of the proposed flexible knitted 'Bellyband' antenna at two different levels of stretch in a reverberation chamber. This work validates the reverberation chamber measurements through comparison with simulations and anechoic chamber measurements at 900 MHz. Moreover, this work demonstrates how the approach can be used to quantify bellyband antenna efficiency in the vicinity of a human body. Finally, the efficiency results were used to predict the read range of Bellyband radio frequency identification technology.

Total Radiated Power Measurement in an Uncalibrated Reverberation Chamber

With the increased use of wireless communication in recent years, the use of reverberation chamber (RC) has increased to a great extent. Reverberation chambers have been eminently used for EMC testing and shielding effectiveness. The environment it provides is very similar to the reverberant surroundings that antenna undergoes in real life use. An experiment to measure total radiated power of antenna, antenna efficiency and quality factor of chamber in indoor environment is proposed. This will make the measurement very simple and inexpensive as designing and calibration of chamber will not be needed. In this paper, we have used three different techniques to compare total radiated power, quality factor, Rician K factor and efficiency of a patch antenna measured in indoor environment with RC data. The three method used include plate stirring method and two time domain methods. The time domain methods use modulated pulse and Gaussian pulse respectively for the measurement. The antenna and chamber parameters are measured in the real time and the data matched well with the RC data for different techniques.

Flexibility in over‐the‐air testing of receiver sensitivity with reverberation chambers

The next generation of wireless device utilises higher frequencies and a large array of form factors. With miniaturisation of devices and the growth of internet-of-things applications having no connectors for testing, the use of conducted tests is no longer an option and verification of this equipment requires over-the-air measurements. Additionally, some important metrics such as receiver sensitivity requires an actual communication link. Reverberation chambers loaded with lossy absorbers provide an efficient environment to measure such averaged quantities with flexibility in a test volume as well as device size, shape, and placement. Here, the authors present an overview of OTA measurements in reverberation chambers compared to anechoic measurements. The authors show the consistency and flexibility of reverberation chambers through measurements of a wireless device in three different reverberation chamber setups and compare to an anechoic chamber measurement. By using a proper chamber setup, and by showing a coherence bandwidth (CBW) as a universal metric for comparing chamber loading between setups, excellent chamber-to-chamber consistency is shown, without precise placement of the device in each chamber. The authors show empirical evidence for the choice of threshold in determining the CBW.

Improving Field Uniformity Using Source Stirring With Orbital Angular Momentum Modes in a Reverberation Chamber

The orbital angular momentum (OAM) offers new degrees of freedom and finds application in communications and radar imaging. In this letter, the OAM is introduced for reverberation chamber (RC) measurements using OAM metasurfaces. Because of the space limitation of the used RC, the OAM source has to be located near a sidewall, which inevitably causes some degradation of the OAM purity. Nevertheless, our experimental results show that the field uniformity of the RC can be improved effectively by using the OAM metasurfaces.

On Near-Field and Far-Field Correlations in Reverberation Chambers

A common question for the reverberation chamber (RC) measurement is whether the Fraunhofer far-field distance is satisfied between the device under test and other objects (e.g., RC walls and mode stirrers). In this letter, it is shown that far-field sampling may not be needed for most RC measurements that rely on the statistics of the stirred field. For these kinds of measurements, measurement uncertainties in near field and far field are the same.