Passive Intermodulation Research Articles

Additive manufacturing of a compact Ku-band orthomode transducer

This work presents a compact ortho-mode transducer (OMT) built by additive manufacturing in a single-block, reducing the number of parts and flanges and improving the losses and power handling capability. The single-block approach also reduces potential passive intermodulation issues since the number of interfaces between parts is minimized. The presented OMT is based on the T-junction topology, with a short-circuited common circular waveguide where the two rectangular waveguides ports are attached. Both ports are arranged in opposite direction to maintain a symmetry plane for the whole structure in order to obtain a high isolation. Specific matching elements are introduced for each polarization to route the orthogonal modes to the common waveguide. The device is built by Selective Laser Melting (SLM), which imposes a set of specific mechanical restrictions to the 3D model of the OMT. The proposed design incorporates those restrictions, simplifying the geometry of the OMT as much as possible (especially the routing elements) to simplify the manufacturing. After this process, the experimental results show an OMT working in the band from 13.4 to 15.6 GHz (15.2%) with a return loss level higher than 20 dB for both polarizations, insertion loss lower than 0.18 dB and isolation between polarizations better than 45 dB. The OMT has also been tested in radiation connected to a reference horn, measuring a cross-polarization lower than -45 dB. This experimental performance shows that the proposed combination of compact design with single-block SLM manufacturing provides tested results similar to those obtained by high-accuracy milling or spark erosion suitable for satellite applications.

Assessment of Various Window Functions in Spectral Identification of Passive Intermodulation

Passive Intermodulation (PIM) distortion is a major problem in wireless communications which limits cell coverage and data rates. Passive nonlinearities result in weak intermodulation (IMD) products that are very difficult to diagnose, troubleshoot and model. To predict PIM, behavioral models are used where spectral components of PIM are estimated from the power spectral density at the output of the model. The primary goal of this paper is to study the effect of window functions on the capability of the power spectral density computed from the periodogram of signal realizations to predict low power PIM components in a wideband multichannel communication system. Different window functions are analyzed and it is shown that windows with high side-lobe level fail to predict PIM components which are close to the main channels due to their high spectral leakage; while window functions with low roll-off rate of the side lobes fail to predict low power higher order PIM components especially when the frequency separation between the main carriers is high. These results are supported by simulations of the power spectral density computed using signal realizations of an LTE carrier aggregated system.

Experimental study on the practical mitigation of passive intermodulation for time and temperature in cavity duplexer

Practical methods to mitigate level and variation of passive intermodulation with respect to time and temperature of coaxial cavity type duplexer are proposed through a variety of experimental results. From the findings derived in this study, a stable passive intermodulation response with less than 1 dB variation was shown in the environment temperature range of −10–85 °C. The duplexer with time-stable PIM response at room temperature had a stable PIM value even in the environment temperature range.

A Method for Measuring the Maximum Measurable Gain of a Passive Intermodulation Chamber

This paper presents an approximate method that allows the calculation of the maximum measurable gain (MMG) in an anechoic chamber. This method is realized by using a low passive intermodulation (PIM) medium-gain directional antenna. By reducing the distance between the antenna and the wall of the chamber to reduce path loss, the purpose of replacing a high-gain antenna with a medium-gain antenna is achieved. The specific relationship between distance and equivalent gain is given in this paper. The measurement interval is determined by the 3 dB beamwidth of the measurement antenna to scan the whole chamber. A set of corresponding data for the residual PIM level and the MMG of the chamber can be obtained by the method of measurement outlined herein. The feasibility of this method was verified by measurements in two PIM measurement chambers.

Analysis and experiment on multicarrier passive intermodulation in high power component

Analysis and experiment on multicarrier passive intermodulation in high power component

Novel cost-effective passive intermodulation measurement technique using a single power amplifier

The growing number of cellular devices and the introduction of high-speed data affect the network performance of mobile communication systems. Evaluation of passive intermodulation (PIM) phenomenon in the field of microwaves and mobile communications improve system performance. Typically, the PIM of the device-under-test (DUT) is measured using a set-up that involves two power amplifiers (PA), making it an expensive measurement set-up. This research aims to make PIM measurements cost-effective by using a single PA. Reflected (RFL) PIM distortion measured for two different DUTs, i.e., circulator and a microstrip (MS) line containing radio frequency interference (RFI) shielding gasket, using the presented and standard techniques. The presented PIM analyzer has a residue PIM of -104.5dBm. Comparison and error analysis suggested that the presented single PA method is feasible for DUTs having PIM between -60 and -94.5dBm at 43dBm input power. The residual PIM of the presented analyzer is high compared to the standard PIM analyzers; it is investigated that IM generated by the PA is the leading cause of this higher residual PIM product (RPIM). The proposed method will reduce overall system cost and size; it will also improve the battery efficiency of portable and desktop PIM analyzers.

무선 중계 시스템에서 2차 수동 상호 변조 왜곡 모델링 비교

PIMD (Passive Intermodulation Distortion)는 무선 통신 시스템의 수동 소자의 비선형성으로 인해 2개 이상의 전송 신호주파수가 서로 간섭하여 원하지 않는 신호가 발생하는 현상입니다. 이러한 상호 변조 왜곡은 무선 통신 시스템의 수신 주파수대역에서 잡음 레벨을 증가시켜 수신기의 성능을 저하시킨다. 본 논문에서는 수정된 Quadratic Volterra 필터와 시간 지연 신경망을 기반으로 한 PIMD 모델링을 조사하여 수동 상호 변조 왜곡 수준을 줄여 업 링크 수신 성능을 향상 시킨다. 모델링 성능은 정규화된 평균 제곱 오차 측면에서 메모리 길이 및 시간 불일치에 따라 LTE 시스템의 중계기에서 샘플링된 데이터에 모델을 적용하여 평가된다.

Evaluation of Passive Intermodulation From Multiple Connectors With Generalized Network Method

Connectors are significant contributors to passive intermodulation (PIM) in microwave systems. In this letter, a generalized network method is presented for evaluating the PIM from multiple connectors. Taking a structure composed of two connectors and a transmission line as an example, a generalized network model describing both linear and nonlinear relationships is developed and an expression of the third-order PIM is given. Finally, PIM measurements of a microstrip line with two subminiature version A (SMA) connectors are reported to validate the network model.

A New Model to Determine Passive Intermodulation Terms When Non-Contributing Carriers are Added to Classical Scenarios

This paper provides a new theoretical model for the characterization of passive intermodulation behaviour when non-contributing carriers (i.e., carriers which do not directly affect the particular non-linear contribution under analysis) are added to classical two or three carriers scenarios. According to traditional models, the power of a particular passive intermodulation frequency term is only related to the contributing carriers which combines in non-linear way to generate this term, thus the presence of additional (i.e., non-contributing) carriers should not affect to the predicted power of such an intermodulation product. However, recent laboratory tests reveal that non-contributing carriers do reduce the power of unrelated passive intermodulation terms. A heuristic model is proposed in this work to explain this effect, which shows a good matching with measured results already published in the technical literature. The study is focused in third order non-linear contributions, as being the most critical in practical applications.

Frequency and Power Dependence of Passive Intermodulation in Connectors With Contact-Material Nonlinear Modeling

Passive intermodulation (PIM) is generally regarded as an unavoidable interference in radio frequency (RF) circuit and system, which exhibits dynamic characteristics with the change of signal frequencies and powers. In this letter, a prediction model of frequency and power dependence of PIM is theoretically proposed and experimentally verified. Physical nonlinearities of truncated distortion and tunneling effect are introduced to provide explanations for the contact surface-induced PIM. Considering nickel is the nonlinear material source in connector coating, the impact of signal frequency on PIM is investigated by calculating the current density in the magnetic area coupled with skin effect theory. Through the combination of two PIM generation mechanisms, a novel equation-based equivalent circuit model is provided to simulate the third-order intermodulation (IM3) powers in connector resulting from different signal frequencies and powers. The frequency and power dependence characteristics are observed from both the theoretical predictions and measurements.

분산 안테나 시스템에서 3차 수동소자 상호변조 왜곡추정을 위한 신경망과 Cubic Volterra 필터의 성능 비교

분산 안테나 시스템에서 3차 수동소자 상호변조 왜곡추정을 위한 신경망과 Cubic Volterra 필터의 성능 비교

Empirical Passive Intermodulation Multiphysics Modeling Using Design of Experiment Method

This work presents a numerical modeling of passive intermodulation (PIM) product on contact junction under the multiphysics condition. The carrier power, temperature, and pressure impacts are combined into the model using design of experiment method. Based on several sets of optimized PIM test conditions, an empirical PIM model versus multiphysics can be established. The experiment demonstrates that the proposed PIM modeling method can predict PIM product with the max error of 7 dB.

Modeling of Signal Distortion Caused by Passive Intermodulation and Cross Modulation in Coaxial Connectors

Coaxial connectors play an important role in communication systems. However, passive intermodulation (PIM) and cross modulation resulting from slight nonlinear characteristics in coaxial connectors may negatively impact signal transmission quality. In addition, degradation of coaxial connectors tends to exacerbate such effects. In this work, the nonlinear characteristics of pristine coaxial connectors are analyzed and modeled. From these results, a new polynomial behavior model is developed to describe the nonlinear characteristics for connectors with various levels of degradation for the first time. A series of two-tone PIM tests on pristine connectors and degraded connectors is conducted with the input power from 35 to 44 dBm. The test results validate the model for pristine connectors and the model for connectors with various levels of degradation. The error vector magnitude (EVM) is selected as the index to quantify the level of signal distortion. The power of PIM and cross modulation interference and the impacts of PIM and cross modulation on EVM are theoretically calculated and simulated. This work serves to provide a behavior model that describes the nonlinear characteristics of pristine and degraded connectors, and a theoretical basis for evaluating the impacts of PIM and cross modulation on signal distortion.

Effects of Environmental Temperature on Passive Intermodulation in Electrical Connectors

Environmental temperatures may affect the high-frequency electrical performance of passive components, including passive intermodulation (PIM) characteristics. This work proposes a theoretical model of the third-order intermodulation distortion (IM3) signal as a function of the material properties and contact characteristics in a connector. A cross-sectional model of the inner conductor supported by finite-element analysis (FEA) simulations was used to investigate the effect of temperature on the current density distribution in different material regions and its impact on nonlinear performance. Based on electrical contact theory and temperature-dependent modeling, mathematical expressions were developed to calculate the impact of the various contact parameters, such as contact pressure, contact area, and contact resistance. Combining the effects of both material nonlinearity and contact nonlinearity, the IM3 product powers resulting from coaxial connectors associated with temperature change were predicted theoretically. A series of experiments were designed and conducted to measure the IM3 product powers of connectors for a variety of environmental temperatures. The model predictions show a good correlation with these experimental results.

A Dual-Broadband Dual-Polarized Directional Antenna for All-Spectrum Access Base Station Applications

A dual-broadband, dual-polarized directional antenna with an electrical down-tilted beam is designed for all-spectrum access base station applications. Three columns of elements are deployed in the manner that six lower-band elements are placed in the center column while the eleven upper-band elements are arranged in the other two columns. Moreover, each column of elements are fed by two ports, which makes the proposed antenna suitable for multiple-input-multiple-output (MIMO) applications. To verify the design concept, a prototype is fabricated and measured. The measurement results confirm that the proposed antenna achieves a bandwidth of 31.6% (698-960 MHz) for VSWR<; 1.5 in the lower band and a bandwidth of 44.5% (1710-2690 MHz) for VSWR<; 1.5 in the upper band, covering all the frequency bands for 2G/3G/LTE systems and 700 MHz/2.6 GHz frequency bands. The measured gains are 17.4 ± 0.9 dBi, 17.3 ± 0.8 dBi, and 14.4 ± 0.9 dBi for the antenna port1, port2, and port3, respectively, while the third-order passive intermodulation (PIM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) is larger than 115 dBm at all frequencies. With these distinctive advantages, the proposed antenna is suitable for multi-band base stations.

Directional passive intermodulation effect by distributed nonlinearities in two‐port network

AbstractThis work demonstrated the directional passive intermodulation (PIM) product distribution effect in a microwave two‐port network using a distributed nonlinear model. The analysis is demonstrated through the comparison of PIM products at the double ends of a general two‐port network by a microstrip line. The reverse and forward PIM products are compared with the nonlinear source distribution and carrier frequency, the model and experiment demonstrates that the directional PIM effect (the difference by reverse and forward PIM) is caused by the PIM accumulation effect from multiple nonlinear sources, what is more, the frequency dependent phase relation can be one reason for the frequency dependent PIM product on wideband transmission line.

Passive Intermodulation in Simultaneous Transmit–Receive Systems: Modeling and Digital Cancellation Methods

This article presents novel solutions for suppressing passive intermodulation (PIM) distortion in frequency-division duplexing (FDD)-based radio transceivers, stemming from nonlinear radio frequency (RF) components and simultaneous transmission and reception, with special emphasis on modern carrier aggregation networks. With certain transmission band combinations in, e.g., long-term evolution (LTE) or the emerging 5G new radio (NR) mobile radio systems, the nonlinear distortion produced by the passive components of the transceiver RF front end can result in intermodulation (IM) distortion that falls within one of the configured reception bands. While the traditional solution to mitigate this problem is to reduce the transmit power, we take an alternative approach and seek to cancel such PIM in the transceiver digital front end by using the original transmit data as a reference. To generate as accurate cancellation signal as possible, we derive different advanced signal models for the observable IM distortion at own receiver band that incorporates also the power amplifier (PA) nonlinearities, together with the passive component nonlinearities and the frequency-selective responses of the duplex filters. The performance and the processing complexity of the devised digital cancellation and parameter estimation solutions are evaluated with real-life RF measurements, where an actual LTE-Advanced user equipment (UE)-type transceiver system is utilized. The obtained results show that the proposed cancellers are implementation feasible and can suppress the self-interference by over 20 dB, canceling the distortion nearly perfectly up to UE transmit powers of +24 dBm. The results also indicate that, in many cases, it is necessary to model the nonlinear distortion effects produced by the PAs, even if the individual component carriers are combined after the amplification stage.

Wide-band high dynamic range variable passive intermodulation generator using fabric-over-foam gasket

Accurate measurement of Radio Frequency (RF) component’s passive intermodulation (PIM) and systematic evaluation of PIM’s effect on mobile communication require flexible PIM generator. This work presents a wide-band high dynamic-range PIM generator using a fabric-over-foam (also known as RF Interference Shielding Gasket) as superstrate of low-PIM microstrip line (MS-line). An air gap of 1 mm introduced between the foam and the MS-line to ensure long-term repeatability by avoiding the nonlinearity that can be generated by the metal-to-metal contact. To make the PIM generator adjustable, the foam moved transversely over the MS-line. The size of the gasket selected in such a way that it slightly changes the S-parameters of the MS-line. Due to the variation of the exposure area of the foam (which is an electromagnetic magnetic material) to the electromagnetic (EM) field generated by the MS-line; the PIM level of the MS-line changes with the foam’s location. The minimum PIM level of the proposed generator determined by the PIM level of the MS-line, which is no more than -125dBm @2×43dBm. However, the maximum PIM level can be as high as -48dBm @2×43dBm, achieved by placing a gasket piece on the top of the MS-line. The proposed generator verified from 700MHz to 2.7GHz and all of the measurement results show its feasibility. Moreover, presented PIM generator can be used a single-port or two-port calibration device.

Simulation analysis of passive intermodulation mechanism of transceiver link

The electromagnetic environment of modern battle-field is complex and the interference intensity is large. Under the influence of high field electromagnetic interference, airborne RF equipment becomes the short board of electromagnetic environment adaptability of aircraft. Because of the wide application of military aircraft platform, the external climate and electromagnetic environment are very complex. At the same time, the interior space of military aircraft is narrow and there are many high-power frequency equipment, which lead to a very complex electromagnetic environment inside the aircraft. Under the irradiation of high-power electromagnetic field, passive intermodulation products will be produced in the receiving antenna, transmission line, connector, filter and even some non-metallic conductive materials and aircraft skin under certain conditions. The passive intermodulation products will cause great interference to the airborne high sensitivity receiving equipment, and even burn the receiving equipment in extreme conditions. Therefore, the research on passive intermodulation products of airborne equipment is very urgent.

Small Anechoic Chamber Design Method for On-Line and On-Site Passive Intermodulation Measurement

A design method is proposed in this article for on-line (online measurement: measurement when producing devices) and on-site (on-site measurement: measurement when using devices) passive intermodulation (PIM) measurement with a small-size anechoic chamber. In a PIM measurement, the role of an anechoic chamber is to shield extraneous electromagnetic signal and reduce reflection, so that an accurate measurement result can be obtained. However, the chamber itself may introduce new PIM sources due to the nonlinearity in absorbing material and shielding enclosure. The traditional method to lower the influence of chamber to PIM measurement is to design a large-size PIM test chamber in order to reduce the PIM level by path loss. In this article, an innovative design concept of the PIM test chamber is presented by controlling the PIM source level of the anechoic chamber instead of increasing the size of the chamber. It can significantly reduce the chamber size using this method, particularly suitable for on-line and on-site PIM measurement. To implement the concept, absorbing material and shielding enclosure structure with extremely low PIM are developed. The design concept includes defining the worst-case reflection level, estimating the PIM level due to reflected signals, and determining the smallest size of the chamber that meets the design specifications. Finally, two PIM test chambers are built to demonstrate the design concept. PIM measurement results are given to verify the theoretical analysis.