Passive Intermodulation Measurement Research Articles

System Residual Noise Reduction for Non-Contact Passive Intermodulation Measurements of Array Antennas Using Multiple Testers

System Residual Noise Reduction for Non-Contact Passive Intermodulation Measurements of Array Antennas Using Multiple Testers

Determination of Optimal Sample Length for Passive Intermodulation Measurements in Microstrip Lines with Non-Contact Connections

Determination of Optimal Sample Length for Passive Intermodulation Measurements in Microstrip Lines with Non-Contact Connections

Novel Passive Intermodulation Measurement Platform for Planar Microwave Circuit

Novel Passive Intermodulation Measurement Platform for Planar Microwave Circuit

Passive Intermodulation Measurement System

The Passive Intermodulation (PIM) phenomenon occurs because of non-linarites in antennas or passive components. When data rates are increasing in next generation communication systems like in 4G, 5G and 6G, PIM is getting even crucial and costly problem. Unwanted harmonics and intermodulation products might occur in communication band and this can take down the communication network. This work presents PIM mechanisms, and PIM measurement test setups for any communication standards (4G, 5G and 6G).

Passive Intermodulation Measurement: Challenges and Solutions

In modern wireless communication systems, the signal-to-noise ratio (SNR) is one of the most important performance indicators. When the other radio frequency (RF) performance of the components is well designed, passive intermodulation (PIM) interference may become an important factor limiting the system’s SNR. Whether it is a base station, an indoor distributed antenna system, or a satellite system, there are stringent PIM level requirements to minimize interference and enhance network capacity in multicarrier networks. Especially for systems of high power and wide bandwidth such as 5G wireless communication, PIM interference is even more serious. Due to the complexity and uncertainty of PIM, measurement is the most important means to study and evaluate the PIM performance of wireless communication systems. In this review, the current main PIM measurement methods recommended by International Electrotechnical Commission (IEC) and other standard organizations are introduced, and several key challenges in PIM measurement and their solutions (including the design of PIM tester, the location of the PIM sources, the design of compact PIM anechoic chambers, and the evaluation methods of PIM anechoic chambers) are highlighted. These challenges are of great significance to solve PIM problems that may arise during device characterization and verification in real wireless communication systems.

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.

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.

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.

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.

Compact Intermodulation Modulator for Phase Reference in Passive Intermodulation Measurements

In this article, a modular designed intermodulation (IM) modulator based on coherent nonlinearity superposition is demonstrated. A coherent combination network for two nonlinear sources (diodes) is composed, excited by a set of two-tone carriers, and the IM products generated from two diodes are inherently coherent. The designed IM phase and magnitude are synthesized by using two IM signals with different magnitudes. The modulator is achieved by modular design, and the manufacture is simplified by using low IM laminate and normal FR-4 substrate to the section with different IM sensitive levels. The work proves a solution to generate an IM phase and magnitude tunable reference for IM measurement, which also proves that IM can be used to carry some information.

Measurement and Analysis of Passive Intermodulation Induced by Additional Impedance in Loose Contact Coaxial Connector

The mechanism of the passive intermodulation (PIM) distortion caused by the loose contact in coaxial connectors is of great importance in wireless communication systems. This paper focuses on the effect of the additional impedance on PIM levels in coaxial connectors. First, the additional impedance and PIM levels of the loose contact coaxial connectors are measured. Then, the S-parameters and the additional impedance with three different loose contact cases are simulated. Finally, a modified coaxial connector with different values of conductive tapes is designed to verify the additional impedance impacts on the PIM level. It is shown that the increase of the additional impedance of the metal–metal loose contact will lead to the deterioration of PIM levels in the coaxial connector.

A Segmented Polynomial Model to Evaluate Passive Intermodulation Products From Low-Order PIM Measurements

In this letter, a segmented polynomial model is developed to accurately calculate the powers of high-order passive intermodulation (PIM) products from the measurement data of low-order PIM product. This proposed model fully considers the dependence of dominant PIM generation mechanism on carrier power. An example of a microstrip transmission line is given to verify the effectiveness of this model, and excellent agreement between the measured data and calculated results has been achieved. Compared to the common-used polynomial model, not only the order adopted in the segmented model is much lower but also the number of measured data required for evaluation is much less. Moreover, this presented model can calculate the more accurate high-order PIM products than any other model. Thus, it can provide a theoretical guidance for PIM performance assessment in engineering application.

Notice of Retraction: A Measurement Method for Passive Intermodulation Chamber Performance Evaluation

With emerging wireless communication technology, undesired interference caused by passive intermodulation (PIM) becomes a major obstacle to the network efficiency and the electromagnetic compatibility (EMC) of the devices. Accurate measurement techniques for PIM are fundamental to identify and characterize the electromagnetic interferences (EMIs). Anechoic chambers are commonly used in the antenna PIM test. The noise of PIM test chamber usually determines the accuracy of PIM measurement. The additional PIM sources caused by the chamber can be classified into three classes: those due to the absorbers, those due to the design of the shielded enclosure, and those due to the extraneous signals caused by an ineffective shielding. In this paper, a measurement method is proposed for the performance verification of PIM test chamber. The proposed method has two key points. One is adjusting the measurement distance between antenna and absorbers for gain compensation, which can evaluate the maximum measurable antenna gain of the PIM test chamber. The other is choosing the multiple measurement points according to the beamwidth of antenna, which can define the PIM level in worse case of the chamber. Theoretical analysis of the proposed method and measured results for two PIM chambers by the method are given in this paper.

Prediction of Passive Intermodulation on Mesh Reflector Antenna Using Collaborative Simulation: Multiscale Equivalent Method and Nonlinear Model

We propose a collaborative simulation method for evaluation of passive intermodulation (PIM) on mesh reflector antenna. The electric field on the surface of reflector can be simulated exactly and conveniently by the multiscale equivalent method and finite-element method solver. Meanwhile, the coefficients in the nonlinear model of PIM characteristic on mesh structure are determined from the experimental results on mesh reflector sample, so that the PIM of mesh reflector antenna with the same braided form and working conditions can be calculated using the same nonlinear model. In simulation, the PIM calculations of mesh reflector samples based on the actual woven form and with dimensions of $0.25 \times 0.25\,\,\text{m}^{2}$ at C-band and Ku-band have been established, indicating basically consistent to the measured results and validating the effectiveness of this method. Moreover, the PIM prediction of a mesh reflector antenna with the same woven structure is also given. These results show that this method can be utilized to predict PIM level of the mesh reflector antenna under conditions of the precise PIM measurement of mesh reflector sample, also providing reference for designing the low PIM microwave components.

Advanced Compact Setups for Passive Intermodulation Measurements of Satellite Hardware

This paper provides the guidelines for the practical development of novel advanced test beds for passive intermodulation (PIM) measurements. The proposed test beds show high performance and are flexible, allowing for the measurement of several PIM signals of different orders, with two or more input carriers. In contrast to classic test beds for satellite hardware, based on the cascaded connection of several elements, an integrated solution involving the minimum number of hardware pieces is proposed. The result is a lower number of flanged interconnections, thus reducing residual PIM level and insertion losses. In addition, return loss degradation and harmful spurious generation in the interconnections are also avoided. Measurement test beds for conducted and radiated PIM, in both transmitted and reflected directions, are discussed, highlighting the benefits and drawbacks of each configuration. Design guidelines for the key components are fully discussed. Illustrative application examples are also reported. Finally, excellent experimental results obtained from low-PIM measurement setups, working from C-band to Ka-band, are shown, thus fully confirming the validity of the proposed configurations.

How to measure test repeatability when stability and constant variance are not observed

Passive intermodulation (PIM) is a critical measurement for radio frequency (RF) communication networks. Yet PIM measurement inherently has very poor repeatability, which makes product assessment unreliable. The RF industry struggles with the issue since there are no known solutions. With the increasing demand for low-PIM performance, there are pressing demands to address the challenge. Two fundamental problems make traditional gage R&R study invalid for PIM: (1) PIM in nature is unstable and unrepeatable; (2) PIM measurement has inherently inconstant variance at different PIM levels, which is primarily due to limited capability of PIM analyzer. This resulted in several less-known issues significantly impacting the estimation of PIM test repeatability, including sample selection, one-sided spec and differences between test R&R and gage R&R. The paper proposed two fundamental changes when studying R&R of PIM test or tests in general violating constant variance assumptions: (1) sample selection; (2) what measurement to use to better estimate and represent the test repeatability. Special sampling is proposed to minimize the impact of inconstant variance. A more direct R&R measurement, margin of error (MOE), also known as study variation, is proposed to replace traditional gage R&R metrics to more meaningfully represent PIM test R&R. Several statistically based techniques to improve the repeatability and reliability of PIM measurement are also discussed. The study and proposed solutions apply to not only PIM test but also tests in general violating constant variance assumptions.

Broadband Dual-Port Intermodulation Generator for Passive Intermodulation Measurements

A dual-port tunable intermodulation (IM) generator is reported. This IM generator employs a coupling network to distribute carrier waves and IM. The proposed coupling network is constructed by cascading four parallel-coupled directional couplers, while a Schottky diode is working as a tunable IM source. By adjusting the IM performance of diode using bias voltage and distributing IM along symmetrical IM paths, a pair of tunable IM signals is introduced into passive IM (PIM) test loop simultaneously, they can serve as bidirectional IM reference in dual-port PIM measurements. The provided IM3 can be tuned to be lower than −112 dBm at $2 \times 43$ dBm at either 710 or 2550 MHz, while their dynamic IM3 range can reach about 20 dB at $2 \times 43$ dBm. The work promotes the application of IM predistortion techniques and provides a solution to realize dual-port dynamic IM calibration in PIM test.

Reflection modulation basis dual‐port intermodulation generator for dynamic calibration application in passive intermodulation measurements

The high-accuracy passive intermodulation (PIM) measurement shows great significance in PIM quality checking and related researches. As the key point to guarantee the test accuracy, the IM reference is essential in realizing precise PIM calibration. This paper presents a design of dual-port IM generator that can generate dynamic IM reference for this demand. The proposed generator employs a cascaded coupling network consists of two couplers, while a reverse biased Schottky diode is working as a tunable reflection IM source. The bias voltage and coupling coefficient are introduced to regulate the quiescent operation points of diode and the IM signal excitation strength respectively, they work together to put the IM reference signal on a desired level. Specially, a trimming strategy by using a compact terminal matching resistor is adopted to smartly regulate the IM tunable interval. These configurations can promote the utilization of this proposed IM generator to be flexible not only for design engineers but also for commercial users. The test results demonstrate that this generator can provide the tunable IM3 whose dynamic range can reach about 30 dB@2x43 dBm, 50 dB@2 × 31 dBm. Meanwhile, the significance of bidirectional dynamic IM reference signals is demonstrated.

RF Receiver Characterization and Spurious-Free Dynamic Range Enhancement for PIM/Weakly Nonlinear Device Measurements

This paper proposes a technique to characterize and postcompensate for the dynamic nonlinearity exhibited by radio frequency (RF) receivers employed in passive intermodulation (PIM) and weakly nonlinear devices characterization (low gain compression of the order of 0.1 dB). The proposed technique begins by accurately characterizing the RF receiver’s nonlinearity using two-tone and multitone stimuli with swept frequency spacing and input power level. Then, a pruned Volterra series-based postcompensation model is devised to mitigate the dynamic weak nonlinear distortions exhibited by the receiver. The technique was validated in two steps. First, the capacity of the postcompensation technique to separate the RF receiver’s intermodulation from the device under test’s intermodulation was assessed. Second, its capacity to postcompensate for the dynamic nonlinearity exhibited by the RF receiver and to enhance its spurious-free dynamic range (SFDR) was assessed using digitally modulated stimuli. The application of the proposed technique reduced the third-order intermodulation (IMD3) of the RF receiver by an average of 10 dB. This technique can significantly relax the requirements on duplexers and tone injection techniques used to enhance the accuracy of PIM measurements. Furthermore, the receiver’s SFDR was extended by up to 15 dB.

Passive Intermodulation of Analog and Digital Signals on Transmission Lines With Distributed Nonlinearities: Modelling and Characterization

Passive intermodulation (PIM) often limits the performance of communication systems with analog and digitally modulated signals and especially of systems supporting multiple carriers. Since the origins of the apparently multiple physical sources of nonlinearity causing PIM are not fully understood, the behavioral models are frequently used to describe the process of PIM generation. In this paper, a polynomial model of memoryless nonlinearity is deduced from PIM measurements of a microstrip line with distributed nonlinearity with two-tone CW signals. The analytical model of nonlinearity is incorporated in Keysight Technology's ADS simulator to evaluate the metrics of signal fidelity in the receive band for analog and digitally modulated signals. PIM-induced distortion and cross-band interference with modulated signals are compared with those with two-tone CW signals. It is shown that conventional metrics can be applied to quantify the effect of distributed nonlinearities on signal fidelity. It is found that the two-tone CW test provides a worst-case estimate of cross-band interference for two-carrier modulated signals, whereas, with a three-carrier signal, PIM interference in the receive band is noticeably overestimated. The simulated constellation diagrams for QPSK signals demonstrate that PIM interference exhibits the distinctive signatures of correlated distortion and this indicates that there are opportunities for mitigating PIM interference and that PIM interference cannot be treated as noise. One of the interesting results is that PIM distortion on a transmission line results in asymmetrical regrowth of output PIM interference for modulated signals.