Alexey Shitvov

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 Technologys 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.

Effect of Laminate Properties on Passive Intermodulation Generation

The results of experimental study of passive intermodulation (PIM) on printed circuit board (PCB) laminates are reported. Comparison of PIM performance of different materials has shown correlation between the PIM level and the laminate physical properties. The effect of the line length on PIM level has been experimentally observed. A simple model has been devised to describe the relation between the line length, attenuation and PIM performance of the lengths of straight microstrip lines.

Effects of Geometrical Discontinuities on Distributed Passive Intermodulation in Printed Lines

This paper presents the results of experimental study of passive intermodulation (PIM) generation in microstrip lines with U-shaped and meandered strips, impedance tapers, and strips with the profiled edges. It is shown that the geometrical discontinuities in printed circuits may have a noticeable impact on distributed PIM generation even when their effect is indiscernible in the linear regime measurements. A consistent interpretation of the observed phenomena has been proposed on the basis of the phase synchronism in the four-wave mixing process. The results of this study reveal new features of PIM production important for the design and characterization of low-PIM microstrip circuits.

Studies on passive intermodulation phenomena in printed and layered transmission lines

This paper reports on the recent developments into the investigation of passive intermodulation products (PIMs) in PCB based printed and layered transmission lines. Based on the results of earlier experiments, potential sources of PIM generation are examined, and a new semi-phenomenological model of PIM production mechanisms is discussed. The proposed model assumes non-local nature of PIM sources stochastically distributed along the path of high power microwave transmission. A consistent interpretation of the PIM measurements of printed circuits is proposed and further experiments are discussed.

Characterisation of Passive Intermodulation in Passive RF Devices with X-parameter

A novel approach to the modelling of passive intermodulation (PIM) generation in passive components with distributed weak nonlinearities is outlined. Based upon the formalism of X-parameters, it provides a unified framework for co-design of antenna beamforming networks, filters, combiners, phase shifters and other passive and active devices containing nonlinearities at RF front-end. The effects of discontinuities and complex circuit layouts can be efficiently evaluated with the aid of the equivalent networks of the canonical nonlinear elements. The main concepts are illustrated by examples of numerical simulations of PIM generation in the transmission lines and comparison with the measurement results.

Self-induced instability of passive intermodulation in microwave laminates

Passive intermodulation (PIM) generation in microwave laminates presents one of the major challenges to the manufacturers of printed antennas and beamforming networks. Whenever the PIM products, interspersed into the transmitting signals, appear at the receiver input, they degrade the system performance and might even block the channel completely. This is a common problem for GSM base stations, where the third-order PIM products of the transmitted multi-carrier signals appear as spurious signal in the receiver band. This detrimental effect is further aggravated by the inter-band interference in multiband communications systems. The low PIM performance of the printed circuit board (PCB) materials is a key issue for ultra-wide-band communications, co-located antennas, antennas in a complex changing environment and densely-packed electronic front-ends.

On passive intermodulation test of analog and digital systems

This paper presents initial results of evaluating suitability of the conventional two-tone CW passive intermodulation (PIM) test for characterization of modulated signal distortion by passive nonlinearities in base station antennas and RF front-end. A comprehensive analysis of analog and digitally modulated waveforms in the transmission lines with weak distributed nonlinearity has been performed using the harmonic balance analysis and X-parameters in Advanced Design System (ADS) simulator. The nonlinear distortion metrics used in the conventional two-tone CW PIM test have been compared with the respective spectral metrics applied to the modulated waveforms, such as adjacent channel power ratio (ACPR) and error vector magnitude (EVM). It is shown that the results of two-tone CW PIM tests are consistent with the metrics used for assessment of signal integrity of both analog and digitally modulated waveforms.

Distributed passive intermodulation generation in coplanar waveguide transmission lines

A phenomenology of distributed passive intermodulation generation in coplanar waveguide transmission line is presented. The theoretical analysis is based upon the generalised nonlinear transmission line model, which accounts for the coupling of two propagating modes. The case of weak substrate nonlinearity is considered and the model is given qualitative verification through the mapping of passive intermodulation products generated in coplanar waveguide fabricated on a commercial laminate. Implications for future research are discussed.

Characterization of metamaterials made of stacked layers of dogbone conductor pairs

The metamaterials composed of planar arrays of tightly coupled dogbone-shaped conductors have recently attracted considerable interest owing to their potential of supporting backward wave propagation [1]-[3]. The detailed study of the plane wave interaction with a layer of dogbone pairs has been performed in [3], [4] along with the eigenwave analysis of an infinite stack of array layers. In particular, it has been found that such metamaterial may exhibit negative effective refractive index near the transmission resonances.

Beam Index Modulation Wireless Communication With Analog Beamforming

In this paper, we propose a new architecture and performance analysis for beam index modulation (BIM) wireless communication implemented with the aid of analog beamforming. The BIM concept is presented in order to extend and adapt the spatial modulation concept to sparse wireless communication channel environments. It is shown that when compared with the conventional beamforming and SM strategies, the BIM system can achieve higher spectrum efficiency in high signal-to-noise ratio and non-line-of-sight (nLoS) sparse channel scenarios. Furthermore, different to the previous spatial scattering modulation (SSM) work the proposed BIM architectures have reduced hardware complexity, and, importantly, in some nLoS sparse channel conditions the BIM can also outperform the SSM. The above findings were further validated by some demonstrative experiments that were conducted at 10 GHz.