Andrew Christianson

Prediction of Passive Intermodulation From Coaxial Connectors in Microwave Networks

Coaxial connectors are frequently the dominant contributors to passive intermodulation (PIM) distortion in high-frequency networks. This paper reports on a circuit model enabling estimation of PIM distortion by coaxial connectors in the design of high-frequency networks. A method of modeling the effect of multiple point sources of PIM is applied to coaxial connectors, allowing the prediction of the PIM of networks with several connectors. Typical ranges of PIM produced by common connectors in a two-tone test are reported. The stability and repeatability of PIM produced by a single connector is examined. Nonlinear current-voltage curves for coaxial connectors are given that predict the PIM distortion output by coaxial connectors over a broad range of input powers. An experimental verification is given showing that PIM of a system can be predicted if the characteristics of the individual components are known.

Engineered Passive Nonlinearities for Broadband Passive Intermodulation Distortion Mitigation

By adding controlled thicknesses of nickel and gold plating to the conductors of a coaxial transmission line, the magnitude of passive intermodulation produced by the transmission line can be controlled with precision. Theoretical predictions of distortion magnitude as a function of plating thicknesses are presented, along with an experimental validation. These adjustable-magnitude passive intermodulation sources are used to give a fourfold improvement in the bandwidth of techniques presented previously, demonstrating that cancellation can for the first time be achieved in bandwidths needed for cellular systems.

Higher Order Intermodulation Product Measurement of Passive Components

The concept of using high-order intermodulation (IM) products beyond the third order for the analysis of passive intermodulation (PIM) sources is introduced. Current PIM characterization techniques focus on measurements of the third-order IM product. An advantage of observing higher order IM is demonstrated by showing that two simulated models, which predict an identical level of third-order IM, exhibit clear qualitative differences in the higher order IM products. A low residual PIM measurement system is able to measure up to the 29th-order IM product. A full discussion of uncertainties in the measurement technique used is given. The measured high-order IM products of a coaxial connector are used to fit and verify a model for its current-voltage characteristic. This fit model, a hyperbolic tangent function, accurately predicts the behavior of the higher orders as a function of input power and matches the third order over the 21-dB input power range with a root mean squared error of 3.5 dB. For another device, a microwave circulator, higher order IM products are used to deduce a model, which is then confirmed using a traditional measurement of third-order IM. High-order IM products are measured and analyzed in order to aid investigations of the physical processes causing PIM.

In-situ control of tunable evanescent-mode cavity filters using differential mode monitoring

In the present work, a method for tracking the center frequency of a widely tunable evanescent-mode cavity filter in-situ is introduced. The goal is to be able to monitor the performance of a filter without disturbing the fields or degrading the quality. The proposed method is to monitor the resonant frequency of each resonator of the filter independently by inducing higher order differential modes. This method enables a continuous feedback loop to lock in the filter center frequency and shape. An example filter is fabricated to demonstrate the concept and tuned from 1.4 to 3 GHz while monitoring the differential mode at 4 to 6.5 GHz. The ability to independently monitor and control the individual resonators in-situ without disturbing the main mode is demonstrated and is a crucial step towards a robust fielded widely tunable filter.

Measurement of ultra low passive intermodulation with ability to separate current/voltage induced nonlinearities

A measurement technique is proposed which exploits the standing wave in a microstrip resonator to more thoroughly measure passive nonlinearities. The measurement technique is demonstrated to be able to measure the difference between current induced and voltage induced passive nonlinearities. By placing the device under test in a resonator three times higher voltage and current magnitudes are created compared to traveling wave passive intermodulation measurement systems. This increases the effective power only at the device under test and increases the measurement sensitivity of the original traveling wave measurement system. Based on our measurements we project the ability to characterize devices at −196 dBc, a 29 dB improvement over the original system.

Linear–Nonlinear Interaction and Passive Intermodulation Distortion

This paper describes several consequences of a linear-nonlinear interaction that was recently found to be of importance in microwave circuits that produce passive intermodulation (PIM) distortion. This paper briefly discusses how this linear-nonlinear interaction operates in an example system. It then discusses how an understanding of the linear-nonlinear interaction allows us to distinguish between different types of nonlinearities from the power dependence of the third-order intermodulation distortion product. Next, an example uses a multiphysics simulator to demonstrate that electrothermal nonlinearities behave as expected from the linear-nonlinear interaction model. Lastly, it illustrates how simple nonlinear models characterized with one circuit can accurately predict distortion levels when the nonlinearity is placed within a very different circuit, showing that knowledge of the interaction gives the ability to accurately predict the behavior of PIM-producing components in a variety of circuits such as resonators, filters, and matching networks.

Linear-nonlinear interaction's effect on the power dependence of nonlinear distortion products

An unusual power dependence of the nonlinear distortion produced by a two-tone test has been observed in several different physical systems. We show that the interaction between the nonlinear and linear elements of a system can dramatically transform the overall nonlinear behavior of the system from that of the nonlinear component in isolation. In particular, we show that when interaction with the linear elements of an electric circuit is appropriately accounted for, rather simple models of nonlinearity display the unusual power dependence of nonlinear distortion products observed in a variety of physical systems.

Cancellation of Passive Intermodulation Distortion in Microwave Networks

Passive nonlinearities in high-power transmit/receive wireless systems cause undesirable transfer of power from high-power transmit carriers to the receive band, causing levels of interference and distortion that can severely limit or debilitate the system. This paper proposes a method of cancellation that allows for reduction of nonlinear distortion due to passive nonlinearities in high-power wireless systems. We verify the method by experiment, resulting in almost 40 dB of cancellation of nonlinear distortion in a high-power radio frequency system, effectively eliminating the reflected passive intermodulation from the system.

Dynamic Visualization of Antenna Patterns and Phased-Array Beam Steering [Education Column]

Antenna education is often hindered by the fact that radiation patterns are hard to visualize (RF electromagnetic waves are invisible), and require complicated, expensive equipment to measure. To solve this problem, we have built a wall that illuminates when RF energy is directed at it. This allows immediate visualization of an antennas radiation pattern, and of concepts such as wireless propagation, polarization, gain, and even phased-array beam steering. A low-power LED rectenna array is used to create the visualization wall. We have also developed a multicolor LED rectenna that changes color depending on incident power. Also included is a phased-array antenna with a crystal oscillator and power amplifi er on the same board. This board allows the user to control the phase of signals fed to each of four columns of a 16-element patch array using only a rotary selector switch. This allows instructors to demonstrate the principles of phased-array beam steering in real time, all with a self-contained system cost of less than

Matching network design for passive intermodulation distortion reduction

1500.