Antenna Systems & Technology - Winter 2013 - (Page 16)

FEATURE ARTICLE Improve Passive Intermodulation Performance with Specialty High-Frequency Circuit Materials By John Coonrod, Market Development Engineer - Rogers Corp., Advanced Circuit Materials Division Passive intermodulation (PIM) is a form of distortion that can affect many different highfrequency passive components, including antennas. Any passive component that handles signals with multiple tones, such as antennas, cables, connectors, couplers and filters, can fall victim to PIM. For example, excessive levels of PIM generated by antennas and distributed antenna systems (DAS) in cellular communications infrastructure networks can degrade the quality of the voice, data and video communications in those systems. It is not difficult to understand how PIM is caused in antennas and other components, but managing the different variables and their interactions that lead to PIM can be challenging. For printed-circuit-board (PCB) antennas, at least, the choice of circuit material can play a significant role in the PIM performance from an antenna fabricated on that material. PCB antennas are growing in popularity for many wireless communications applications, and this is easy to understand. They are small, can be readily integrated into many compact electronic designs, and they can provide improved performance with high reliability over a wide range of environmental conditions. But PCB antenna can also be affected by PIM, which can significantly reduce the expected performance levels of a PCB antenna. A number of different circuit material properties and circuit fabrication issues can impact the PIM behavior of a PCB antenna, and some of those circuit material properties and circuit fabrication practices will be reviewed here. But first, it may help to better understand a bit more about PIM and how it is generated. PIM is essentially caused by the nonlinear mixing of two or more signals in a passive component, where additional, unwanted signals are produced as harmonics or as sums and/or differences of the fundamental frequencies of the mixing signals. The nonlinear behavior can be the result of material characteristics, such as the composition of a PCB material or the blend of metals in a connector interface, or even a poor interface between modules, such as interfaces between a filter, a transmission line and an antenna. Although these unwanted PIM signals are at lower amplitudes than the tones that produce them, they can still cause interference with a desired receiver or with other systems. PIM may not be a problem for a single-channel radio system with an antenna set high on a mountain top, far from signals generated by other radio systems. But for most radio systems, such as cellular communications networks, the addition of PIM signals can cause interference and loss of performance. The interference generated by PIM can decrease receiver sensitivity along with causing other problems, such as increasing the number of signal sidebands and increasing the occupied bandwidth of a communications system, leading to adjacentchannel interference for other systems. The harmonic and spurious signal products, created by the two original fundamental frequency signals, are generally thought of as odd-order signal products, with the lower-order products, such as the third-order products, being at higher amplitudes than the higher-order products, such as the fifthand seventh-order products, and posing more Figure 1. This plot shows how two fundamental-frequency problems in communications systems, such as in signals (f1 and f2) can combine to generate the spurious signal products associated with PIM. terms of degrading receiver sensitivity. When the interference due to PIM falls within the bandwidth range of a receiver and is accepted by the receiver (as noise), it elevates the noise floor of the receiver. The results of the elevated noise floor in terms of wireless communications systems include reduced digital data rates and an increased number of dropped calls. Figure 1 offers a graphic representation of how two fundamental-frequency tones (f1 and f2) can generate a larger number of harmonic and spurious intermodulation products. 16 Antenna Systems & Technology Winter 2013 www.AntennasOnline.com http://www.AntennasOnline.com

Table of Contents for the Digital Edition of Antenna Systems & Technology - Winter 2013

Editor’s Choice
LTE's MIMO Requirement Strengthens the Need for Active Antenna Systems
Optimizing Cell and Tower Sites During the Data Explosion
Improve Passive Intermodulation Performance with Specialty High-Frequency Circuit Materials
Next Generation Wireless: Riding on the Back of Millimeter Waves
Antennas
Components/Subsystems
Test & Measurement
Marketplace
Industry News

Antenna Systems & Technology - Winter 2013