Terminations matter for wireless

MANY PROBLEMS IN HARD-WIRED INDUSTRIAL NETWORKS can be tracked to wiring and connectors. Running all that wire and cable from field devices to I/O racks, multiplexers, fieldbuses or device networks, and then to a control system involves miles of wire, dozens of connectors, and hundreds of chances to screw something up. All this time, trouble and expense is driving many industrial OEMs and end users to consider wireless communications, in hopes of eliminating wiring and connector problems forever.It isn’t quite that easy. Although the number of connections may be fewer, their quality is critical.Craig Skarpiak, product line manager, wireless products, at B&B Electronics (www.bb-elect.com) says the cellular industry learned this first. “The cellular industry learned that as the digital-modulation schemes increase data throughput and capacity, care in termination of the RF path becomes critical to the error performance of the system,” explains Skarpiak. “Improper termination could lead to increased system noise due to antenna system VSWR [voltage standing wave ratio—ed.] and spurious distortion noise. Additional system noise takes away from the overhead of the digital error-correction feature, thus eroding its ability to correct errors.”This isn’t a just problem with wireless, of course. “When an RF circuit becomes functional, the signal enters the antenna and gets transformed into a current that needs to be carried via a conductor,” says Davis Mathews, product marketing manager for wireless devices at Phoenix Contact (www.phoenixcon.com). “The conductor makes termination very critical in any electrical installation. It's not only the cables, waveguides and circuit traces that matter, but also the wire connection points used, power terminations and adapters.”If given a good signal to work with, modern error-correction schemes work well. “Digital communication systems like WiFi 802.11, Bluetooth, and Zigbee 802.15.4 are inherently robust when it comes to rejection of RF noise and interference,” says Skarpiak. “The digital error-correction algorithms and spread-spectrum techniques embedded within these RF devices help create this robustness. However, like most technologies, they do have limits to their performance.”The trick is to keep signal loss to minimum. All the usual techniques for minimizing loss should be used when leading up to the wireless system. These include using low-loss cables and industrial connectors. With wireless systems, you also have to pay attention to the connection and termination of RF components.“Wireless devices terminated to their corresponding antenna systems must have minimal loss within the overall system gain,” says Mathews. “Use the highest-quality low-loss cables between the antenna and the wireless device to obtain better overall system gain. This allows for better transmission of the RF signal.”Skarpiak agrees, adding, “We need to consider the RF termination most importantly when the antenna is separated from the radio device to improve node coverage.”The Antenna Interface Standards Group (AISG, www.aisg.org.uk) wrote the AISG1 standard to address these problems. “This standard has been produced…to facilitate the introduction of antenna line products with remote control and monitoring facilities,” reports AISG. Their membership list has 50+ companies, but almost no control equipment suppliers (with the exception of Siemens). Its Control Interface standard was first issued in 2003 and revised in July 2004, and it incorporates “provisions” from dozens of other specs, including the IEEE connector standard 60130-9.We put out an editorial call to dozens of networking and connector companies, asking if they had any connector products that specifically addressed the wireless issue, and only Lumberg (www.lumberg.com) and Phoenix Contact responded. “We make UMTS radio tower connectors that comply with AISG standards and IEC 60130-9 specifications,” says Ed Gambacini, distribution sales manager for Lumberg. “These circular DIN connectors facilitate remote communications between radio tower base stations and the antennas mounted on top. They come in standard 8-pole configurations and are rated IP67 watertight.” Mathews says there are several types of connectors available in the market for industrial wireless devices. “Some of the most popular types used in the industry today are N, SMA, MCX and BNC,” he advises. “Phoenix Contact offers low insertion loss integrated surge protectors and wireless I/O devices using a combination of these connector types.”Wireless systems reduce the number of terminations and connections you need to make, so you can afford to pay close attention to the few that are left. When connecting wireless devices, use connectors and cables with the lowest loss possible, and get familiar with the AISG and IEEE 60130-9 standards.