Minimizing PIM Generation from RF Cables and Connectors
By Fred Hull, Director of Engineering, San-tron, Inc.
Understanding mechanical tolerances, coaxial design details and connector materials used in cable assemblies helps produce communications equipment with the lowest levels of passive intermodulation (PIM).
Communications system interference can come in many forms, but one of the most difficult to defeat is passive intermodulation (PIM) distortion. Unlike noise, which tends to cover broad bandwidths and is easily filtered, PIM results in discrete signals that fall within the operating bandwidth of a wireless communications receiver, capable of blocking desired signals. At sufficiently high magnitudes, PIM distortion can desensitize a communications receiver, resulting in poor quality of calls in a cellular system or even dropped calls. Reliably good communications system performance depends on minimizing PIM. And choosing the right cable assemblies can go a long way towards helping to shave PIM distortion.
What is PIM? It is the result of the nonlinear characteristics of the components and materials that channel communications signals. It is caused when multiple tones mix to generate unwanted harmonic signals. Sometimes referred to as “ghost” signals, these harmonic signals can interfere with the proper operation of a communications system when they occur at high enough levels. For example, for a wireless cell tower and its base transceiver station (BTS) attempting to process the very low-level signals from handheld portable cell phones, unwanted PIM harmonic signals can block the desired cell phone signals from ever reaching the cell tower receiver.
When two signals, f1 and f2, coexist, they can mix and produce third-order, fifth-order, seventh-order, and higher-order harmonic signal products. The third-order products are at the highest power levels and are the greatest concern as potential interference. The other harmonic mixing products decrease in level with increasing order. Third-order PIM products can be produced by a combination of 2f2 – f1 or 2f1 – f2. For example, in the PCS band, cellular communications towers transmit very strong signals at 1930 to 1990 MHz and listen for (receive) return signals in the band from 1850 to 1910 MHz. The strong transmit signals can generate PIM “ghost” signals from 1870 to 1910 MHz [such as 2(1930) – 1990 = 1870 MHz]. When the strength of these “ghost” signals rivals the thermal noise floor of the receiver system (-120 dBm), the receiver can become confused, not being able to tell the difference between a real phone call signal and a “ghost” signal, resulting in a dropped phone call and lost communications system capacity. This condition causes frustration for cell-phone users and lost revenue for cellular service providers.
Wireless communications systems work over a wide dynamic range of signal levels, often with moderate to extremely low-power signal levels. The noise floor of a BTS is dependent upon the modulation technique; however, a typical noise floor is generally -120 dBm. Therefore, typical wireless BTS receiver sensitivity is about -110 dBm.
Given that a typical cell phone can transmit signal power levels to about +22 dBm (about 0.16 W), and assuming a drop in signal level due to free-space path losses for an average one-mile distance between the cell phone and the BTS antenna tower, a signal from a cell phone arriving at a cellular tower site may have a power level of about -109 dBm (about 0.000000000000013 W). For such a cellular communications system to operate effectively, any unwanted “ghost” signals such as PIM signals must be maintained below the noise floor of -120 dBm. A cellular BTS that transmits a downlink signal of +30 dBm (1 W) per channel provides more operating margin at the receiver than the lower-power handheld cell phone, yielding a lower-level limit of about -150 dBc, requiring that PIM signal levels be controlled or suppressed to -150 dBc or more, an extremely low signal level by any standard.
PIM is caused by the nonlinear characteristics of different components in a communications system, and those nonlinear characteristics can stem from the effects of corona generation, the use of paramagnetic materials, and the effects of current saturation. Corona generation can occur at high power levels due to sharp edges at metal-to-metal junctions. Although it is not extremely common, it should be considered as a possibility during the design process for any component intended for communications system use. Under high-voltage conditions, electrons will congregate at sharp corners, with the high power levels driving these electrons to the edge of the electrical junction, causing the dielectric material to become locally ionized. When tested in a vacuum the corona will display this ionization as a glowing purple light cloud similar to the Northern Lights. Corona generation can be avoided by developing RF geometries that support the expected power levels of an application.
Paramagnetic materials exhibit magnetic behavior in an applied AC field. Such materials should not be used in any components intended for low-PIM communications applications. Stainless steel components, silver-clad steel center conductors, and nickel plating are the most common types of paramagnetic materials in communications system components. A permanent magnet will not typically attach itself to these materials, but when an RF signal is applied to a paramagnetic material, the same type of hysteresis is developed which can initiate PIM harmonics. Care must also be taken when considering sources of brass, as impurities within the brass can promote PIM generation. This has been particularly troublesome to manufacturers of magnetic-resonance-imaging (MRI) equipment, where any paramagnetic impurities can hinder proper performance. In fabricating components for minimal PIM in communications systems, some of the safe materials to choose are brass, phosphor bronze, beryllium-copper (BeCu), aluminum, silver, gold, and albaloy (white bronze plating), using certified ASTM specified materials whenever possible.
Current saturation is a phenomenon that occurs at the microscopic level and is one of the top contributors to PIM generation. At junctions between conductors, electrical connections are not often smooth and uniform across the components. Due to irregular surface finish (surface roughness similar to microscopic sandpaper), an electrical junction can include a small group of high points like micro-mountain tops. As a result, the current flow is not distributed efficiently and evenly across the mating components. However, with additional mating force between these surfaces, the micro-mountain tops can be partially flattened to increase the contact surface area and reduce current saturation within the junction or connection. This can help minimize or negate the generation of PIM.
Oxide layers that develop on plating surfaces of mating components can also impact current saturation. These oxides establish an insulating layer between conductors that results in reduced surface area for current flow. Through the use of material combinations of hard and soft materials with strong wiping action between mating surfaces, the action of mating these components can break down the oxide barriers, establish an improved connection, and further negate PIM generation.
The use of coaxial cable assemblies with braids can present challenges when attempting to achieve low PIM performance in a communication system. When such cables undergo flexure, the braids rub over each other and are constantly repositioning themselves. Because of such dynamic conditions, ground currents are continuously being rerouted through the fine braids, presenting opportunities for current saturation and non-transverse-electromagnetic (non-TEM) modes or eddy currents within the cable braid structure. Although this is not fully modeled, evidence of this effect is apparent during vibration testing of cables while measuring PIM levels.
For example, consider evaluations of three different cable types terminated in industry standard Type N plugs. An RG-400 coaxial cable has a dual braid structure of woven round conductors.
Under a static condition, this cable will support PIM performance of -145 dBc. But during vibration, this type of cable becomes unstable and the PIM performance drops to -90 dBc.
When the vibration is removed, the PIM performance returns to -145 dBc. Another braided cable type, flexible-141, has a flat ribbon braid followed by a woven round overbraid. When tested under a static condition, it supports PIM performance of -155 dBc. Under vibration, the PIM performance drops to -148 dBc. When the vibration is removed, the PIM performance returns to -155 dBc. A third braided cable type, TCOM-240, has a flat ribbon braid, followed by an intermediate foil, followed by a woven round overbraid. Tested under a static condition, the PIM performance is -155 dBc. With vibration, the cable remains extremely stable, with PIM performance degrading only to -153 dBc. Once the vibration is removed, the PIM performance returns to a level of -155 dBc. As these three examples show, braid structure is extremely important to the development of low-PIM system integration and one of the more critical elements in designing and deploying reliable, robust cellular communications sites.
Minimizing current saturation is one key to minimizing PIM generation in coaxial cable assemblies. Some of the design considerations for minimizing current saturation include fabricating smooth surface finishes, using single-body constructions and, when multiple-body constructions are required, not using threads. Use heavy press fits for pins and sockets to maximize the number of socket tines to increase the number of contact points. Also important is the choice of high-quality, engineered cables. To support these efforts, San-tron has developed a suite of low-PIM cable assemblies called SRX™ cables to support system integrators. Seven different deployment scenarios encompassing cabinet integration, short cable runs or jumper applications, long haul cable runs, indoor use, outdoor use, riser, and plenum environments have been accommodated for. As an example, for in-cabinet and plenum-rated deployments, low-PIM SRX cable assembly solutions are formed through the combination of eSMA™ and eSeries™ connectors and flexible-141, conformable-141, and semirigid RG-402 cables.
The flexible-based solutions support plenum-rated deployments to 150 W at 2.5 GHz. These assemblies exhibit attenuation of 22 dB per 100 ft., with bend radius of 0.2 in. They can handle operating temperatures to +125°C. Raw flexible-141 cables are specified for PIM level of -150 dBc although they have been characterized with typical PIM levels of better than -157 dBc. These cables are light in weight, two of them are flexible, and they support the integration of subsystems and in-cabinet deployments.
For short runs and jumper applications requiring highly flexible, robust assemblies (such as in indoor and outdoor use, risers, and in plenum deployments), San-tron has developed a unique solution for the clamp-type termination of low-PIM cable assemblies to braided, flexible coaxial cables. Basic deployment based on TCOM-240 cable supports power levels to 160 W at 2.5 GHz with attenuation of 13 dB per 100 ft. and bend radius of 2.5 in. These assemblies can handle operating temperatures to +85°C with PIM performance of -155 dBc that is stable with vibration within a 2 dB window. For flame-retardant requirements assemblies employ TCOM-240-FR cables, while for plenum-rated installations, assemblies feature SFT-205 cables with a temperature rating to +165°C.
For long-haul runs requiring robust, low-loss deployments (such as in indoor and outdoor in stations, in risers, and in plenum deployments), San-tron has developed a technique that sets extremely low and stable PIM levels and supports continuous bending and flexure. Basic deployment with TCOM-400 cable supports power levels to 310 W at 2.5 GHz with low attenuation of 7 dB per 100 ft. and a bend radius of 4 in. These assemblies, which are rated to operating temperatures to +85°C, also exhibit PIM performance of -155 dBc that is stable with vibration within a 2 dB window. For flame-retardant requirements assemblies employ TCOM-400-FR cables, while for plenum-rated installations, assemblies feature SFT-393 cables with a temperature rating to +165°C.
In conclusion, the use of San-tron’s SRX coaxial cable assemblies offers deployment advantages when low PIM levels are critical. eSMA series connectors control PIM to less than -153 dBc at frequencies to 20 GHz, while eSeries Type 7/16 connectors operate through 8 GHz with PIM levels of -174 dBc or less. SRX cable assemblies manufactured with these connectors and TCOM and SFTcables support performance through 6 GHz with PIM of less than -155 dBc. SRX low-PIM cable assemblies are available in standard lengths as well as custom configurations (contact factory).
Notes: TCOM and SFT are trademarks of Times Microwave Systems, Wallingford, CT. SRX, eSMA and eSeries are trademarks of San-tron, Inc.
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