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Perusing PCBs For Low PIM Levels
Passive intermodulation (PIM) is the unwanted mixing of two or more signals in a passive circuit or component, resulting in unwanted spurious or harmonic signals. These additional signals can clutter a system’s operating passband and cause interference in a system’s receive band. Although PIM is often associated with certain high-frequency passive components within a system, such as connectors, cables, filters, and couplers, it can start with the printed-circuit-board (PCB) materials, in particular when those materials are used for components critical to communications systems, such as PCB-based antennas and filters. For applications where PIM can be a concern, circuit materials should be selected not only for their desired electrical and mechanical performance levels but for minimal generation of PIM across all the operating conditions of those applications.
PIM is caused by the nonlinear mixing of multiple signals, where unwanted signals are produced as harmonics or as sums and/or differences of the fundamental frequencies of the mixing signals. PIM can stem from poor mechanical junctions between high-frequency passive components, such as connector interfaces and poor solder joints, but can also result from material characteristics, such as the composition of a PCB’s dielectric material or the blend of metals in a PCB’s conductive layer. For example, ferromagnetic materials, such a nickel and steel, are notorious for their capabilities in generating nonlinear responses as reactions to electromagnetic (EM) fields. Nickel, which is often used as a barrier layer in PCBs between copper conductors and gold finish, must be avoided in any circuit where PIM is a concern. The interfaces of a PCB’s material components, such as the interface of a PCB’s dielectric and conductive layers, can also give rise to PIM at levels sufficient to cause interference, for example, in a sensitive receiver.
Careful choice of PCB materials can contribute to minimizing PIM levels, which are typically evaluated at a decibel level relative to a carrier (dBc). Levels of -145 dBc and further below the carrier (more negative) are considered good PIM levels for many communications applications. Of course, a large part of performing a meaningful evaluation of different PCB materials for their PIM performance involves proper maintenance of test fixtures and measurement equipment. Because PIM level are typically low in amplitude, nothing in a test setup can artificially improve a material’s or component’s PIM performance, only degrade it. Something as simple as debris in the connector mating interface area can contribute to the measurement of degraded PIM performance in a test setup. When evaluating components or PCB materials for PIM performance, measurement system maintenance is essential.
PTFE circuit materials have typically been a first choice for passive components such as antennas and filters where PIM performance was critical. But compared to other high-frequency circuit materials, PTFE tends to be expensive, and can require some special handling during circuit fabrication. Fortunately, newer, non-PTFE-based PCB materials have been shown to provide PIM performance that is as good or better than the PIM performance levels possible with PTFE-based materials.
For example, several non-PTFE materials from Rogers Corp. (www.rogerscorp.com), RO4725JXRTM and RO4730JXRTMcircuit materials, have consistently exhibited PIM performance levels of -164 dBc or better when used as a starting point for PCB antennas. Unlike the special treatments required for processing PTFE-based materials, circuit fabrication with these two materials is very much like processing standard PCB materials. The RO4725JXR and RO4730JXR circuit materials exhibit low dielectric-constant (Dk) values, of about 2.55 and 3.00 in the material z axis at 10 GHz, respectively, that make them attractive for many RF/microwave applications.
PIM performance may be important for a PCB antenna, but it may also be affected by other material parameters, such as the temperature coefficient of dielectric constant (TCDk). This parameter, which is a measure of changes in Dk with temperature, is typically high for PTFE-based circuit materials. Ideally, for outdoor applications, the TCDk should be as low as possible to minimize shifts in Dk value with temperature which may or may not impact the PIM performance.
As evidenced by their TCDk values, the RO4725JXR and RO4730JXR materials provide Dk values that are extremely stable with environments of changing temperature. They feature TCDk values of +34 and +32 ppm/°C, respectively, for the RO4725JXR and RO4730JXR circuit materials, indicating that electrical performance should remain stable over a wide range of temperatures, with minimal effect on PIM performance. This stability is related to good TCDk properties only. Dk variations which are possible for a thermoset material exposed to elevated temperatures over a long period of time are not addressed in this blog.
Fortunately, the low PIM levels possible with these two non-PTFE materials are achieved without sacrificing electrical or mechanical performance. The RO4725JXR and RO4730JXR laminates have been formulated as antenna-grade PCB materials, capable of low insertion loss and low loss tangents of 0.0022 measured at 2.5 GHz and 0.0027 or less measured at 10 GHz and room temperature. These are “environmentally friendly” halogen-free materials, RoHS compliant and compatible with high-temperature lead-free processing.
The non-PTFE materials incorporate specially formulated thermoset resins and unique fillers consisting of closed microspheres which contribute to light weight and low density along with the low PIM characteristics. In fact, these laminates are typically about 30% lighter in weight than PCB materials based on PTFE/glass combinations. The RO4700JXR laminates have been formulated for excellent mechanical stability. They feature a low Z-axis coefficient of thermal expansion (CTE) of better than 30 ppm/°C for design flexibility, with CTE of typically 25.6 ppm/°C in the z-axis for RO4725JXR laminates and 21.1 ppm/°C in the z-axis for RO4730JXR laminates, when measured from -55 to +288°C for both materials.
Whether for base-station or other wireless antennas, or for other passive components, such as couplers and filters, for which PIM must be held to a minimum to ensure that the highest quality of voice, data, and video communications is maintained in a system, the choice of PCB material can greatly determine the final level of PIM achieved no matter how careful the circuit design. When other factors are considered, such as outdoor operating temperature range, it is clear to see that achieving a target PIM level in a wireless communications system starts with specifying low-noise PCB materials that have been formulated for that type of performance.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.