Mitigating the Effects of PIM, Multipaction, and Corona Discharge in Satellite Systems
by Kevin Moyher, Product Manager for Commercial Products, Times Microwave Systems
Today’s communications satellites require broader bandwidths, higher levels of functional integration, and higher power handling capability. However, these and other factors create higher electromagnetic field densities inside the devices and can create a variety of issues that can limit the capability of a subsystem to handle high power signals. The most onerous are passive intermodulation (PIM), RF breakdown from corona discharge, and multipaction effects.
PIM is a distortion/noise generated when two or more high-power signals interact with non-linear components or other structures in the receive path and when these signals exist on an RF interconnect, additional frequencies can be created that increase the noise floor. Higher frequencies are more prone to PIM effects because the operating bands are very close to each other.
Ultimately, cables, connectors, and termination workmanship can all play a role in PIM. Performance degradation will occur due to non-linear junctions of materials and components. In other words, junctions where the current does not increase linearly with voltage. This becomes a problem because PIM can create interference that limits receive sensitivity, lowering the system’s reliability, data rate, and capacity.
PIM Performance Testing
PIM is also an excellent criterion for measuring the quality of an interconnect, especially mechanical integrity where VSWR and insertion loss may not detect an issue. In an ideal system, 100% of the energy is transmitted. However, suppose there is a cold solder joint or air pocket in the solder, loose connections, or a related condition. In these cases, they will be easier to detect with passive intermodulation rather than with a baseline electrical measurement.
In the telecommunications industry, it is common practice to test for the magnitude of the third harmonic by placing two 20 W signals onto the RF interconnect. A PIM level requirement of -153 dBc or higher is standard in the industry. Two types of PIM tests are performed. The first test is static (no movement of the cable), which is a fairly easy bar to meet if the proper materials and plating are used and all threaded connections are correctly tightened.
The second test is dynamic, in which connectors are tapped to detect conductive particles within the interface, and the cable is flexed or side-loaded behind each of the connectors. Achieving low PIM in the dynamic test is much more difficult because flexing of the cable will transmit a force to the electrical transition between the cable and connectors to detect any non-linear contact within the transition.
Mitigating the Effects of PIM
The need for RF interconnect solutions that can accommodate these critical connections while minimizing PIM will continue to grow. First, ensure that connectors are securely and adequately tightened and choose suitable materials and plating to help reduce PIM. Next, eliminate any non-linear contacts within the RF interconnect and any poor electrical contacts.
PIM can also be caused by ferrous materials, loose parts, parts with rough surfaces, oxidation, and residual flux. If conductive material is used, conductive particulates on the face of the dielectric or within the interface itself will cause problems and may move directly within the connectors when installed.
Low-PIM coaxial cable assemblies (Figure 1) can also help mitigate the effects of PIM. Several options are available depending on the type of satellite subsystem being designed. Many of the best low-PIM cables are corrugated copper rigid or semi-rigid types, and popular options include a low loss, low-PIM corrugated copper cable available in a fire-retardant version and a helically-corrugated cable that creates a more flexible and rugged cable.
Coaxial cable assemblies that can accommodate tight bend radii are required for installations within smaller internal packages with less physical space. However, flexibility does not typically go hand-in-hand with PIM performance. A traditional low-PIM corrugated copper cable can be easily damaged if the maximum bend radius is exceeded. Once a kink in the cable occurs, it must be replaced.
Kinking can occur in the corrugated copper underneath the cable’s outer jacket. The damage may not be immediately evident until the system degrades and the troubleshooting process begins— potentially creating delays, cost overruns, and more. New low-PIM cable assemblies are emerging that use a tinned, copper, flat-braid outer conductor construction to create an ultra-flexible cable with a durable FEP outer jacket.
The connector forms a critical part of the RF coaxial cable assembly, and as cables get smaller, connectors do as well. As a result, the industry is moving to more miniature connectors, including the NEX 10 or 1.0-2.3 configurations. These connectors are designed for low-PIM performance and are available with a threaded coupling. Snap-on designs are emerging as well.
Multipaction (or the multipactor effect) is a nonlinear electron resonance phenomenon that occurs when RF fields accelerate free electrons in a vacuum and cause them to impact with a surface that, depending on its energy, releases one or more electrons into the vacuum (secondary emission). When the electrons release and timing of the impacts are such that a sustained multiplication of the number of electrons occurs, it can lead to loss and distortion of the RF signal and damage to the RF components or subsystems.
In RF cable assemblies, multipaction can occur between the inner and outer conductors at the connector and can limit the delivery of RF power. It can cause internal breakdown of the dielectrics, erosion of the metal cavities, and can ultimately lead to the melting of internal components.
Multipaction risks can be mitigated by designing mating connectors with overlapping dielectrics to remove any free path between conductors, and by selecting materials that are less prone to secondary emissions. The shape of the paths can also be a factor as it affects the time taken by electrons to travel from the surface from which they are released to the surface they impact.
Corona discharge is a process in which an electron plasma is formed by the ionization of the gas surrounding a high electromagnetic field. Lightning is a good example of this phenomenon. The primary contributing factor to the increase in free electron density is the ionization of the gas molecules. Ionization is how electrons are extracted from a molecule/atom. The colliding electron must transfer enough energy to the molecule to overcome the binding energy of the electron to it to achieve ionization.
High electromagnetic fields result in higher ionization rates as more energy is transferred to electrons in the surrounding gas. When a high-field electromagnetic region is surrounded by a low field region, electron current will flow from the high-field to the low-field zone to compensate for the charge generation. In RF and microwave technology, corona discharge occurs when the high electromagnetic field can ignite, resulting in an electron plasma formation.
However, corona discharge poses less risk to the satellite itself as it cannot occur in a vacuum because a gas is required to generate the electron plasma. However, it could create potential issues for the Telemetry, Tracking, and Control (TTC) subsystem of a satellite that provides the critical connection between the satellite and facilities on the ground.
Ultimately, corona discharge in an RF component is highly damaging because the decrease in transmitted power eventually leads to the complete destruction of the power source. So, from a designer’s point of view, the corona discharge power threshold, or the largest input power that a device can admit without developing a corona discharge, is the critical parameter to consider.
Broader bandwidths, higher component integration, and more power handling capability are required in geosynchronous satellites, which means microwave components that power critical functions must be enhanced. However, these requirements create a higher electromagnetic field density inside the devices that results in a multitude of issues that may limit the capability of the satellite’s subsystem, including interference from PIM, RF breakdown from corona discharge, and multipactor effects.
The occurrence of any of these can severely degrade the functionality of a satellite. Therefore, system designers must work closely with the RF component supplier to ensure they use the optimal RF interconnect solutions to help mitigate the risks posed by these effects.