Kevin Moyher, Product Manager for Commercial Products at Times Microwave Systems, discusses the effects of interference from RF breakdown from corona discharge, Passive Intermodulation (PIM), and multipaction on communications satellite systems.
The RF and microwave applications that power many of the critical functions within today’s communications satellite systems demand a larger bandwidth integration, higher component integration, and more power handling capability. These requirements create a higher electromagnetic field density inside the devices, resulting in many issues that limit the ability of a satellite subsystem to handle high-power signals. The most relevant are interference from RF breakdown from corona discharge, passive intermodulation, and multipaction effects.
In RF and microwave technology, corona discharge occurs when an electron plasma is formed due to the ionization of the gas surrounding a high electromagnetic field. High electromagnetic fields naturally result in higher ionization since more energy is transferred to electrons in the surrounding gas.
Corona discharge poses less risk to the actual satellite itself (it cannot occur in vacuum conditions because gas is required to generate the electron plasma). Still, it does create potential issues for the Telemetry, Tracking, and Control (TTC) subsystem of a satellite which provides the critical connection between the satellite and facilities on the ground. In addition, Corona discharge in an RF component is highly damaging because the decrease in transmitted power eventually leads to the destruction of the power source.
The corona discharge power threshold, or the largest input power that a device can admit without developing a corona discharge, is critical for the RF and microwave component designer to consider. In addition, system designers should work with RF component suppliers to ensure they are using proper materials and have experience in such conditions for optimal results.
Passive Intermodulation (PIM)
Passive Intermodulation (PIM) is a distortion generated by two or more high-power signals interacting with non-linear characteristics in the RF path. When two or more high power frequencies exist on the same RF interconnect, there is the chance to form additional frequencies, which will raise the noise floor.
Higher frequency ranges are more prone to PIM effects because the frequency bands are very close. Ultimately, cables, connectors, and termination quality can all play a role in PIM. Performance degradation will occur due to non-linear junctions of materials and components, or 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.
First, ensure that connectors are securely and adequately tightened and choose suitable materials and platings to help reduce PIM issues. Next, eliminate any non-linear contacts within the RF interconnect and any poor electrical contacts. PIM issues can be caused by ferrous materials, loose parts, parts with rough surfaces, oxidation, residual flux, etc. 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.
The use of low PIM coaxial cable assemblies can also help mitigate the effects of PIM. Several options are available depending on the type of satellite subsystem being designed.Many low-PIM cables are corrugated copper rigid or semi-rigid cables. Popular options include a low loss, low PIM corrugated copper cable, which also comes in a fire-retardant version. Helically corrugated designs also create a more flexible and rugged cable.
However, coaxial cable assemblies that can accommodate tight bend radii are increasingly required for installations within smaller internal packages with less physical space. Flexibility does not typically go along with PIM performance, and a traditional low-PIM corrugated copper cable can be damaged once the maximum bend radius is exceeded.
If a kink occurs, the cable needs to be replaced. However, the damage may not be immediately evident until the system degrades and the troubleshooting process begins. There are new low-PIM cable assemblies available that use a tinned, copper, flat-braid outer conductor construction to create an ultra-flexible cable with a durable FEP outer jacket.
Furthermore,the connector is an important part of the RF cable assembly. As cables get smaller, connectors need to do the same. Therefore, the industry is moving to miniature connector configurations, including the NEX 10 or 1.0-2.3. 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 non-linear electron resonance phenomenon that occurs when RF fields accelerate free electrons in a vacuum and cause them to impact with a surface, which 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. In addition, it can cause the internal breakdown of the dielectrics, erosion of the metal cavities, and 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 selecting materials 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.
RF and microwave applications that power many critical functions within today’s advanced satellite communications systems need to be enhanced. However, a higher electromagnetic field density inside the devices can result in a multitude of issues that may limit the capability of the satellite’s subsystem, including RF breakdown from corona discharge, interference from PIM, 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.