RF Interconnect is another term for RF connections between devices, boards, ports, RF cable assemblies, waveguide, probes, antenna, and etc. As discussed in a previous blog post, What is an RF Connection, RF interconnects are a method of transferring RF signals from one node to another either conductively, through a transmission line (RF coaxial connectors/ coaxial cable assemblies), or through a waveguide. It can be argued that an RF antenna is also a form of interconnect, though it is really a transducer that converts electrical signals carried conductively to a free-space wavefront.
When speaking of RF interconnect, this generally refers to connections between circuit boards, devices, subsystems, systems, and between interconnects. The reason RF interconnects are used, especially transmission lines and waveguides, is because containing the RF signal energy in a transmission line or waveguide is generally a much more efficient method of transferring that signal energy from one point to another, that signal is shielded to some degree from interference/noise injections from outside sources, and these types of connections help to ensure that the signal is harder to intercept, jam, block, or otherwise interfere with.
There are a variety of types of transmission lines and waveguides, as well as other types of RF interconnects. The most common types of RF transmission lines that most engineers and professionals are aware of are coaxial transmission lines. These make up the bulk of RF interconnects between modules, subsystems, and systems. Waveguides such as rectangular, circulatory, and elliptical waveguides are also commonly used for high power, precision, and extremely high frequency signals in the millimeter-wave range. There are other types of transmission lines, including planar transmission lines used on planar substrates/laminates, such as PCB, low-temperature co-fired ceramic (LTCC), high-temperature co-fired ceramic (HTCC), flex-PCBs, semiconductors, and others. Example planar transmission lines are coplanar transmission lines and striplines, of which there are a variety of configurations. There are also planar waveguides, such as a microstrip.
There are also a variety of different connector and adapter styles that should be included in the RF interconnect categories. These include adapters from different transmission line types, such as a planar transmission line to a coaxial transmission line, or between different types of coaxial connector types. There are also adapters between coaxial transmission lines and waveguides. It is important to note here that coaxial transmission lines are broadband RF signal carrying medium, while waveguides are banded based on the geometry and structure. Hence, any adapter between a coaxial transmission line and a waveguide will have a lower cutoff frequency limited by the waveguide and a higher cutoff frequency either limited by the coaxial transmission line or waveguide, whichever is lower.
It can be argued that certain resonant coupling structures can also be considered interconnects, as well as components such as directional couplers and probes. However, passive structures like directional couplers and probes are generally not considered RF interconnects as they lack a conductive path for the signals and rely on coupling to fields external to the conductors of the main RF signal paths.