Bench Talk

RF Connectors Boost Embedded Systems

(Source: zef art - stock.adobe.com)

In recent years, the design of high-quality RF/microwave systems has experienced a significant transformation thanks to four pivotal advancements:

• the emergence of RFICs and monolithic microwave ICs (MMICs)
• the enhancement of design automation through EDA software
• the increased availability of advanced antennas
• the development of superior RF/microwave and data connectors.

At the forefront of these cutting-edge RF/microwave connectors are the VITA OpenVPX-specified devices, which have proven to be invaluable across a wide range of military and industrial applications, paving the way for more efficient and innovative communication systems.

The important Sensor Open Systems Architecture (SOSA) standard leverages the OpenVPX standard to define card profiles with specifications for features such as pinouts, networking capabilities, and serial interfaces. The SOSA Reference Architecture is specifically intended to support US DoD Command, Control, Communications, Computer, Intelligence, Surveillance, Reconnaissance, and Tracking (C4ISRT) systems development, but is very useful in many applications.

The VPX (VITA 46) specification was ratified in 2007 as the next-generation VME backplane. Then, in 2009, VITA introduced OpenVPX (VITA 65) to address interoperability issues. This brought profiles so that one manufacturer’s boards would always talk to another’s in a designated backplane/chassis setup. OpenVPX defined different "planes" in the specification (control, data, expansion, management, and utility planes) that grouped other signals/functions. Then, it defined the signal types in “pipe” classes, which include ultra-thin pipe (x1), thin pipe (x2), fat pipe (x4), and double fat pipe (x8) as the principal groups, as well as a couple of others.

SOSA requirements for 3U and 6U modules refer extensively to the VITA VPX standards. However, SOSA restricts good VPX primary slot profiles to a relatively small set of three 6U and six 3U profiles.

Meanwhile, the full VITA 65 OpenVPX standard provides dozens of 6U VPX and 3U VPX slot profiles with user-defined pins and, therefore, systems with unique backplanes designed to be used with specific modules. SOSA limits this individuality.

The SOSA standard also recognizes the need for excellent cooling methods so that power growth can be accommodated to support future processing needs. SOSA has defined a 1.00” slot pitch for conduction and liquid cooled modules to accommodate various cooling methods and a 1.5" pitch for air flow through modules.

While the SOSA approach establishes guidelines for military applications in command & control, communications, computers, cyber warfare, intelligence, surveillance, and reconnaissance (C5ISR), many civilian/commercial systems applications can benefit from this design philosophy. Aircraft and railroad control designs are sure-fire users, and factory industrial control systems will also help.

First, let's take a look at the four S’s of microwave connectors:

• SOSA - Sensor Open Systems Architecture
• SMP - miniature 50W plug-in RF coaxial connectors for applications up to 40GHz, with easy push-on connect
• SMPM - a smaller 50W coax interface with a frequency range of DC to 65GHz (Figure 1)
• SMPS - even smaller 50W coaxial connectors with a maximum frequency of 65GHz or 100GHz. These connectors are 45 percent smaller than the SMP connectors and 30 percent smaller than the SMPM.

Figure 1: An SMPM vertical PC board mount connector for a single cable connection with a frequency range of DC to 65GHz. (Source: Mouser Electronics)

The most compact connector series requires less than 2mm x 2mm of PCB real estate. Larger connector types are meant to support high-power needs, such as the 5G wireless infrastructure. As an example, the sizes of quad coaxial modules for a backplane vary from the SMP module at 0.890" wide to the SMPM at 0.668" and the SMPS at 0.470” (Figure 2).

Figure 2: RF/coaxial SMPM and SMPS multiport PCB connectors often come in dual and quad configurations and enable extremely high frequencies of DC up to 100GHz. (Source: Mouser Electronics)

Table 1 shows the basic specifications of SM-series miniature 50W plug-in RF coaxial connectors.

Table 1: The basic specifications of SM-series miniature 50W plug-in RF coaxial connectors. (Source: Mouser Electronics)

All three connectors offer a Voltage Standing Wave Ratio (VSWR) (2MHz to 40GHz) of 1.5:1 maximum and insertion loss of 0.12 (as seen in Table 1). They are rated for -65°C to +165°C operation and allow radial misalignment of up to 3°, and axial misalignment of .010” maximum.

For example, the Amphenol VITA 67 coaxial interconnects offer a robust, rugged, high-speed cabled solution (Figure 3). Two specifications are available: VITA 67.1 with 3U, 4-port SMPM configuration and VITA 67.2 with 6U, 8-port SMPM configuration. The floating SMPM coaxial contacts ensure excellent RF performance in any mating condition.

Figure 3: A high-frequency VITA 67 interconnect configuration. (Source: Mouser Electronics)

These parts are also designed for side-by-side implementation with VITA 46 high-speed data connector hardware (Figure 4) and can be connected to .086" and smaller diameter coaxial cable types (Figure 5). The design engineer can also choose a direct-to-PCB configuration—instead of a cabled.

Figure 4: Amphenol’s R-VPX VITA 46 backplane connectors can achieve data rates over 10Gbps. (Source: Mouser Electronics)

Figure 5: Coaxial plug-in modules are available in several sizes. (Source: Mouser Electronics)

Advancements in RF/microwave systems have been made more accessible due to crucial developments such as RFICs/MMICs, improved EDA software, advanced antenna availability, and superior RF/microwave and data connectors. The VITA OpenVPX-specified devices and SOSA architecture play a vital role in providing standardized solutions for military, industrial, and civilian applications. The SOSA standard offers excellent cooling methods and promotes interoperability, ensuring future-proof and efficient designs. The range of microwave connectors, including SMP, SMPM, and SMPS, offers varied capabilities to cater to diverse application requirements.

As the demand for quality RF/microwave connections continues to rise in military, SATcom, and 5G/6G wireless applications, companies like Amphenol are providing a wide range of reliable and high-performance products. These innovative developments have the potential to significantly impact various industries, including IoT, satellite, networking, aerospace, aircraft and train control, and telecommunications.

Jim Harrison is an electronics engineer and has held senior design engineering positions with industrial automation and scientific instrumentation companies since 1989. In 2004 he moved to writing and was a Sr. Editor with Hearst Business Media, Electronics Products Magazine for 14 years. He is now a consultant with Lincoln Technology Communications.