Increasing Flexibility in Rugged Defense Embedded Systems Using RF and Optical Connections in Open Architectures

by Ken Grob, Director of Embedded Computing Products, Elma Electronic

In January 2019, the secretaries of the U.S. Navy, Army and Air Force released a joint statement recognizing the importance of the development work being done cooperatively between industry and defense on open standards. This milestone document included specific recognition of several standards, including Sensor Open Systems Architecture (SOSA), and their importance in supporting the Modular Open Systems Approach (MOSA). 

The letter specifically states, “MOSA supporting standards should be included in all requirements, programming and development activities for future weapon system modifications and new start development programs to the maximum extent possible.”

OpenVPX: The Basis for Standardization

Interoperability is the primary focus of SOSA, driven by the realization of Army, Air Force and Navy officials that, by working together to standardize hardware interfaces, they could reduce costs across the board. This conceptual evolution brought with it a large number of interested parties, representing all facets of the industry—from the acquisition authorities (government), to integrators and contractors, to the product and services vendors. 

OpenVPX is a strong catalyst to the current open architecture standards development within SOSA.  The OpenVPX ecosystem is a robust group of embedded technology companies that have worked in the development of industry standards for decades.  By tapping into that knowledge base, SOSA has quick access to products and resources from a diverse group of manufacturers that have been working in tandem with one another in a cooperative as well as competitive environment.

Increasing Backplane Connectivity

As SOSA begins to gain traction, a subset of the OpenVPX community—those working on the VITA 65 specification—were particularly impacted. In fact, VITA 65, which is that standard’s identification for OpenVPX, is the foundation of the SOSA initiative. This open architecture will help improve the efficiency and speed of technology refreshes, while lowering long-term life cycle costs for warfighter systems such as radar, electronic warfare and signals intelligence equipment.

One significant development assisting the adoption of OpenVPX to the SOSA standards is the addition of apertures, which are the openings in VPX backplane slots that enable the use of blind-mate RF and optical connectors. This allows embedded, OpenVPX-based systems to integrate higher speed data connections that match the growing processing needs of these highly complex, data-sensitive applications. 

VITA 67.3, a standard developed under the VITA 65 umbrella to add the apertures, represents the one of the most significant advances in backplane connectivity.  New configurations in coaxial and optical connections can now pass signals through the OpenVPX backplane, providing enhanced interoperability and flexibility in a design that will absolutely benefit prime contractors designing systems for the warfighter.

How are Apertures Located in a Slot? 

VITA 67.3 defines the required mounting holes for each size aperture, the location contact type, and all allowed locations within a slot. C size apertures can be located in the J2 position for 3U backplanes or the J2, J3, J4, J5, or J6 position for 6U backplanes. The location of plug-in modules is driven by design from the backplane location. 

The standard’s documents also show the locations for full, half, and full+half size modules, and similar location charts can be found for necessary aperture locations in other related VITA standards (VITA 67.1, 67.2, 66.1, 66.4, 66.5). Using the daughter card contact requirements, the module designer determines connector location, whereas designers using 67.3 must drive their requirements afterwards.

Improving System Flexibility

The importance of these backplane connector modules is that they can be installed and replaced by the end user. The VITA 67.3 backplane apertures can support modules with different arrangements of RF and optical connectors for each aperture size. (Figure 1) 

 Connector modules incorporating optical ferrules, or a combination of optical ferrules and RF contacts, are defined in the VITA 66.5 standard, but utilize the apertures defined in VITA 67.3. These two standards work in tandem to define how to document new RF and optical module configurations.  These backplane connector modules can be installed and replaced by the system integrator or the end user. More importantly, VITA 67.3 backplane apertures can support modules with different arrangements of RF and optical contacts for each aperture size.

The apertures enable access to module RF and optical signals through the backplane, which requires a matching connector module to populate the apertures. Plug-in cards can be populated with an assembled connector module with multiple contacts or can be configured to directly launch from individual contacts mounted on the base card and mezzanine(s). 

System flexibility is what drives most open architecture initiatives for defense electronics, and VITA 67.3 and VITA 66.5 are no different, a main driver as to why the connector modules can be removed and replaced from a backplane to support different configurations of coaxial connectors or optical modules. The end user can change the RF or optical module to accommodate different plug-in cards, if a slot has this feature.

VITA 67.3 Supports RF Field Replaceable Modules with Direct Launch

VITA 67.3 and 66.5 were developed to define modules with contacts that could be directly launched from the base card or from one or more mezzanines on a base card.  To allow contact alignment during engagement, the contacts on the daughter card needed to be fixed and the mating contacts on the backplane had to be sprung.  Positioning and location would be determined by the spacing of the contacts within a row and by the spacing between the mezzanine cards and the base card and the location. All the permitted locations of a VITA 66 or 67 connector module within a slot is given, defined within their respective standard. (Figure 2)

VITA 67.3 enables flexibility in the design by allowing two different types of contacts to be used on the apertures. In this example, four cable assemblies (upper right-hand corner) can be snapped into the top row. Below that, edge launch contacts that might be used to launch directly from the mezzanine or baseboard are included. The contact row spaces can be unique to a user’s application. 

The purpose of supporting these apertures was to move coaxial cables for RF signals from the front of systems to the rear of systems so that the cables don’t have to be moved to access adjacent modules—in the past they were just draped across the front of plug-in modules. This also enables module wiring to leverage rigid and rigid-flex instead of just flexible cabling for higher speed and more demanding applications. This setup improves system reliability and allows more precise control of RF signals.

New connectors have now become available that fit as many as 16 RF contacts in a typical full-size VITA 67.3 module. The RF contacts that have thus far been specified or proposed include SMPM, SMPS, NanoRF™, and both 1 and 2 row MT/MPO optical ferrules. The SMPS and NanoRF contacts are much smaller to accommodate many more contacts per interface.

Upgrading to Leverage VITA 67.3

Any upgrade like the one to 67.3 requires new backplanes, but such a move is made easier by the availability of optical components by such suppliers as Reflex Photonics, which can incorporate optical technology while using very little board space.  

With VITA 65 apertures, designers will be able make changes as new cards become available without requiring a change to the actual backplane PCB. Apertures solve a myriad of problems; simply having all cables coming off the rear of the card solves human factor and reliability issues. VITA 67.3 and VITA 66.5 enable flexible arrangements of contacts to accommodate the board to mezzanine space. 

The Importance of Open Standards Ecosystems

One of the biggest advantages in adopting open standards is the widespread support from a healthy range of vendors. Currently, there are at least 44 different companies producing VPX modules for the open market and at least six connector companies that make various connector modules used in VPX products (Figure 3). There are also several larger suppliers building special VPX modules for their own customer programs.

OpenVPX is extensively documented, with more than 47 ANSI-VITA standards supporting every aspect of the VPX architecture family. As capabilities increase, new guidance is added to support them. This sharing of information is important for users who want to define a specific module for possible use by other board designers who may wish to have more than one source. 

Benefiting the Warfighter

Previously, one of the biggest criticisms of VPX was that, although cards may offer similar capabilities defined only by location of the data and control planes, all other backplane signals were unique to each card supplier. Adding apertures (VITA 67.3) to the VITA 65 standard was done to guarantee more hardware interoperability, a goal of the VITA community, the SOSA organization and the defense entities involved in this open architecture initiative. 

With fully defined slot profiles, different cards with similar functions can populate the same slot profile and interoperate with cards in other slots, enabling easier upgrades. RF contacts bring far more functionality and I/O to boost performance of defense-related embedded systems.

As VPX migrates to 25-gigabit channels, a great deal of engineering will be required to ensure that all daughter cards have command over the backplane. With VITA 67.3, designers can implement high-speed links with optics, saving design time and reducing the demands on the PCBs.

Making the system more reconfigurable via common profiles means a system can be configured for one environment today, then reconfigured with new radar or electronic warfare features that may lend itself to a different warfighter environment in the future.  In short, VITA 67.3 enables a more flexible system, supporting different types of sensors, for multiple defense environments requiring cost-effective, reliable embedded computing.

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