Implementing optics for Level Four electronic subassembly packaging in VPX systems enables significantly higher bandwidth over purely copper interconnects.
The signal-processing requirements of today’s active electronically scanned antenna (AESA) radar, infrared search and track (IRST), and surveillance systems are demanding. This situation challenges designers to squeeze more processing power and electronics into VPX (ANSI/VITA 46) and OpenVPX (ANSI/VITA 65) systems. One answer is to use a new generation of 3U-form factor VPX cards that offer significantly more bandwidth thanks to several new optical interconnects that conform to the forthcoming VITA 66.5 standard. These interconnects – integrating high-density MT ferrule interfaces and 24-lane optical transceivers – in Level Four electronic packaging will enable up to 900 Gb/sec duplex data throughput within a half-size connector module. That rate nearly quadruples the linear bandwidth capability compared to backplanes using the latest copper-based VITA 46.30-compliant interconnects.
Bringing Level Four electronic packaging for VPX up to light speed
While VITA standards continue to improve copper alloy-based interconnects used in electronic packaging for VPX systems, evolving VITA 66 “dot” standards are doing the same for optical interconnects. Advanced optical connectors give VPX backplanes and modules the benefits of higher bandwidth and other inherent advantages of optical technology, which include:
- Low signal loss: Propagating light signals through optical fiber requires far less amplification than moving electrons through copper. As a result, attenuation (loss of power) is significantly less likely.
- Less electromagnetic interference (EMI) and crosstalk: Nonmetallic fiber-optic cables are inherently EMI-resistant and require no electromagnetic shielding. Crosstalk is negligible between signals in different fiber-optic cables.
- Size and weight reductions: With lighter and more compact components, optical technologies minimize space and mass.
- Safety: Fiber-optic bundles are not electrically conductive and are inherently resistant to indirect lightning strike effects. The pure-glass (SiO2) core withstands common wet and corrosive environments without degrading.
- Security: Light traveling through glass does not radiate signals, making optical fiber extremely difficult to tap.
- Flexibility: Optical fiber’s small diameter, high strength, and various jacketing options provide flexibility and resistance to cracking and breaking. The typical minimum recommended long-term bend radius for conventional fiber optic cable is ten times the outside cable diameter. Optical flex circuits can accommodate smaller bend radii and multiple stacked layers and fiber crossings.
- Cost: Reductions in material cost of fiber cable, components, and hardware give fiber optics a price/performance advantage in many applications.
Along with high signal integrity and robustness, fiber optics offer density and bandwidth advantages that are extremely attractive in data-intense VPX applications.
Consider, for example, a 3U VPX module with a VITA 46.30-compliant MULTIGIG RT 3 high-speed backplane connector employing copper-alloy contacts. Each half-size connector uses eight printed circuit board (PCB) wafers that deliver 16 differential pairs. With each pair capable of supporting duplex bandwidth of 25 Gb/sec, the total duplex bandwidth amounts to 200 Gb/sec.
First compare that to a 3U VPX module employing a forthcoming half-size VITA 66.5 Style E (a.k.a. 66.4 HD derivative) fiber-optic connector for 0.800-inch card-to-card pitch applications. The smaller size allows a higher density in which two 24-fiber MT ferrules enable 24 duplex channels. With each channel capable of providing duplex bandwidth of 25 Gb/sec, the total duplex bandwidth amounts to 600 Gb/sec, a bandwidth increase of 300% percent over the latest copper interconnects.
The forthcoming VITA 66.5 Style D fiber-optic connectors provide additional bandwidth capability for 1.000-inch pitch applications (Figure 1). These connectors accommodate up to three 24-fiber MT ferrules within a half-width connector module, providing up to 36 duplex channels, enabling up to 900 Gbps in the same linear space, a 450 percent increase in bandwidth compared to VITA 46.30-compliant copper interconnects.
[Figure 1 | VITA 66.5 Style D – 3MT half-size connector. Courtesy TE Connectivity.]
This substantial increase in bandwidth is achieved at Levels Three, Four, and Five in the electronic packaging of a VPX system.
By convention, interconnections occur between two levels of the electronic packaging scheme. At Level One, a basic circuit element, such as an integrated-circuit (IC) chip, is permanently and directly connected to its leads, such as the frame of a chip carrier. At Level Two, the device is connected to a PCB. At Level Three, boards are connected together. Subsystems are connected together at Level Four, while at Level Five, subsystems are connected with a system’s input/output (I/O) interface. Finally, at Level Six, systems are physically connected from box to box across distances ranging from centimeters to kilometers.
Besides the obvious use of optics in board-mount I/O modules at Level Five and fiber-optic networking at Level Six, optics can be employed at other electronic-packaging levels:
- At Level One: Applying the optical equivalent of a chip-on-a-chip carrier involves direct connection of a laser diode or a photodetector chip connected to a waveguide. A photonic “flip-chip” technique may be used, or cleaved fibers may be aligned in these interconnects.
- At Level Two: Implementing optics may involve pluggable transceivers – such as single-channel SFP+ or MT-based parallel mid-board transceivers – and can include the subassemblies inside those transceivers.
- At Levels Three and Four: Advances in optical interconnects make it possible to combine Level Three board-edge solutions with Level Four subsystems. These advanced optical interconnects between VPX plug-in modules and the backplane significantly expand bandwidth.
This bandwidth boost is the direct result of developments in VITA standards for optical interconnects. In particular, VITA 66.5 addresses Level Three and Four interfaces, while VITA 87 defines interfaces for Level Five.
Arrays and parallel transceivers
To increase density, board and system-level designs are generally moving away from bulky discrete connectors to array-based solutions using smaller and more densely packaged circular and rectangular connectors. New connector module designs can integrate optical signals in a common block for the backplane interface. The block uses less space in a slot versus conventional side-by-side solutions. This trend is affecting Levels Three, Four, and Five within VPX systems. For high fiber counts in small spaces, optical flex circuits may be used to support tight-bend radii and simplify routing by accommodating up to 12 layers stacked and up to six fiber crossings.
To increase bandwidth, both card-edge and midboard parallel optical transceivers can be used to allow simultaneous transmission and reception of data over multiple fibers. VITA 66.5-compliant MT optical interfaces also support this application.
VITA 66.5 evolves
The ANSI/VITA 66.0 base standard defines a family of blind-mate fiber optic interconnects for ANSI/VITA 46 VPX backplanes and plug-in modules (Table 1.). Each termini style offers different benefits in terms of density, ruggedness, repairability, and other characteristics. The forthcoming VITA 66.5 standard defines the high-density fiber-optic interconnects for ANSI/VITA 65 OpenVPX architectures.
[Table 1 | Family of VITA 66 standards. Courtesy TE Connectivity.]
Blind-mate transceivers on the card edge
An active blind-mate optical interconnect design offers several benefits. A floating insert within the backplane connector module provides prealignment before the MT ferrules start to engage (Figure 2). Additionally, the interaction of the card-level guide pins and guide modules provides error-free mating and prevents damage to the connectors. Moreover, the system can be assembled only one way to avoid miswiring and mishandling. The low-profile VITA 66.5 Style A plug-in module connector and transceiver are integrated. The module is screwed onto the board edge along with an interposer; this assembly style is designed to be removable to enable future upgrades. The design also eliminates the need to locate transceivers midboard, which saves board space and eliminates fiber cable routing on board.
[Figure 2 | Ruggedized blind-mate MT optical modular/backplane interconnects that meet VITA 66.1 and 66.4 open architecture specifications for VPX systems. Blind-mate transceiver shown in large detail. (Courtesy TE Connectivity.)
Features of the backplane optical connector module include a floating insert and pins or a blade that align in X and Y directions with the MT ferrule mating interface, a design that complies with VITA 66.5 mating requirements. Features of the plug-in module connector include the optical transceiver with its MT ferrule, an LGA interposer, and the front connector housing with its corresponding alignment feature(s).
The soon-to-be-released VITA 66.5 standard defines two different blind-mate optical connector interface configurations. Connectors incorporating edge-mounted transceivers contain fixed MT ferrule interfaces on the plug-in module and spring-supported (floating-displacement) MT ferrules on the backplane. Connectors containing cable-to-cable variants incorporate spring-supported MT ferrules within both the backplane and plug-in module connector halves.
What’s new is the increase in density possible with VITA 66.5 Style D and Style E connector variants. This standard defines both half-width and full-width connector modules for dual MT ferrules. The dual-MT ferrule combination can provide up to 48 (2 × 24) lanes, while Style D supports up to 72 (3 × 24) lanes.
This MT development not only boosts bandwidth, but the forthcoming VITA 66.5 standard also makes other critical contributions by supporting:
- Multifiber termination (MT) ferrules: The MT interconnect for MT ferrules was described in the first “dot” specification published (VITA 66.1). Of all industry-standard ferrules, the MT provides the highest-density interconnections for both multimode and single-mode fibers. It serves as the foundational ferrule type for VITA 66.5. Over time, MT modules for multifiber terminal ferrule connectors at the VPX backplane interface have doubled or tripled in the same physical space on the boards. Fiber counts per MT module are increasing from 12 to 24 to 48 to 72. MT is a physical-contact-style terminus, meaning the glass end faces are in direct contact for very low loss. VITA 66.5-compliant designs use a spring-loaded MT backplane connector to provide mating force between the backplane and the plug-in module. The spring-loaded interface ensures secure MT-to-MT mating to minimize return loss and the effects of vibration.
- Hybrid RF/optical modules: The forthcoming VITA 66.5 standard encompasses more than optics: The standard also aims to achieve further density with hybrid RF/optical modules by combining the RF and optical links within the same connector module. The floating insert can house both an array of RF contacts andMT ferrules, adapting up to 10 NanoRF contacts and an MT within a 3U half-module space.
- Higher-density VPX enclosure optical I/O connectivity: The VITA 87 standard for optical MT circular connectors is also under development. This standard will define circular connector shells compliant to MIL-STD-38999 (Figure 3). Options for 12 or 24 fibers per MT and for physical contact or lensed MT will be available.
[Figure 3 | D38999 Series III Style circular connectors with up to four MT ferrules accommodating up to 96 optical channels. (Courtesy TE Connectivity.)]
Entering the future: VITA and SOSA working together
The development of the VITA 66.5 and VITA 87 standards has been leveraged in another influential standards organization – the Sensor Open Systems Architecture (SOSA) Consortium. Comprised of U.S. military, government, and related industry representatives, the SOSA Consortium aims to develop interoperability standards for next-gen sensor systems. SOSA-aligned architectures are being developed for signal intelligence (SIGINT), electronic warfare (EW), radar, and communications applications. SOSA is adopting ANSI/VITA 65 OpenVPX architecture within the boxes and collaborating with VITA to leverage existing standards while also driving new VITA standards.
Evolving VITA standards have driven the development of valuable optical and RF technologies for VPX systems. As a result, tomorrow’s VPX systems can keep pace with demanding signal-processing requirements by adding more fast lanes to boost bandwidth.