Miniature connectors evolve for high-speed I/O in SFF embedded systems

3The world of rugged Small Form Factor (SFF) embedded computers is rapidly growing. Powerful architectures such as VPX remain the standard solution for High Performance Embedded Computing (HPEC), but designers are looking toward smaller, lighter I/O solutions to save space and weight. VPX is modular and scalable, therefore well suited to SFF systems. However, unleashing embedded computers’ horsepower and achieving end-to-end high-speed performance pose another challenge. A new generation of miniature and nanominiature connectors now supports the needs of high-speed I/O for 100 ohm data busses like Gigabit and 10 G Ethernet. Such connectors use robust designs already time-tested in adverse environments to cancel noise, decrease crosstalk, and maintain signal integrity.

There is nothing new in the constant drive in electronics to make systems that are smaller, faster, and more efficient. The same is true with embedded computing, where the drive for smaller, lighter, and less power-hungry systems opens up new application possibilities. Small Form Factor (SFF) systems enable use in satellites, smart munitions, missile defense, aircraft communications, Unmanned Aerial Vehicles (UAVs), and similar applications. Such applications are highly constrained by Size, Weight, and Power (SWaP) requirements. The intention is to create systems small enough and rugged enough for mobile field deployment.

From a connector perspective, SFF systems require connectivity that’s smaller and lighter, but still capable of multigigabit speeds, and as rugged as existing military-style connectors. Other desirable attributes, such as appropriate materials selection, field reparability, and reuse of known components, lend to the success of SFF destined interconnects.

Achieving signal integrity in compact connectors

Applications are continually being driven faster and required to support multiple high-speed protocols. Designers must address resulting phenomena in interconnections, such as insertion loss, return loss, and crosstalk to maintain signal integrity. The name of the game in high-speed data transfer is preserving signal integrity, and the I/O challenge is to keep the I/O interface from being the weak link in the impedance-matched chain.

Maintaining signal integrity in high-speed connectors centers primarily on two issues: maintaining constant impedance to prevent reflections and crosstalk, and dealing with external noise factors. The connector’s characteristic impedance must be the same as the attached cable’s impedance since any impedance discontinuity will affect the waveform and impart loss. The discontinuity cannot only increase crosstalk, but can distort the signal since the high-frequency harmonics in the signal will be reflected more strongly. The severity of the mismatch depends on both the mismatch’s magnitude and electrical length in relation to the signal’s frequency. As the data rates increase, the disruption seen by the signal becomes progressively larger in relation to the signal’s risetime. A mismatch may pass a 100 Mbps signal fine, present a minor disruption to a 1 Gbps signal, and choke on a 10 Gbps signal. Connectors designed for high speeds must consequently be designed more carefully. With the widespread use of Ethernet, which specifies an impedance of 100 ± 10 ohms, system architects are looking toward a common 100 ohm interconnection platform to simplify interconnections of multiple protocols. Since Ethernet specifications have relatively strict electrical requirements, cables and connectors are readily adapted to other 100 ohm protocols.

Good impedance matching also accommodates the minor degradations that can occur through poor workmanship, repeated mating cycles, and other wear and tear.

A large high-speed connector cannot be simply downsized. Traditional MIL-DTL-38999 connectors, which were not originally designed with today’s high speeds in mind, achieve high speeds by keeping contact pairs well separated from other pairs. Selectively pinning an existing 38999 can minimize crosstalk through isolation. The contact system is designed to be mechanically robust and repairable, which meets the requirements of the connector’s original design many years ago. However, these solutions do not address requirements of contact geometry and location for matched impedance and superior return loss, which is critical for the connector to satisfy high-speed requirements.

Noise and signal integrity

Noise, either generated internally in the cable as crosstalk or externally from other sources, is primarily dealt with through differential signals. Differential noise is canceled at the receiver. Common-mode noise, on the other hand, is harder to deal with.

Shielding can function both to prevent external noise by conducting it to ground and to help maintain the characteristic impedance. While military applications prefer braid shields over the more fragile and harder to work with foil shields, braids result in a bulkier cable. The drive to SFF systems makes smaller cables desirable. One approach to achieving high-speed signals is to carry the shield through the connector, keeping it close to the contacts instead of terminating it at the backshell. The advantage is that the impedance variation is minimized and shielding is maintained. This allows excellent electrical performance, but places limits on reducing the connector size.

As a connector is miniaturized, the contacts are brought closer together. Since crosstalk depends in part on distance, closer proximity can increase its effects. Crosstalk can be canceled by having adjacent differential pairs at right angles to one another. This can be done as shown in Figure 1. On the left side, pairs are placed around the periphery of the insert. Since adjacent pairs are at high angles, crosstalk is minimal between them. Contacts on opposite sides, although parallel, are more widely separated. As the connector is miniaturized, these parallel pairs are more susceptible to crosstalk. A T-shaped contact pattern, shown on the right side of Figure 1, maintains high signal integrity by keeping all pairs at right angles. The T-pattern minimizes the effects of crosstalk by symmetrical noise cancellation. By arranging the interface with a symmetrical contact pair pattern, the full effects of cancellation can be realized, reducing the amount of required pair separation. This has the added advantage of allowing for higher contact density.

21
Figure 1: Contact arrangements for 10 G connectors.
(Click graphic to zoom by 1.8x)

As protocol data rates have risen, a gap in rugged copper interconnect capabilities was created. To address this gap in connectivity, TE Connectivity has recently introduced three families of CeeLok products capable of 10 Gbps performance, each of which offers specific advantages to designers in performance and size. Figure 2 shows the ongoing miniaturization enabled by the new designs. Note that the 38999 configuration is for Gigabit Ethernet (with red indicating the selective pinning).

CeeLok FAS-X connectors use an innovative method to maintain shield continuity through the connector. As a result, the connectors can be concatenated multiple times without degrading performance. The connector is somewhat larger than the other two discussed here, but has the highest signal integrity, while still offering field reparability. The connectors support a single 10 G Ethernet channel in a size 11 shell or four channels in a size 25 shell.

CeeLok FAS-T connectors are smaller – an eight-position connector in a size 8 shell. The connector’s T-shaped contact pattern provides noise cancellation and decoupling to minimize crosstalk and increase signal integrity. The backshell is integrated into the plug body to help provide low profile, low cost, low weight strain relief and EMI protection. The connector is field terminable and repairable.

CeeLok FAS-T Nano connectors use the same T-shaped contact pattern in a nanominiature size – plugs are only 0.3 inches in diameter, with a choice of push-on or threaded coupling. Unlike the larger CeeLok FAS-T connectors, the nano version is factory wired rather than field terminable. Along with small size comes weight savings: a one-meter cable assembly with a push-pull connector coupling weighs only 13 grams. The connectors are based on the well-established environmentally sealed nanominiature connectors, but with an insert designed for high speeds.

22
Figure 2: The 10 Gbps connector families give designers more choices in meeting SWaP goals.
(Click graphic to zoom by 1.9x)

10 G SFF connectors: Rugged and ready for prime time

Despite their small size, SFF connectors can be designed for rugged applications, including the capability to withstand significant levels of shock and vibration, salt spray, oil and solvent immersion, and other environmental and mechanical hazards. In addition, connector materials can be selected which exhibit low outgassing as required for space applications. The bottom line is that the new generation of SFF connectors are very robust and capable of meeting a wide range of applications where weight and space savings are achieved without compromising performance.

Gregory Powers serves as Market Development Manager for the Electronic Systems and Space segments within the Global Aerospace, Defense & Marine business unit of TE Connectivity. He received a B.S. in mechanical engineering from Syracuse University, has completed numerous graduate-level studies, and holds two patents relative to optical datacom devices. Connect with Greg Powers at www.DesignSmarterFaster.com.

TE Connectivity 610-893-9800 www.te.com