Advances in MIL-D-38999L connectors lead to smaller, lighter, and greater bandwidth

The circular have been around since the beginning of time, or so it seems. They were developed in the 1930s for Army and Navy severe applications. The “AN” Series of Circular connectors, as they were known, originated in the 1930s as a dependable, rugged military electrical connector that has evolved into the 38999 electrical interconnect system today that now has many more demands on it than simple point-to-point . It has been utilized by not only the military, but has also found its way into Industrial, Aerospace, and Automotive applications where dependability is paramount.

The latest revision, MIL-DTL-38999L series III, standard describes a connector suitable for blind mating, high-vibration, and elevated temperatures. They are also suitable for “severe wind and moisture problem” (SWAMP). The contacts are removable crimp style pin and tubular socket style or solder type pin and socket contacts.

Little innovation has taken place in the contact design during the first approximately 70 years of the life of military circular connectors. It wasn’t until increased bandwidth requirements proved the ever-popular pin and socket contacts to be inadequate for the performance needed. It is only in the last ~15 years that bandwidth requirements have increased dramatically driven by Ethernet, DisplayPort, USB, SATA, InfiniBand, etc. that has forced innovation in contact design and contact arrangement.

The Quadrax contact is an early example of increased bandwidth by rearranging the signal and ground contacts within a 38999 circular connector to isolate the signals from one another for improved isolation from crosstalk rather than redesigning the contact itself. The Quadrax design, while increasing bandwidth capability, does sacrifice pin count and pin density. For example, a 38999 size 23 shell will accommodate six (6) Quadrax contacts for a total of six bidirectional lanes each capable of approximately 2.5 Gb/s. (See Figure 1).

Figure 1: Size 23 Shell with 6 Quadrax Contacts.

In 2015, VITA adopted VITA 76.0: High Performance Cable – Ruggedized 10 Gbaud Bulkhead Connector for Cu and AOC Cables as a standard. It included a new 38999-style connector utilizing a new innovative contact design and arrangement due to a need for even higher performance and density than Quadrax could meet. VITA 76.0 was adopted as an ANSI standard (ANSI/VITA 76.0-2016) in early 2016.

The advantages to the new contact design, pioneered and commercialized by Meritec of Painesville, Ohio, are many including increased density, increased pin count, increased bandwidth per contact, and increased aggregate bandwidth per connector. System engineers looking for size, weight, and power (SWaP) improvements within their systems can use the VITA 76.0 standard that has been utilized in many designs.

The new contact design differs from the traditional pin and socket implementations in that it is a flat hermaphroditic contact. The flat hermaphroditic contact interface is identical in both the cable plug and the receptacle. When mated, the flat mating surfaces provide two points of contact and when combined with the wire termination techniques utilized, provide a virtually transparent signal impedance path. There are no electrical stubs that are typically found in pin and socket contact designs. And while many attempts have been made to embed commercial connectors such as Ethernet and USB within 38999 shells to make them stand up to the rugged military environment, the new hermaphroditic contact design is the solution for a smaller, lighter solution with bandwidth capability for accommodating the various protocols.

The advantages of the increased pin density and pin count are made obvious by comparison. For example, the VITA 76.0 contact interface in a 38999 shell size 23 contains a total of 145 contacts that will accommodate 22 bidirectional lanes at 10 Gb/s per lane for an aggregate bandwidth of 220 Gb/s per connector. (See Figure 2).

Figure 2: Size 23 Shell with 44 Differential Data Pairs (22 Lanes).

More recently, a presentation was made to the VITA membership at its November 2016 standards meeting by Meritec, in conjunction with Reflex Photonics of Quebec, Canada. Evidence was presented that suggests the possibility of a new standard, VITA 76.1, which would be an Active Optic Module (AOM) version of the copper-based VITA 76.0. The VITA 76.1 standard would utilize the same VITA 76.0 copper contact interface in both the cable plug and the receptacle. Thus, the receptacle would accept either a copper cable OR optic cable. VITA 76.0 is specifically designed to stand up to the challenging environmental requirements of today and into the future.

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