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Shrinking connectors uphold MIL-style requirements

Thu, 08/07/2014 - 11:01am
Klaus Montoya, Senior Cable Design Engineer, Elma Electronic

Physical characteristics, such as size and weight, aren’t the only real problem with using traditional MIL connectors in the growing number of small form factor (SFF) and unmanned applications. There is a significant technology challenge designers are facing, as well. 

These older, clunky connectors just don’t keep up with some of the high-speed signal connectors being designed for modern chassis systems, causing a breakdown in system functionality.

Today’s military and aerospace environments are calling for connector series that mimic the electrical and mechanical performance of traditional connectors, such as D38999, yet meet the ever-increasing requirements for high-speed interconnect solutions, including Ethernet 10GBase-T, USB 3.0, DVI, Display Port, SATA 3.0 and HDMI.

Traditional MIL connectors fall in to a few different, well-known categories:

·         “Standard” MIL-DTL-5015, MIL-DTL-22992;

·         “Miniature” MIL-DTL-26482, MIL-DTL-26500, MIL-DTL-83723;

·         “Subminiature” MIL-DTL-38999 Series III 

While these workhorses have long provided solid performance in harsh military applications, they have limitations with regards to the high-speed and high-bandwidth data requirements in today’s defense environments. This inevitably leads to limitations and technical challenges when they are used in chassis being designed to handle modern communications and control systems.

The technology trends in the military and aerospace markets are driving the increased use of ruggedized, miniaturized connectors. It all relates to the technology within the connectors as well as the cables required to harness this advancement of more sophisticated electronic equipment.  For example, compare the data and graphic throughput requirements of Ethernet 10GBase-T, USB 3.0, SATA 3.0, DVI or HDMI to older technologies, such as 10/100 Ethernet, PATA, VGA, etc.

Connections in a shrinking world
The large size and weight of traditional D38999 and similar styles of connectors preclude them from being used in these military systems continually looking to optimize SWaP.  To accommodate the smaller connections being designed into these types of SFF and UAV systems, manufacturers have developed several new interconnect series—from miniature, ultraminiature and nanominiature—that still meet defined guidelines.

The challenges don’t stop with the mere dimensions or bulk of the device either.  These connectors need to be extremely rugged, withstanding severe shock, vibration and temperature extremes. Thankfully, newer connector styles that boast weight and size reductions of 50% aren’t sacrificing high performance, reliability and signal integrity…it’s SWaP to the extreme.

Get it right from the get-go
Part of the process when choosing a connector is to make sure all the right questions were asked before settling on a given connector family.  When looking for a connector solution, determining the following needs upfront will enable you to focus on the other design issues that will inevitably come into play:

· Number of contacts

· Level of sealing

· Number of mating cycles

· Temperature range

· Material & locking types

· Shielding requirements

When placing connectors onto a PCB or panel, certain design criteria standards, known as MIL-STD-1472G for human engineering, exist for the design and development of military systems, to ensure an actual human can operate the equipment. In some cases, especially with the shrinking size of systems, this is not possible, so a compromise needs to be reached.  

For example, there is a recommended minimum spacing of one inch between two adjacent connectors to ensure a hand can physically fit within the space.  Yet, one may have to reduce the gap so everything fits in the amount of space within a given application.  The designer ultimately will have to make the final decision and deviations may be required.  Sometimes, it’s unavoidable.

Working with design challenges
Even though connectors designed for high speed and high reliability have been around for a few years, there are times when certain touted design parameters have not been used in the real world.  You may find yourself being the first to use them.  That’s when the details of the design become apparent and you will need to work with the manufacturer to enhance or tweak the design so it is manufacturable.

There are times when your manufacturing group is using these new connectors; they discover things that would be better if only this was done instead of that.  An experienced manufacturer will incorporate your feedback into the connector designs to provide a more optimum solution.

The notion that smaller equals cheaper is not often the case.  Although these new connectors will meet the high-speed demands, there is an associated cost for that accomplishment.  Currently, the cost of some of these connectors is high and could be very hard to justify.  But as time goes on, like anything else, the more it’s used, the lower the cost will be.

Cabling comes into play
As new connectors are designed to perform for modern high-speed protocols, standard cables may not work as well.  Finding a special cable can be somewhat difficult, since cable manufacturers lag a little behind with new cable designs that will work for the new connectors.

The design challenges with RF cables need to take into account several issues: connector types, such as straight or right angle as well as cable types such as regular coaxial, semi rigid and rigid.  Also there may be requirements for length matching and in some cases, phase matching. 

Length matching is, when all the routing is complete for all the cables used in a chassis,  the longest cable will determine the length of the others in that part of the group—they must all be the same length.

Phase matching is a different and more critical situation.  In today’s world of microwave needs as well as the inevitable variations in connectors and manufacturing processes, phase matching can be considered an art more than a technology.  It mandates experienced designers and state of the art RF instruments.  Typically it requires a phase spec, calling out the degrees and the specific frequency to match to.

The key to incorporating the right RF cables is to align yourself with suppliers that specialize in the field.  They have the knowledge and know-how and the latest test equipment to produce a finished product that will meet the needs of a given project.

Rigid cabling has its own design requirements, such as using 3D modeling to determine the actual layout of all the cables needed in a chassis.  The cables need to be laid out in a manner that can be converted to a 2D drawing for each cable assembly.  Tight tolerances and special manufacturing sequences are needed when installing these types of cables into a chassis. 

Special tooling is required when fabricating rigid cable assemblies to control the actual bends in specific locations along with the tight tolerances needed.  This ensures the accuracies of the finished assembly, so it can be installed as designed.  Figure 1 depicts rigid RF cabling in a chassis.

Figure 1. Example of proper rigid RF cabling.

A real-world application
A recent rugged chassis design required several types of MIL-style connectors be placed on a panel that would interface to, and communicate with, the outside world using several signal types including one, which was a USB/SATA application.  The I/O and cabling needs required an intense review.

All the other connectors in the chassis were of the traditional D38999 style, but the USB/SATA connector type still needed to be defined, ideally a connector solution that would handle the USB/SATA in the same connector. 

Since room for the connectors on the front panel was limited, the team focused on incorporating smaller connectors, which led to using smaller ones for the other GPIO signals, as well.  A review of available rugged SFF connectors uncovered a few types that would meet the application’s requirements. 

New high speed connectors that offer virtually equal performance to MIL-DTL-38999 interconnects can provide up to 70% weight and 50% size reductions for similar contact layouts.  These reductions are not true for all connector sizes, of course, and will vary depending on final configuration, but the ability to significantly reduce connector size and weight still holds true.  See Figure 2a & 2b as some of the available rugged connectors featuring compact footprints.


Figures 2a & 2b. New rugged connector styles are now available in much smaller footprints, saving on size and weight.

The initial thought for the I/O connectors was to use traditional cabling: select a connector, select the wire type and do the traditional, cut, crimp and poke.  The question came up, is it possible to go “board-to-board”?  If so, board mount versions, which most manufactures offer, would be needed. 

Now the trick was to make sure all the connectors lined up on the same plane, so the front panel would not need any special machining and to make sure the mating connectors would work properly with no interference.  Close collaboration among the design engineers and the suppliers produced a family of connectors that met all the application needs. 

In addition to the I/O cabling requirements, the chassis also needed RF cables, two of which needed to be part of the custom I/O board—the 1553 signals.  Again, the collaboration paid off, and a solution was developed. 

Now came the fun part: working the mechanical design to place all the connectors in the physical space of the chassis.  When arranging the connectors onto the panel, other issues arose, such as making sure there was enough clearance for the mating connectors as well as ensuring the operator’s ability to install and remove the mating connectors without any difficulties, avoiding what is known as a “Knuckle Buster”.

Once the mechanical design was solidified, the overall footprint of the PCB was sent to the electrical board design team.  Knowing the locations of the connectors on this new PCB, along with the interface connectors to the backplane, they did their magic and worked through final design issues, such as relocating connectors to optimize signal integrity. (Figure 3)

Figure 3. Typical layout of a panel that uses rugged, smaller connectors, as well as RF connectors.

The upfront planning and initial research helped minimize design changes in these final stages to provide a cost-effective, solid chassis suited to the customer’s expectations.

Mission accomplished
Smart design engineers are always looking for better results, whether it be smaller, faster, more reliable, cheaper or a combination of these benefits—of course, they’d take it all if they could. And in many cases, most needs can be met, and in some cases, only some. 

But there inevitably comes a time when compromise is the key to the best solution, and a manufacturer who can weigh the pros and cons for a specific application will be a tremendous asset to any company looking to find that optimal balance between the ideal and the practical solution.

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