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High speed data through Pandect SRA’s. 1 – Gigabit Ethernet

Many defence applications need the transmission of high data rates through rotary joints.        

Gigabit Ethernet (GE)  is an increasingly popular choice.

Another popular choice is HD-SDI and Pandect’s solutions for this will be discussed in a subsequent case study.

The most popular variant of Gigabit Ethernet in defence applications that require rotary joints is 1000BASE T, which is defined by the IEEE802.3ab standard and it has superseded “Fast Internet”.

If the customer’s specification allows Pandect to develop the SRA with a bore along its axis Pandect may choose to incorporate a fibre optic rotary joint (FORJ). FORJ’s are very adaptable and capable of carrying many channels of ethernet but they do require the central axis of a unit or a slip ring with a bore. Also, using a FORJ with 1000BASE T means that at one side of the FORJ an electrical to optical media card is needed and at the other side of the FORJ an optical to electrical media card is needed, for an electrical signal to enter the SRA and for an electrical signal to emerge at the other side. Media cards require space and media cards+ a FORJ add cost.

A non-FORJ solution is possible in small diameter slip rings where a bore is not available or where the cost of a FORJ solution is unattractive. In this sort of solution 8 rings are used per GE channel, arranged into 4 pairs. The spacing between the pairs and  the dielectric of the material between them control  crosstalk  between the pairs in a GE channel. If more than one GE channel is being carried guard rings are designed and built between them. Guard rings at either end are used to  keep crosstalk  from extraneous sources within satisfactory limits. These are some of the considerations that Pandect’s experience allows us to address in our designs.

Here is a photo of a Pandect SRA design in volume production for a military application. This SRA has a great deal of functionality, including Gigabit Ethernet using the sort of non-FORJ solution above.  GE is also suitable for very much simpler SRA’s where the only need is to carry high-definition video.

Pandect defence application SRA including Gigabit Ethernet that doesn't use a FORJ..jpg

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