[Ground-station] FMC-Ultra96 adapter board proposal

[Michelle Thompson]

Comment and critique and double-checking welcome and encouraged.

This adaptor board was discussed in Slack starting in May 2018. I have
collected the links and the definitions, but this writeup needs careful
checking before we engage in layout efforts, in order to avoid mistakes.

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Proposal:

Open Research Institute to build a board for Phase 4 Ground. This is a
mezzanine according to the 96Boards standard.

https://github.com/96boards/mezzanine-community/blob/master/CONTRIBUTE.md

The function of the board is an adapter between the Ultra96 development
board...

https://www.avnet.com/opasdata/d120001/medias/docus/187/AES-ULTRA96-G_2018-03-19.pdf#%5B%7B%22num%22%3A37%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22Fit%22%7D%5D

...and FMC daughter cards.

The standard for FMC is described here:

https://www.samtec.com/standards/vita/fmc

Specifically, we want to enable the Ultra96 to talk to the AD-FMCOMMS4-EBZ

https://wiki.analog.com/resources/eval/user-guides/ad-fmcomms4-ebz/hardware

Schematic of the connector here:

https://wiki.analog.com/_media/resources/eval/user-guides/ad-fmcomms1-ebz/fmcomms1_revc_sch.pdf#page=12

Layout of differential pairs needs to ensure that the lines are equal
length.

Initial discussions with 96Boards indicated no one else is working on this
and that the effort would be welcome and appreciated.

We have a volunteer with time to take a crack at the layout next week.
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