[Gb-ccb] connectors

[Brian Mason]

For reference I have also posted John's sketch to my development web
page.

 -Brian

John Ford writes:
 > 
 > Martin,
 > 
 > This is indeed a tricky business.  The skewclear cables seem like the
 > best approach, but the crosstalk spec you have is certainly pretty
 > tight.
 > 
 > First off, we'll connect a big fat ground bus between the receiver and
 > the backend, which should eliminate nearly all common mode error
 > between the 2 chassis.
 > 
 > I would, for the sake of sanity, separate out the control signals from
 > the data lines.  That is, the controls for the phase switches and cal
 > signals should have their own connector.  This connector is
 > non-critical.  I suggest a standard DB-25 Spectrum Control 700 series
 > filtered connector with either twisted pair individual cables, or some
 > of the spectra-strip twisted pair flat cable.  Use an overall shield
 > if you like, but I don't think it's necessary.
 > 
 > For the data cables, you will have to decide on an approach, either a
 > couple of large cables, or a multitude of shielded twisted pairs...
 > Both have their strong points.
 > 
 > I think that the most workable approach would be to build a PC board
 > attached to an FCN-260 connector that you've suggested, with the RFI
 > chip filters (note that a capacitor is not sufficient. You'll need a
 > ferrite bead at least and I'd use an LC or Pi section filter.  See
 > http://www.spectrumcontrol.com/pdfs/emi4-8.pdf for example) Then, put
 > a metal can over the connector and filter assembly.  From this board,
 > to the A/D board, then, you could either use a card-adge connector and
 > plug the A/D board diractly into this card, build the A/D's directly
 > on this card, or use some more of the skewclear.  The skewclear is not
 > terribly flexible, so I would recommend against doing this.  I would
 > also use 2 connectors for the data.  Each FCN-260 infiniband connector
 > can hold either 8 or 24 pairs, so 2 of the 8 pair connectors would
 > work fine.  It seems like it would be impossible for a mere mortal to
 > build these inifiniband connectors, so we should stick to commercially
 > available configurations.  
 > 
 > See my sketch for some ideas.  We think that your best chance for
 > success and the least cost, least fuss option is to build the A/D
 > section with the connectors on that board.  The less distance from the
 > connector to the A/D system the better from a noise perspective.
 > 
 > Obviously, the other idea is to use individual shielded twisted pairs
 > for the data signals.  This has some drawbacks as well: Bundling up
 > the cables creates the same problems as putting them inside the
 > skewclear, you still need to filter them, you still have the problem
 > of handling them inside the chassis, and now there are lots more
 > mechanical connections to deal with.  In any event, I would not
 > attempt to capture all these connectors in a bolt-down plate or set of
 > plates.  We're smart enough to wire it up right, and the ability to
 > change the data inputs from one to another comes in handy from time to
 > time for troubleshooting.  I hate this idea.  It seems it would be a
 > nightmare inside the chassis and out.
 > 
 > At the end of the day, the connectors used are probably not the
 > limiting factor in the low-noise, crosstalk, and accuracy departments.
 > The noise inside the enclosure, the analog circuit designs, and the
 > digitization will contribute to all of this.  It may be that simply
 > using standard spectrastrip mass-terminated cables, and using every
 > other pair as a ground, would give good enough performance.  That
 > would have to be verified via a test on the bench.  Whatever solution
 > you choose, it should be tested on the bench.
 > 
 > I think that we could live with whatever you think will give the best
 > performance.  Keep in mind that the RFI levels are a specification
 > that cannot be violated.
 > 
 > John
 > 
 > 
 > ccb.ps file in separate mail...
 > 
 > 
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 > gb-ccb at listmgr.cv.nrao.edu
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Brian Mason                           | office: +1(304)456-2338
Assistant Scientist                   | fax:    +1(304)456-2229
National Radio Astronomy Observatory  | mail:   PO Box 2
bmason at gb.nrao.edu                    |         Green Bank, WV 24944
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