[Gb-ccb] Filtering connectors on the PCB

[John Ford]

Hi Martin.  Could you give us information on the cable and connectors
you wanted to use before the RFI question arose?  I seem to remember
it was a mass-terminated type twisted pair cable and associated
connectors.  It's quite possible we have a source for such a beast in
the RFI flavor..
.
Please give us the details of the cable and connectors.

We'll discuss this a bit and give you a coherent response tomorrow.

John


Martin Shepherd writes:
 > Brian asked that I send multiple questions at a time, so here is a set
 > of 3 (albeit related) CCB questions.
 > 
 > 1. Regarding the control and signal lines connecting the CCB and RX
 >    boxes, my original plan was to mount bulkhead connectors through
 >    the walls of the CCB case, and use shielded patch cables on either
 >    side of these connectors, connecting to the RX box on one side, and
 >    the CCB PCB on the other side. Using patch cables between the case
 >    and the PCB, rather than soldering wires to case-mounted sockets,
 >    would make it easy to remove the PCB for maintenance, and also
 >    reduce RFI problems inside the CCB box. Unfortunately, not only
 >    haven't I been able to find any suitable filtered bulkhead
 >    connectors, but I haven't managed to find any type of filtered
 >    twisted-pair connectors, bulkhead or not. For this reason I am now
 >    considering a slightly different approach.
 > 
 >    My idea is to use shielded, but unfiltered bulkhead feedthrough
 >    connectors, going through the walls of the CCB box, have these
 >    connect to PCB mounted sockets on the CCB PCB via shielded cables
 >    and connectors, and have RFI filter capacitors soldered to where
 >    the pins of the connectors are soldered to the PCB. Given the use
 >    of shielded connectors and cables, the only point at which RFI
 >    could get onto or off the cable would be via the pins of the
 >    PCB-mounted connectors.  However, provided that these pins were
 >    very short, and that the filter capacitors were either soldered
 >    directly to them, somehow embedded in the board as feedthrough
 >    capacitors, or connected via matched stripline or micro-stripline
 >    PCB tracks, I imagine that this wouldn't be significant. Would this
 >    be an acceptable solution?
 > 
 > 2. My second question is whether anybody has any experience with
 >    Sabritec's line of ultra-miniature concentric triaxial
 >    connectors/cables (or can suggest something better)?
 > 
 >     http://www.sabritec.com/technotes/triaxconnectors.htm
 > 
 >    Given the difficulty of finding an off-the-shelf RFI filtering
 >    solution for the high density connectors that I was looking at
 >    previously for the differential signal lines, the difficulty of
 >    building custom filters for high-density connectors, and worries
 >    about crosstalk within a high-density connector, I am now back to
 >    the idea of using triax/twinax shielded twisted pair cables. The
 >    problem with this is that the standard triax/twinax connectors are
 >    very bulky, and I don't want to have to devote a large area of the
 >    main CCB PCB to 16 of them. Sabritec's ultra-miniature triax
 >    connectors are much smaller, so I am considering using them
 >    instead.
 > 
 > 3. My final question is whether push-on connectors (like SMB
 >    connectors) must be avoided. If so, would this still be the case if
 >    a number of push-on plugs were attached to a screw-down plate? I am
 >    wondering about having the plugs at each end of the 16 triax cables
 >    that carry the detected signals, all go through a single screw-down
 >    plate. This would reduce the amount of space required to plug in
 >    the connectors, make connection and disconnection easier, and make
 >    it almost impossible to accidentally cross-wire the 16 cables. I
 >    probably wouldn't actually afix the plate to the connectors. The
 >    plate would probably just be threaded over the cables, and press
 >    down on the connectors from behind. This would allow individual
 >    cables to be intentionally cross-wired while the plate wasn't
 >    screwed down, and thus facilitate debugging.
 > 
 > Martin
 > 
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