[Gb-ccb] Information and advice on optic-fibre ethernet

[Martin Shepherd]

On Wed, 13 Aug 2003, John Ford wrote:
>...
>  > Note that I plan to use metal fiber-optic bulkhead couplers to
>  > pass the TX and RX fibers through the wall of the CCB chassis.
>
> For best RFI shielding, you should consider using a waveguide beyond
> cutoff farrangement to get the fibers into the system.

I have been searching the web for any mention of explicitly
RFI/shielded couplers, but not found anything. Is the solution that
you are suggesting available in an off-the-shelf item, or is it
custom? Could you give me the appropriate part numbers please?

>  If you are
> sure your metal connectors will work, then that would be fine, too.

No. I don't have any idea how well they would work. They were just the
best off-the-shelf solution that I could find on the web.

> On the subject of RFi shielding, have you a block diagram of the
> system and the interconnections between assemblies that we could look
> at to pick out any RFI problems?

I'm working on documentation of the external interfaces at the moment,
so this is still a work in progress (hence the question about
ethernet). The following is a list of the external connections:

 Ethernet input/output:
   2 x optic fibre, using some kind of bulkhead feedthrough connector.

 1-PPS signal input:
   I am guessing that you use 50-Ohm RG58 coax to distribute the
   1PPS signal around the focus cabin, and have thus tentatively
   decided on bayonet BNC connectors.

 Opto-isolated digital phase-switch and cal-diode control signals outputs:
   I am planning on using off-the-shelf IEEE-1284 parallel-port cables
   for this. The IEEE-1284 standard mandates overall foil and braid
   shields, connected around 360 degrees to the shielded male DB25
   plugs at each end. These shields enclose 8 independent unshielded
   twisted pairs. There are also 9 other twisted pairs, but these
   share ground-return wires with the 8 independent ones, so I don't
   plan to use them. In fact I only need 4 of the 8 that can be used,
   so there will be a comfortable margin of 4 extra independent pairs
   available for potential future expansion. The standard mandates
   that there be no more than 10% crosstalk between neighboring pairs.
   It doesn't say whether this is near-end or far-end crosstalk, but
   the fact that these cables are used to exchange TTL signals between
   computers and peripherals at up to 2MHz suggests that this level of
   crosstalk won't be a problem.

   Alternatively I could use 4 individual coax cables for the 4
   control signals, as Tim has suggested, but I would prefer to have
   as few cables as possible.

 DC power cables:
   I haven't got to looking for suitable connectors and cables for
   this yet. I understand that at in the focus cabin you have +/-15V
   and 5V power supplies that can be used by any instrument. What
   types of cables and connectors do you usually use when connecting
   these to other instruments? Also, how much current will we be
   allowed to draw from these supplies?

By the way. I am spending huge amounts of time searching the web for
individual electronic components, connectors and cables, plus
associated data-sheets. Do you have any suggestions for a few
convenient one-stop shops, so that I don't have to waste so much time?
In particular, Newark's online catalog is horrendously inefficient for
finding things (and Newark still hasn't sent me the paper catalog that
I requested months ago).

Martin