[Gb-ccb] Caltech Backend Telecon Monday 07 July 4pm EDT

[Richard Lacasse]

Martin,

Martin Shepherd wrote:
> 
> On Fri, 4 Jul 2003, John Ford wrote:
> 
>>Richard Lacasse writes:
>>
>>>Martin Shepherd wrote:
>>>
>>>>... assuming that by opto-isolation, we are talking about optic fibres,
>>>>rather than opto-isolator chips and wires,
>>>>...
>>>
>>>All I had in mind were wires and opto-isolator chips.  A differential
>>>driver would drive a shielded twisted pair and at the far end there
>>>would be a termination network and opto-isolator chip.  No current
>>>has to flow through the grounds or power supplies of the end with
>>>the opto-isolator chip, so shield currents are minimal.
>>
>>This is also what I had in mind.
>>
>>The optical fiber approach would also work, but would probably cost
>>more.  Maybe not too bad if we used the plastic AMP fiber system.  The
>>waveguide beyond cutoff ports in the RFI boxes are the way to go in
>>that case for shielding the penetrations.
>>
>>I don't think we can isolate the 2 chassis, so it's probably not worth
>>the extra cost and trouble to use optical fiber for digital control
>>signals, rather opto-isolators are probably the way to go.
> 
> 
> Understood. However this leaves me puzzled.  Now that it has been
> all-but-decided that the shields of the analog signal cables are going
> to connect the two chassis, I don't see any reason to opto-isolate the
> digital signaling cables at all, unless optic fibers are used. Surely
> LVDS cables would generate less RFI than unbalanced cables driven by
> opto-isolators, and the existing plan calls for using LVDS, not only
> to reduce RFI, but also because the tri-state nature of LVDS will give
> us the ability to remotely relinquish control of the receiver to
> another backend.
> 
I might be missing something, but here's my take on the above...

With respect to the analog signal cables, it is possible to have two shields, 
one for RFI protection which connects to both ends and another that serves as an 
analog guard that connects to one end.  We should probably discuss the merits of 
this and other shielding approaches during the videocon.

With respect to the opto-isolators... In the non-isolated case current flows 
mainly through the termination.  A small amount of current also through the LVDS 
receiver, through it's supply and ground system and must return to the driver 
side supply/ground system.  This small return current will flow on the shields 
of the digital and analog cables.  This return current would be much smaller 
using opto-isolators.