Connectors and cables for high-speed digital signals

Per Torstein =?iso-8859-1?Q?R=F8ine?= (perr@ifi.uio.no)
26 Jul 1996 14:42:19 +0200

I am currently designing a prototype for evaluating several
possibilities for high-speed low-swing digital interconnects. This
includes differential signals in parallel and a new method using
3-wire pseudo-differential signals (patent pending). On the PCB, 50
Ohm microstrips or striplines will be used, with on-chip termination.
Frequency range is 150-500MHz (0.3-1Gb/s), distance less than 3m.

I need to connect the prototype boards together, and to an
oscilloscope. In order to do comparisions, I would like the cables and
connectors to have similar properties. As I have little experience in
this field, I need your comments and answers to some questions.

One option is to use coax cables and SMB connectors (or similar). This
will be a bit clumsy and expensive, as 32 connectors are needed per
board. I think skew may be a problem. What kind of skew can be
expected? (for connectors only, and per meter of cable). What kind of
delay differences can I expect between pre-made cables from the same
vendor/batch? I also need addresses of companies that can supply such
cables, preferrably in Europe.

For the differential signals, twin BNC connectors can be used. This
cuts down the number of connectors to 22 per board. I guess that skew
between the two signals of each differential pair is better controlled
this way, but delay differences between cables is still a problem. Are
there any reason that it should be worse/better than for coax cables?
Where can I buy pre-made twin BNC cables? Are there any other similar
concepts that I can use? I think the best would be a cable with 2-3
100/50 Ohm shielded twisted pairs.

For the three-wire signals, I would use triple BNC connectors if they
existed. Instead, I think I have to treat them as three separate 50
Ohm single-ended signals, and use three coax cables per signal. Other
suggestions?

Another option is to use some kind of high-speed connectors and ribbon
cables, like Amphenol Champ or something. Twisted pair ribbon cables
is probably good for differential signals, but what about the other
ones? Do you know of any microstrip or stripline connector systems
(like AMP Z-PACK) that can be used with cables? I have seen references
to Skewclear from Amphenol and something called Molex-Tensolite
milli-Z, but I have problems finding any more information. Again,
addresses to producers and distributors will be helpful.

What is _your_ favourite board-to-board interconnection system for
things like ECL, PECL or LVDS?

Thanks,

-- 
Per Torstein Roeine                            email: perr@ifi.uio.no
University of Oslo                             phone: +47 22 85 24 52
Dept. of Informatics, Microelectronics Group     fax: +47 22 85 24 01
Box 1080 Blindern, N-0316 OSLO, NORWAY