Electrical gigabit transmission ?

[Erik Magnuson]

From the RF design point of view, one should remember that the power
is no longer transmitted with the conductors, but instead propagates
as a field between the conductor and ground plane (or between
conductors in a balanced system). Thus, the dielectric losses of the
PCB or coaxial cable insulation materials will be important, so
ordinary glass fiber boards and PE insulated cables may be
inappropriate at higher frequencies and more expensive materials may
have to be used.

PE is still a pretty good dielectric in the GHz range, but FR-4
substrate starts hurting by the time you gets to a GHz (Del Cecchi made
similar rude noises about FR-4 as well - wonder what he would think of
PVC...). The microwave guys have had a lot of experience with PCB
performance in the GHz range - dielectric loss will have a much worse
effect on a stripline filter than it will on a digital signal.

- Erik

 

[Rob Warnock]

+---------------
| QAM and other modulation schemes have been proposed but never really
| caught on. ... QAM only halves the baud or symbol rate compared to
| the data rate by encoding 2 bits per baud.
+---------------

True, QAM is seldom used in baseband copper, though note that GbE
uses PAM-5 (2 b/Baud + some slight coding). Where QAM *has* been
really pushed is in CATV RF nets, where QAM-64 (6 b/Baud) and
QAM-256 (8 b/Baud) are fairly common.

Of course, now that the RF nets (including RF-over-fiber) are being
displaced by digital fiber... ;-}


-Rob

 

[Rob Warnock]

+---------------
| The fastest signaling over copper that I'm (being a software guy, and not
| involved in bleeding edge hardware development) aware of (in production)
| is 3Gig SAS/SATA cables. I'm not sure what the "baud" of the protocol is.
+---------------

10GCBASE-CX4 runs each of its four pairs at 3.125 GBaud
(2.5 Gb/s net after 4b/5b coding [or is it 8b/10b?]).

+---------------
| Perhaps Infiniband is faster?
+---------------

Original Infiniband was only 2.5 GBaud/lane (2.0 Gb/s/lane net after
8b/10b coding), but DDR is readily available and QDR is "coming"(?).


-Rob