[SI-LIST] Re: si-list Digest V14 #214

  • From: "Chen, Sherman" <sherman.chen@xxxxxxx>
  • To: "eric.tollefson@xxxxxxxxx" <eric.tollefson@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 19 Sep 2014 16:28:48 +0000

Joel,

802.3ap only specifies that AC coupling is required between transmitter and 
receiver. If there is no high common potential that could be induced onto the 
Giga Ethernet HS pair, a cap coupling would suffice. 
For active channel usually external termination is not required - they are 
taken care by the ASIC. For idle pairs on transform (if you do have reason to 
use them), personally I think a 100 Ohm across the pair with one lead of the 
pair connected to ground via a hi potential cap would be good. 

I also wonder how the Bob-Smith termination was verified in the first place, 
and whether the article "Bob_Smith_Termination_vs_Proper_Termination ", which 
is against this termination method, is correct. Anyone has done such 
measurement on CAT6E or other cables? I might spend some time making a VNA 
measurement on the CM impd. of the pair...

Best Regards,

Sherman Chen 
Signal Integrity 
EMC Global Hardware Engineering
Tel: +86 21 60951100-3329 


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Eric Tollefson
Sent: Wednesday, September 17, 2014 7:52 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: si-list Digest V14 #214

Joel,
In addition to Steve's reference to 802.3ap, I would also refer you to ANSI 
VITA 46.6 (if you have access), which discusses Gigabit Ethernet over VPX 
backplanes.  There are two standard variants, one which uses the "standard"
4-pair 1000BASE-T format and the other which uses the 2-pair 1000BASE-KX 
discussed in 802.3ap (basically, SERDES Ethernet).  VITA 46.6 calls for 
magnetics on BASE-T ports, which I presume is to ensure drop-in compatibility 
of standard Ethernet parts and interfaces, and 100-ohm differential traces on 
the backplane with no coupling caps.

Unfortunately, I don't have any particular expertise in designing termination 
for Ethernet ports, so I can't offer much there.

Hope that helps,
Eric


Msg: #7 in digest
> Date: Tue, 16 Sep 2014 15:24:32 -0700
> Subject: [SI-LIST] Question about common mode termination of Ethernet 
> transform
> From: Joel Brown <joel@xxxxxxxxxx>
>
> I am working on a design that has gigibit Ethernet running between two 
> cards connected by a backplane using 100 ohm differential traces 
> approximately 12 inches in length. My understanding is that actual 
> data rate per pair is 125 Mb/S. My first thought was just to use AC 
> coupling capacitors between the two parts (Ethernet controller and Ethernet 
> switch).
> Since Ethernet parts are usually connected through transformers to 
> provide isolation and filtering I decided I should include the option 
> for either the AC coupling capacitors or a transformer.
> Most of the time the cable side of Ethernet transformer center taps 
> have a common mode termination of 75 ohms connected to ground  through 
> a 1000pF capacitor (aka as the Bob Smith termination). I have read 
> some things that claim 75 ohms isn't the optimum value and that the 
> common mode termination may not be needed at all (see 
> people.eecs.ku.edu/~callen/713/Bohnert.pptx)
> I am sure that 100 differential traces are more like 25 ohms common 
> mode impedance, so I am wondering if I should use 25 ohms instead of 
> 75 ohms or just not have any common mode termination.
> Thanks - Joel
>


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