[SI-LIST] Re: High Speed traces and connectors

Thanks David. Yes. I agree. I think we are all directly or indirectly talking 
about impedance here. My point is that differential signals are not purely 
differential mode. 
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx on behalf of David Instone
Sent: Wed 11/12/2008 5:56 AM
To: Cotton, Chris
Cc: Lakshmi N. Sundararajan - PTU; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: High Speed traces and connectors
 
The width of the fingers is set by the requirement to mate with the PCI 
connector.  The required width is such that if you have any plane 
anywhere under them their differential impedance will be much lower than 
the 100 Ohms required, thus in the area of the PCB which is under those 
fingers there must not be any planes, or traces either.  (That's a plane 
anywhere in the stackup under those fingers, the board has a thickness 
defined by the need to mate with that connector.)    You can easily see 
the effect on a TDR.  Getting the differential impedance right is much 
more important than any effect on the common mode. 

Regards
Dave Instone
+44 (0)1235 824963

OXFORD SEMICONDUCTOR LIMITED
25 MILTON PARK
ABINGDON
OXFORDSHIRE
OX14 4SH
Registered in England no 2733820
Registered Address: As above 



Cotton, Chris wrote:
> Correction - The differential mode current should not be
> affected.
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Cotton, Chris
> Sent: Tuesday, November 11, 2008 1:31 PM
> To: Lakshmi N. Sundararajan - PTU; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: High Speed traces and connectors
>
> This is not something that I generally do. That having been said, I
> assume they are only asking you to remove the immediate or adjacent
> pwr/gnd plane. If this is the case, the rationale is that the connector
> pads are much larger than the trace and will look like a capacitive
> load. Removing the immediate plane will lower the capacitance.
>
> The CM return current has to find a GND pin anyway so I don't think it
> will be severely impacted. The differential mode current will be
> affected but difficult to quantify to what degree. I would not make
> these changes without them providing a technical rationale and
> assessment of risk.
>
> Chris
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Lakshmi N. Sundararajan - PTU
> Sent: Tuesday, November 11, 2008 12:58 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] High Speed traces and connectors
>
> Hi Team,
> I had a curious review mentioned on my board for high speed traces
> for which I could not get a satisfactory explanation.
>
> For all high speed traces from a connector, the review mentioned
> to remove GND/PWR plane from under the *signal pads* under connector.
> Even for the PCIE connector, for all the signal pads, the GND/
> PWR plane was requested to be removed.
> Now, these are 6Gig and 5Gig high speed signals. If there isnt GND/PWR
> plane then definetely I would assume there isnt any reference plane for
> return current.
>
> So, can you experts suggest, whether this is a right thing to do
> and if so why?
>
> Thanks
> -LN
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
>                 http://www.si-list.net
>
> List archives are viewable at:     
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>   
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
>                 http://www.si-list.net
>
> List archives are viewable at:     
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>   
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
>                 http://www.si-list.net
>
> List archives are viewable at:     
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>   
>
>
>   

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  



------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: