[SI-LIST] Re: To remove reference planes under the gold finger areas
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: "steve weir" <weirsi@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 16 Jun 2005 08:50:25 -0700
Sorry if it seemed like I made some kind of blanket statement that
altering the image plane never makes a difference in crosstalk. I meant
to apply the statement only for this particular case - ground plane
under the gold fingers of an FBD connector. Before performing the
measurements, I wouldn't have bet tons of money that the crosstalk to
the other side of the connector wouldn't increase appreciably (though I
didn't think it would). My measurements showed it didn't.
I want to also note that I am not responsible for the generic FBD
recommendation to cut back those fingers. My specific area of interest
involved a connector we enabled for Logic Analyzers. I assume, however,
that the folks responsible for the generic solution had similar
experience to mine. I offered an affirmation to the original
question/answer based on similar personal experience.
I believe the reason we don't see much increase in crosstalk is due to
the fact that: 1) the connector makes contact above where the ground
plane is trimmed back, 2) the majority of the return current at this
point is through the coplanar fingers, and 3) the path was already so
disrupted... Mostly conjecture, based on trying to "Be the signal" (as
Eric would say - maybe his next book will be "The inner game of S.I."?).
Jeff Loyer
-----Original Message-----
From: steve weir [mailto:weirsi@xxxxxxxxxx]=20
Sent: Thursday, June 16, 2005 7:58 AM
To: Loyer, Jeff; kalevi@xxxxxxxxxx; Cortex.Chen@xxxxxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: To remove reference planes under the gold
finger areas
Jeff, I agree that big pads translate to significant capacitance and=20
without compensation, impedance dips. But I am quite wary of your
blanket=20
statement declaring no increase in crosstalk when we alter the image=20
plane. To believe that, I would have to think either that there is
another=20
path(s) back to the original return that does not involve nearby signal=20
traces, or that the path was already so badly disrutpted by other
problems=20
that the crosstalk just can't get much worse. Do you see something
different?
Steve.
At 02:49 PM 6/15/2005 -0700, Loyer, Jeff wrote:
>This agrees with my knowledge/experience. Big pads look like WIDE =3D
>traces which equals very low impedance, compared to 50ohm traces. Some
=3D
>of that can be reduced by trimming back the ground plane below the
pads. =3D
> Experiments showed no increase in crosstalk with the ground planes =
=3D
>trimmed back.
>
>Jeff Loyer
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx] =3D
>On Behalf Of Kai Keskinen
>Sent: Wednesday, June 15, 2005 1:39 PM
>To: Cortex.Chen@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: To remove reference planes under the gold finger
=3D
>areas
>
>I would think it is to reduce the capacitance so that the signals don't
=3D
>see
>a big localized impedance change.
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Cortex Chen
>Sent: Wednesday, June 15, 2005 12:17 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] To remove reference planes under the gold finger
>areas
>
>
>Dear SI experts,
>I'm working to design a FB-DIMM add-in card. To recall previous =3D
>experiences
>for PCI-E, I'm confused why it's recommended to remove reference planes
>under the gold finger areas of an add-in card. In detail, refer to =3D
>section
>2.3.2.6 (page 28 ~ 29) of PCI Express Design Guide (rev. no, 0.5, =3D
>12978).
>Who can tell me what the main reason is?
>
>By the way, refer to section 6.11.10 (page 417 ~ 418) from Howard =3D
>Johnson's
>new book, I like to quote some sentences for your reference as below:
>"......In a differential-pcb pair, most of the returning current from =
=3D
>each
>trace still flows on the solid plane, not the other trace, because each
>differential trace couples much more strongly to the big, solid nearby
=3D
>plane
>than it does to its little, skinny differential buddy....".
>
>If it's true, then it will cause some effects like "Differential =3D
>U-Turn"?
>Please give me your comments.
>
>Thanks & Sincerely
>
> Cortex Chen =3DB3=3DAF=3DA5=3DC3=3DAAN
>
>
>
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