[SI-LIST] Re: Remove Ground underneath Differential signal is deserved or not?
- From: "Knighten, Jim L" <JK100005@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
- To: Ibrahim Khan <ikhan@xxxxxxxxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Fri, 26 Jul 2002 14:16:04 -0400
Ibrahim,
If the ground plane is an integral part of the differential transmission
line, then removing it changes the impedance of both the even and odd modes.
If your differential lines are loosely coupled, commonly found, then the
individual traces in the pair are more tightly coupled to the ground plane
than they are to each other and even the odd mode produces circulating
currents flowing in the ground plane. So, removing the ground plane will
disturb this and produce an impedance mismatch in both modes.
This effect is probably less pronounced in tightly coupled lines, since the
coupling between traces is greater than coupling from trace to ground plane.
Jim
Jim Knighten, Ph.D.
Teradata, a Division of NCR http://www.ncr.com
17095 Via Del Campo
San Diego, CA 92127
USA
Tel: 858-485-2537
Fax: 858-485-3788
jim.knighten@xxxxxxx
-----Original Message-----
From: Ibrahim Khan [mailto:ikhan@xxxxxxxxxxxxxxxxxxx]
Sent: Friday, July 26, 2002 11:05 AM
To: Knighten, Jim L
Subject: RE: [SI-LIST] Re: Remove Ground underneath Differential
signal is deserved or not?
Jim,
Thanks for your clarification but what happens to the diff. pair
impedance.
Regards
Ibrahim Khan
-----Original Message-----
From: Knighten, Jim L [mailto:JK100005@xxxxxxxxxxxxxxxxxxxxxxxxxxx]
Sent: Friday, July 26, 2002 1:03 PM
To: jian@xxxxxxxxxx; binchuan2000@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Remove Ground underneath Differential signal is
deserved or not?
Dr. Zheng,
I think it is more precise to say that you will suppress the even mode
that flows on the ground plane. Since the even mode is caused by
characteristics of the signal (imbalance), it cannot be done away with.
It will find a path for propagation, just not on the ground plane.
My two cents,
Jim
Jim Knighten, Ph.D.
Teradata, a Division of NCR http://www.ncr.com
17095 Via Del Campo
San Diego, CA 92127
USA
Tel: 858-485-2537
Fax: 858-485-3788
jim.knighten@xxxxxxx
-----Original Message-----
From: Jian X. Zheng [mailto:jian@xxxxxxxxxx]
Sent: Friday, July 26, 2002 9:38 AM
To: binchuan2000@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Remove Ground underneath Differential
signal
is deserved or not?
Hi, Binchuan:
Removing the ground plane for differential pair will suppress the even
mode. Basically, the even mode will not be propagating. Its energy will
not be transmitted. The even mode signal, which is considered as noise,
will be reduced in your circuit. Thanks!
Best regards,
-----------------------------------------------------------------------
Jian-X. Zheng, Ph.D
Zeland Software, Inc., 48890 Milmont Drive, 105D, Fremont, CA 94538,
U.S.A.
Tel: 510-623-7162, Fax: 510-623-7135, Web: http://www.zeland.com
---------------------------------------------------------------------
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of binchuan2000
> Sent: Thursday, July 25, 2002 7:20 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Remove Ground underneath Differential signal is
> deserved or not?
>
>
>
> I am confused that I read some book about remove Ground under
> Differential could reduce the induced Current from Differential pair,
> but the question is how we could control its impedance(Differential
> impedance) although Differential doesn't reference to Ground. If I
> follow this content to REMOVE Ground under Differential pair how could
> I control its Zodd impedance and its Zdiff impedance. Does it will
> generate any other problem and if it does, why some textbook advise
> it?
>
> Remove Ground is worth or not for Differential signal ?
>
>
>
> Thanks,
>
>
>
> Binchuan.Wang
>
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