[SI-LIST] Re: "Ground" strips between LVDS pairs
- From: Julia Nekrylova <fialka113@xxxxxxxxx>
- To: Jason Young <Jason.Young@xxxxxxxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 26 Oct 2011 12:29:06 -0700 (PDT)
Jason,
I remember when I started in 1996 all the clocks on the board were with guard
bands (ground strips).
It provided help in fighting the emissions (EMI).
Steve,
You mentioned unintentional tuned couplers... If you ground strips very
liberally (lambda/20 or so), how do you create tuned couplers?
Thanks,
Julia Nekrylova
74ze
From: Jason Young <Jason.Young@xxxxxxxxxxxxxxx>
To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
Sent: Monday, October 24, 2011 3:49 AM
Subject: [SI-LIST] Re: "Ground" strips between LVDS pairs
Thanks Steve,
This was my suspicion but with so many others doing it I started to have
doubts. I'm familiar with guard traces in audio design when they are driven to
be a similar potential to the high impedance input - yep, possibly this is the
source of the folklore. Thank you for putting my mind at ease.
Jason
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of steve weir
Sent: 24 October 2011 11:20
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: "Ground" strips between LVDS pairs
Bad habits die hard. One bad habit you want to break for certain is
copying magical mystery folklore design techniques.
At audio frequencies shield traces work great. They are wonderful for
keeping junk out of high impedance Op-Amps. High speed digital
transmission lines are a different animal. As has been covered on this
reflector many times: Inserting guard traces is usually ineffective and
can actually create a tuned coupler.
The most effective way to isolate signals that run over a reflection
plane is to minimize the height over the plane and maximize the
separation between aggressors and victims. If you can't meet your cross
talk numbers that way, consider altering your stack-up, routing layer
assignments, etc. One option always is co-planar wave guides. If you
know how to design them, then you can pull out that option when it is
appropriate.
Steve
On 10/24/2011 2:56 AM, Jason Young wrote:
> Dear Experts,
> Often on boards with LVDS signals, for instance connecting an LVDS
> transmitter to a LCD panel, I see thin 'ground' strips running between the
> differential microstrips. These strips have a sprinkling of vias connecting
> them to the reference plane on layer 2. Either end generally has a via and
> they extend most of the way between the source and termination. I'm told the
> reason is to reduce crosstalk between the pairs. Can someone please explain
> this reasoning?
> Yours
> Jason
> Ps. Sorry for using the 'G' word.
>
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--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
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