[SI-LIST] Re: "Ground" strips between LVDS pairs
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Julia Nekrylova" <fialka113@xxxxxxxxx>, "Jason Young" <Jason.Young@xxxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 27 Oct 2011 09:37:41 -0700
Boy! This one dies hard! Microwave engineers construct band pass filters
on purpose by laying traces side by side. That is how you accidentally
create cross talk with "guard" traces.
I am mystified how someone could advise this as a means for controlling
cross talk no matter how many vias are used. Traces are LC networks, no
matter what their ends are connected to. They are never "ground" at the
frequencies where cross talk is an issue.
This is like the myths surrounding right angle bends. They have lives of
their own and seem to be impossible to drive out of the profession.
If you worry about cross talk, simply impose the spacing between traces that
achieve the level of cross talk you are targeting and leave out the
complications associated with "guard" traces.
--------------------------------------------------
From: "Julia Nekrylova" <fialka113@xxxxxxxxx>
Sent: Wednesday, October 26, 2011 12:29 PM
To: "Jason Young" <Jason.Young@xxxxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: "Ground" strips between LVDS pairs
> 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
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