[SI-LIST] Re: Some reference on reference planes
- From: Mark Grobman <markgrobman@xxxxxxxxx>
- To: Jory McKinley <jory_mckinley@xxxxxxxxx>
- Date: Tue, 8 Nov 2011 06:45:55 +0200
Hey all,
I'm sorry - it seems I've confused all you (perhaps this is a reflaction of
my own confusion).
my stackup is 18 layers and is perfectly symmetrical. The stackup I wrote
down is a "zoom" on the problematic area just so you could understand where
my problem is.
As for the combining of power planes: As I stated - the situation cannot be
avoided without increasing the number of layers ( i tried combining but it
doesn't work) which I am reluctant to do due to reliablity issues caused by
the via's aspect ratio.
What I really want to understand is: If I was making a DC board than we
wouldn't be having this discussion. Since this is a digital board life
isn't so simple (don't tell any DC engineers I implied their job was easy)
- what is the area of rise times where problems start surfacing?
Can I help mitage this issue *without *changing the stackup?
And should I worry about interfernces throughout the transmission line or
just near the edges/where the via's are?
Thanks again for all the help,
Mark
On Tue, Nov 8, 2011 at 3:17 AM, Jory McKinley <jory_mckinley@xxxxxxxxx>wrote:
> Hello Mark,
> Assuming you have meaningful energy on this trace, I would highly
> recommend modifying your stack which seems to be unbalanced without
> appropriate ground planes. The single ground plane at the bottom of your
> stack is not recommended and could create all sorts of unwanted resonances
> as your return current jumps to the bottom through power cavities. I would
> look to pair your power planes and combine on two of the layers not four as
> you have. I would either add another layer and sandwich 3 grounds OR
> remove a layer and have the other layer another ground. I do not like
> power on top or bottom due to potential radiation. Something like:
>
> TOP/LowSpeed Signal 3
> Power 1(Real Ref.)/Power2
> GND
> Signal 1
> Signal 2
> GND
> Power 3/Power4
> BOTTOM/LowSpeed Signal 3
>
> The potential issues you face with improper return path can create signal
> integrity and timing issues on your path and potential resonances through
> the power cavities. You will not only have to decouple near the TX and RX
> of the path from reference to ground but also decouple very close to the
> via transitions of your trace.
>
> -Jory
>
>
>
> ------------------------------
> *From:* Mark Grobman <markgrobman@xxxxxxxxx>
> *To:* Rick Collins <gnuarm.2006@xxxxxxxxx>
> *Cc:* si-list@xxxxxxxxxxxxx
> *Sent:* Monday, November 7, 2011 4:40 PM
>
> *Subject:* [SI-LIST] Re: Some reference on reference planes
>
> Thanks for the quick reply's.
> Paul - I read in Bogatain's book that this method is not effective -It was
> mentioned under what happens when you run over a gap in the return plane
> but as I understand the physics is essentilay the same. To the best of my
> understanding the current will "find it's way back" in a radiative manner
> so that as long as the capacitance between the relevent planes is suffiecnt
> it should be ok above a certain rise time - I just don't know the numbers.
> Is this method effective from your experince? what's the range of Rt for
> which it works.
>
> Rick - Your absoulty right. I've been a bit vauge. The setup I'm talking
> > about is something like this:
> >
> Power 1(Real Ref.)
> Signal 1
> Power 2
> Signal 2
> Power 3
> Signal 3
> Power 4(GND)
>
> And the relevent Signal layer is "Signal 2". The distance between different
> layers is 5 mil on each side.
>
> Mark
>
> >
> On Mon, Nov 7, 2011 at 11:24 PM, Rick Collins <gnuarm.2006@xxxxxxxxx>
> wrote:
>
> > I recall from a course I took that if the plane of the stripline is
> > tightly coupled to the reference plane, you should not have a
> > problem. But "tightly coupled" may not be what you have. I think
> > the context of what I learned was when there was a separation in a
> > power plane or even a signal passing across a gap between two
> > separate power planes, but in both cases the power planes were
> > opposite a ground plane and so were "tightly coupled" acting just
> > like the ground plane.
> >
> > Where is your driver's "reference plane" that it does not interact
> > with the signal? Can you give us a better picture of what you are
> > designing rather than talking in the abstract?
> >
> >
> > At 04:15 PM 11/7/2011, Mark Grobman wrote:
> > >Hello experts,
> > >I require some help on the subject of reference planes. I'm designing a
> > >board and despite my best efforts i'm stuck with a situation where I'm
> > >forced to conduct a signal using a stripline neither of whose planes are
> > >the reference planes of the signal's driver (not the driver's ground or
> > >VCC).
> > >
> > >Now I know from various App. notes and books that this sort of situation
> > >should be avoided and that I have been a bad engineer indeed.
> > >
> > >Still, assuming the situation cannot be avoided I was hoping to get
> > >some quantitative approximation to how bad of an idea this is.Sadly
> > >speaking I don't have access to a 3d simulator which can give me exact
> > >results so I'm going for best effort design methods. I would love to get
> > >your input on the following issues:
> > >
> > >1. Does the interference caused by not using the correct ref. planes
> carry
> > >throughout the transmission line or does it occur only at the edges
> where
> > >the current "jumps" back to the correct ref. planes?
> > >2. Is there a merit figure of RiseTime/planes capacitance/???? for
> which
> > >the situation isn't problematic?
> > >3. Will using diff. lines improve the situation?
> > >4. Suggested reading on the matter.
> > >5. Highly insightful remarks which will blow my mind.
> > >
> > >Cheers,
> > >Mark
> > >
> > >
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