[SI-LIST] Re: Current Return Vias

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: leeritchey@xxxxxxxxxxxxx,"Ihsan Erdin" <erdinih@xxxxxxxxx>, giovanni.guasti@xxxxxxxxxx
  • Date: Tue, 25 Jul 2006 10:55:47 -0700

Lee, when contemplating layer switches, one can readily use a field 
solver to establish cross talk coefficients for any stack-up and edge 
rate.  At this time, we do not share ours ( Teraspeed's ) with the 
general public.   What we have published is an examination of via 
distance effects for a single signal via and plane to plane stitch 
via.  That publication should provide some insights into when to 
start looking at these things.

Regards,


Steve.
At 08:39 AM 7/25/2006, Lee Ritchey wrote:
>Steve,
>
>Complex answer, but good.  Problem with this answer is it confuses me.  At
>what frequencies does an engineer have to do the special via treatment?
>I'm sure it is above where most logic operates and is likely around 10GE
>launches, etc.
>
>
> > [Original Message]
> > From: steve weir <weirsi@xxxxxxxxxx>
> > To: <leeritchey@xxxxxxxxxxxxx>; Ihsan Erdin <erdinih@xxxxxxxxx>;
><giovanni.guasti@xxxxxxxxxx>
> > Cc: <si-list@xxxxxxxxxxxxx>
> > Date: 7/24/2006 10:05:43 PM
> > Subject: [SI-LIST] Re: Current Return Vias
> >
> > Lee, et-al I agree that there is a good deal of misinformation that
> > circulates on this subject.  It is another topic that ultimately
> > comes down to the numbers.  Single stitch vias are incapable of
> > "maintaining return path continuity".   What they can do is act in
> > concert with other vias and planes, so that the net scattering has
> > acceptably low loss.
> >
> >  From my experience path impedance continuity to high frequencies as
> > determined by low, linear S21 loss, and good S11 loss requires a
> > pattern of vias associated with each launch. Teraspeed designs such
> > launches everyday.  But a return via pattern on a signal by signal
> > basis eats up expensive real estate far too quickly to be practical
> > for managing routing layer transitions.
> >
> > Usually we see these circumstances / effects from stitch vias:
> >
> > For boards without either multiple Vss layers, or puddles, there is
> > nothing to stitch.  It is down to bypass caps.
> >
> > For boards with multiple Vss layers and modest edge rates, the Vss
> > vias of the bypass capacitors and components are usually well within
> > the quasi static extents.  Additional stitch vias directed at
> > individual signals have very little impact. If for some reason a PCB
> > has a lot of open real-estate, ( more often test vehicles than real
> > PCBs ) then the situation changes and distributing stitch vias
> > provides quantifiable benefits.  For most practical designs the
> > density and distribution is usually OK without adding extra stitch
> > vias.  When in doubt, do the homework.
> >
> > For boards with very fast edge rates, the quasi static extents are
> > small, and stitch vias associated with individual signals display
> > more pronounced effects.  But, the dimensions become such that we
> > really have to throw away the quasi static model and look at the
> > various wave modes.  This requires a 3D full wave tool like HFSS, or
> > CST.  For really fast signals, I reiterate that I cannot think of a
> > case where one stitch via associated with one signal could make the
> > difference between acceptable and unacceptable S parameter
> > results.  Either a pattern of vias is needed to make the difference,
> > ( sic launch ) or none at all.
> >
> > Regards,
> >
> >
> > Steve.
> >
> > At 03:34 PM 7/24/2006, Lee Ritchey wrote:
> > >Ihsan,
> > >
> > >Well put.
> > >
> > >
> > > > [Original Message]
> > > > From: Ihsan Erdin <erdinih@xxxxxxxxx>
> > > > To: <giovanni.guasti@xxxxxxxxxx>
> > > > Cc: <si-list@xxxxxxxxxxxxx>
> > > > Date: 7/23/2006 7:36:40 AM
> > > > Subject: [SI-LIST] Re: Current Return Vias
> > > >
> > > > Giovanni,
> > > > The EMC justification of a ground via in the close proximity of a
> > >switching
> > > > via is to minimize radial waves to the edges of the card by providing
>a
> > > > "return path" (I hate this phrase...) A two-wire kind of transmission
>line
> > > > -as you put it- would be another rationalization against the impedance
> > > > discontinuity for high-speed signals. In practice, however, the
>placement
> > >of
> > > > a ground via close enough to a switching via in order to provide a
> > >matching
> > > > impedance to -say 50 ohm- or to mitigate radial wave propagation is in
> > >most
> > > > cases -if not all- physically impossible. I think this kind of SI/EMI
> > > > rule-of-thumbs are based on a qualitative understanding of
>electromagnetic
> > > > theory rather than rigorous research results. In this context, I share
> > >Lee's
> > > > stance to debunk these recommendations because they have significant
> > >effect
> > > > on the design cost by closing routing channels and eating up on the
> > >valuable
> > > > board real-estate. If anybody has come across any research that states
> > > > otherwise in a peer-refereed publication I'd like to hear that.
> > > >
> > > > Regards
> > > >
> > > > Ihsan
> > > >
> > > >
> > > > On 7/22/06, Giovanni Guasti <giovanni.guasti@xxxxxxxxxx> wrote:
> > > > >
> > > > > Kenny,
> > > > > GND vias near the signal are not dedicated to return currents, but
>they
> > > > > are often used to optimize the impedance of the via.
> > > > > As the via is a short transmission line, only the higher speed
>signals
> > > > > can benefit of the difference between an optimized via and a "usual"
> > > > > via.
> > > > >
> > > > > You have to compare the higher frequency component of your signal,
>its
> > > > > wavelength and the via length. This will give you an idea of the
> > > > > effective needing to optimize this short transmission line.
> > > > >
> > > > > Of course you could have a 133MHz signal with very sharp edges and
>high
> > > > > frequency components, even if it seems very unusual... In this case
>it
> > > > > would be wise to choose a slower transmitter!
> > > > >
> > > > > The rule is to understand if the via behaves like a transmission
>line
> > > > > for your signal or not, and in the first case to do the best to
>reduce
> > > > > impedance discontinuities.
> > > > > Best regards,
> > > > >         Giovanni
> > > > >
> > > > >
> > > > > -----Original Message-----
> > > > > From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]
> > > > > On Behalf Of Lee Ritchey
> > > > > Sent: 22 July 2006 18:36
> > > > > To: Kenny Frohlich; si-list@xxxxxxxxxxxxx
> > > > > Subject: [SI-LIST] Re: Current Return Vias
> > > > >
> > > > > Kenny,
> > > > >
> > > > > It is not true that you need a "return current" via next to each
>layer
> > > > > changing signal  via.  I continue to be amazed that engineers who
>are
> > > > > looked upon as SI experts say such things.
> > > > >
> > > > > Imagine you have a 4 layer PCB, such as the mother  board in a PC,
>where
> > > > > there are only two planes, one Vdd and one ground, where would such
>vias
> > > > > connect?  There have been billions of these made to date that work
>just
> > > > > fine and have very fast signals on them.  The return currents you
>are
> > > > > concerned about find their way from plane to plane through the
> > > > > collection
> > > > > of decoupling capacitors and interplane capacitance that you had to
> > > > > engineer into the power delivery system in order to make it stable.
> > > > > Focus
> > > > > on this and the return currents take care of themselves.  EMI is
> > > > > minimized
> > > > > he same way..
> > > > >
> > > > >
> > > > >
> > > > >
> > > > > > [Original Message]
> > > > > > From: Kenny Frohlich <kenny_frohlich@xxxxxxxxx>
> > > > > > To: <si-list@xxxxxxxxxxxxx>
> > > > > > Date: 7/22/2006 6:45:56 AM
> > > > > > Subject: [SI-LIST] Current Return Vias
> > > > > >
> > > > > > Dear Experts,
> > > > > >   I understand that I need to provide ground vias next to via
> > > > > explictly
> > > > > for the purpose of letting return currents jump between layers.  I
>know
> > > > > it's a requirement for high speed signals, especially differrential
> > > > > signals.  Is this also required for low speed single-ended signals
> > > > > (133Mhz
> > > > > or slower)? =20
> > > > > >   If this is a requirement, what would be a good signal via to
>ground
> > > > > via
> > > > > ratio? For example,  there are five signal vias within a 1 inch
>area,
> > > > > how
> > > > > many ground vias do I need?
> > > > > >   =20
> > > > > >   Thank you
> > > > > >   Kenny
> > > > > >   =20
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