[SI-LIST] Re: Current Return Vias
- From: "Ihsan Erdin" <erdinih@xxxxxxxxx>
- To: giovanni.guasti@xxxxxxxxxx
- Date: Sun, 23 Jul 2006 10:33:02 -0400
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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