[SI-LIST] Re: Current Return Vias

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: giovanni.guasti@xxxxxxxxxx
  • Date: Sun, 23 Jul 2006 12:32:01 -0400

Giovanni
Several clarifications:

   1. If you are optimizing the impedance of a via transition with
      ground vias, then you are controlling  the return path.  They are
      one in the same.
   2. A via is quite a complex transition line.  The closest parallel in
      planar transmission line theory is that of a trace crossing a
      split plane.

best regards,

Scott

Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
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Giovanni Guasti 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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