[SI-LIST] Re: Current Return Vias
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: leeritchey@xxxxxxxxxxxxx
- Date: Mon, 24 Jul 2006 19:06:32 -0400
A better way to look at this might be the density of signal vias per
power/ground via pair in any particular area of a board. This is not
unlike the BGA packaging problem. As the ratio increases, noise
increases. In most cases, power/ground pairs at the packages, along
with those associated with bypass capacitors, provide the necessary low
ratios. However, I have seen designs where signals were transitioned in
areas where there were no power/ground vias nearby, with associated
coupled crosstalk in the signals, and additional noise in the power
cavities.
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
Teraspeed Consulting Group LLC
Lee Ritchey wrote:
> It is not so much that the simulations shows that the ground vias improve
> the signal quality as it is, does the improvement warrant the extra effort?
> In my experience, it does not for logic signals. When one is trying to
> achive a very flat bandpass at 10 GHz it does as do other things such as
> coplanar waveguides.
>
> The original question was about memory logic.
>
>
>
>> [Original Message]
>> From: Ihsan Erdin <erdinih@xxxxxxxxx>
>> To: Giovanni Guasti <giovanni.guasti@xxxxxxxxxx>
>> Cc: SI freelist <si-list@xxxxxxxxxxxxx>
>> Date: 7/24/2006 8:38:41 AM
>> Subject: [SI-LIST] Re: Current Return Vias
>>
>> Giovanni,
>> I received the file and skimmed it through. First of all, kudos for the
>>
> very
>
>> good 3D simulation results. From my experience with HFSS, I know how much
>> time it takes to properly set up each case, let alone the run time.
>> As for the ground via example, I remember at least one publication where
>>
> the
>
>> authors' recommendations were about surrounding a switching via with 4
>> ground vias in order to improve the signal quality (I can -hopefully- dig
>> that up if you wish). Basically, they were burying the switching via in a
>> virtual cage. Albeit not to that extend, your 2 gnd via example is kind of
>> reminiscent of that scenario. The simulation results show beyond any
>> doubt they improve the signal quality. I'm guessing one may still
>> obtain some improvement with a single via to a lesser degree but would the
>> gain be worth the pain we'd brace for? Even for a single gnd via layout
>> designers groan for the board space. These 2 or 4 gnd via examples are
>>
> good
>
>> show-cases but in reality they're still hard sells for dozens of high
>>
> speed
>
>> signals crammed into tight spaces.
>> As SI engineers, despite our obsession with board related EMI issues, my
>> observations are that majority of the radiated emissions are coming from
>>
> the
>
>> ICs on the boards and we can hardly help it except for brute force
>> shielding. No doubt optimizing the board design will save us a little bit
>>
> of
>
>> margin that may make a difference between an EMC pass or fail criterion
>>
> but
>
>> as engineers we also have the responsibility of using the resources
>> judiciously or the cost will go through the roof.
>> As much as I respect your hard work for putting that effort together and
>> trust the accuracy of your results I still think we need more test cases
>>
> in
>
>> order to prove the feasibility of ground vias as reference conductors to
>>
> the
>
>> switching vias.
>> Regards
>>
>> Ihsan
>>
>>
>> On 7/24/06, Giovanni Guasti <giovanni.guasti@xxxxxxxxxx> wrote:
>>
>>> Ihsan,
>>>
>>> Please tell me if you received the file -- and of course I am interested
>>>
> in
>
>>> your opinion!
>>>
>>> Best regards,
>>>
>>> Giovanni
>>>
>>>
>>> ------------------------------
>>>
>>> *From:* Ihsan Erdin [mailto:erdinih@xxxxxxxxx]
>>> *Sent:* 24 July 2006 15:17
>>> *To:* Giovanni Guasti
>>> *Subject:* Re: [SI-LIST] Re: Current Return Vias
>>>
>>>
>>>
>>> Giovanni,
>>>
>>>
>>>
>>> Although I don't understand the text the pictures tell a lot and the
>>> first doc looks very interesting. I'd be very interested to see the
>>>
> other
>
>>> document if it is related to the topic we discuss.
>>>
>>>
>>>
>>> Thanks
>>>
>>>
>>>
>>> Ihsan
>>>
>>>
>>> On 7/24/06, *Giovanni Guasti* <giovanni.guasti@xxxxxxxxxx> wrote:
>>>
>>> Ihsan,
>>>
>>> I do not think this is just a rule of thumb: this is an experience that
>>> comes from measurements and simulations done in the last years.
>>>
>>> Please give a look to the following link, I used HFSS and Hspice to
>>> simulate. I do not like rules of thumb, this was the reason of a short
>>> presentation for ANSOFT Partners in design -- "Analisi critica delle
>>>
> rergole
>
>>> di buon progetto" (critical analysis of "rules of thumb")
>>>
>>> http://www.ansoft.com/partnersindesign/IT_guasti_italtel.pdf
>>>
>>> Another presentation during the EMC of Rome 2005 "The strategic role of
>>>
> EM
>
>>> field solver in Power and Signal Integrity" spent some slides on the
>>> optimization of vias.
>>>
>>> I would be glad to send you the file if you are interested in.
>>>
>>> I do not want with this to convince anyone. But at this point, in order
>>>
> to
>
>>> change my opinion, I need a research that states otherwise in a
>>> peer-refereed publication. J
>>>
>>> Design costs: in my mail I spoke about an analysis needed first, to
>>> understand if a gnd via can be useful or not. In the most complex PCB I
>>> followed there were only four differential pairs that could benefit from
>>> this technique. So this is not a high cost, I think.
>>>
>>> Board thickness and signal frequency spectrum must always be
>>>
> considered. I
>
>>> would never suggest to double the number of via!
>>>
>>>
>>>
>>> Best regards,
>>>
>>> Giovanni
>>>
>>>
>>> ------------------------------
>>>
>>> *From:* Ihsan Erdin [mailto:erdinih@xxxxxxxxx]
>>> *Sent:* 23 July 2006 16:33
>>> *To:* Giovanni Guasti
>>> *Cc:* si-list@xxxxxxxxxxxxx
>>> *Subject:* Re: [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
>>>> __________________________________________________
>>>>
>>>> Do You Yahoo!?
>>>>
>>>> Tired of spam? Yahoo! Mail has the best spam protection around=20
>>>>
>>>
>>>> http://mail.yahoo.com=20
>>>>
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