[SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines

Requiring length matching to 1 mil is excessively restrictive and not
attainable due to unknown lengths in IC packages and on die lengths.  The
fact that a layout person can do this on the PCB isn't a reason to impose
such a restrictive matching rule.  I'd be willing to bet that the real
length matching on the PCB is not the claimed 1 mil.  That's just what the
tool reports.


> [Original Message]
> From: <Ravinder.Ajmani@xxxxxxxxxxxxxx>
> To: <wdowsley@xxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Date: 5/31/2007 9:13:25 AM
> Subject: [SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines
>
> Bill,
> While I agree with almost all of your recommendations, I do not
understand 
> the need to match the length of differential pairs to within 1 mil.  Will 
> you please give your reasoning for this.
>
> Thanks.
>
> Regards, Ravinder
> Server PCB Development
> Hitachi Global Storage Technologies
>
>
> Email: Ravinder.Ajmani@xxxxxxxxxxxxxx
>
>
>
> Bill Owsley <wdowsley@xxxxxxxxx> 
> Sent by: si-list-bounce@xxxxxxxxxxxxx
> 05/30/2007 08:13 PM
> Please respond to
> wdowsley@xxxxxxxxx
>
>
> To
> Sam.Charles@xxxxxxxx, jeff.loyer@xxxxxxxxx
> cc
> si-list@xxxxxxxxxxxxx, si-list-bounce@xxxxxxxxxxxxx, weirsi@xxxxxxxxxx
> Subject
> [SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines
>
>
>
>
>
>
> I went below to check what Mr.Voorhies asked for;
>   "...have had a very difficult time finding exact numbers for trace 
> spacing/individual trace impedence and so on."
>  
>   And now I don't find in any of the suggestions (or in Mr.Voorhies note) 
> anything about copper weight, dielectric constant at what frequency,
trace 
> width, distance above or between reference planes, length matching,
number 
> of vias across how many layers, crosstalk coupling factor and it's effect 
> on even/odd mode impedance, pre-emphasis to compensate for longer lengths 
> at higher frequencies, various "cute tricks" to emulate differential 
> signals (one I really like, nearly eliminates most of the above), 
> differential drivers or complementary drivers or current steering
drivers, 
> differential to common mode conversion factor, terminations, decoupling, 
> and so on.  All the books/sources referenced have all the conflicting 
> information that Mr. Voorhies has already discovered. 
> For us, immersed in the culture (or lack of it on my part) the references 
> are like preaching to the choir, we nod our collective heads in agreement 
> that those are indeed good sources and Mr. Voorhies says to himself, Hey
I 
> can catch lightning in a jar, but I still don't WTF they just said, thus 
> the reason for his request.  Everyday we work with digit heads, okay, 
> digital guys, who connect to the dots and then look to the SI and EMC
guys 
> to make the board work.  ps. we work well together cuz the only dots I 
> connect are in my granddaughters coloring book.
>   So given the dearth of details here's one answer (of many) he can use;
>   50 ohm surface traces (no vias), spaced 5 times the distance to the 
> reference plane, no breaks in the plane below the traces, matched lengths 
> to within 1 mil. (my layout guys can do that in about 30 seconds all day, 
> another "cute trick"), and all added parts are absolute mirrow symmetric 
> (the parts might need vias). 
>  
>  
>  
>  
>  
>  
>  
> Sam.Charles@xxxxxxxx wrote:
>   I second that.
>
>
>
>
>
> "Loyer, Jeff" 
>
> Sent by: To 
> si-list-bounce@xxxxxxxxxxxxx 
> cc 
>
> 05/30/2007 02:24 PM Subject 
> [SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines 
>
> Please respond to 
> jeff.loyer@xxxxxxxxx 
>
>
>
>
>
>
>
> THANK YOU!!!
>
> A response absent of sarcasm and/or scorn, and with some useful
> information. A pleasant change from the recent postings on the forum.
>
> Thanks Steve,
>
> Jeff Loyer
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of steve weir
> Sent: Wednesday, May 30, 2007 1:39 PM
> To: voorhies@xxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines
>
> Randolph, I suggest any of the following four books would be helpful to
> y=3D
> ou:
>
> Signal Integrity Simplified, Bogatin
> High Speed Signal Propagation, Johnson and Graham
> Digital Techniques for High Speed Design, Granberg
> Right the First Time Vol II, Ritchey
>
> Lee Ritchey self publishes so if you want Right the First Time you will
> =3D
>
> need to go to www.speedingedge.com. Your alternative is to rely on
> PHY=3D20
> manufacturer guidelines.
>
> The basic guide that I can offer you is this:
>
> * Loosely coupled pairs are easier to manage than tightly coupled=3D20
> pairs. If you don't specifically need tight coupling for density=3D20
> reasons, you will be less likely to go wrong with loosely coupled pairs.
> =3D
>
>
> * The crude approximation for a loosely coupled pair is Zodd =3D3D
> 2*Zeven.=3D
> =3D20
> Ie two loosely coupled 50 ohm traces will have about a 100ohm odd
> mode=3D20
> impedance.=3D20
>
> * Maintaining a uniform impedance and matched electrical length will
> do=3D20
> much to get you were you need to go. If you are a tiny bit sloppy,
> it=3D20
> shouldn't matter for a student project. Your project doesn't have =
> to=3D20
> pass FCC part B, and it is unlikely that you have to support =
> maximal=3D20
> length links.
>
> Eric Bogatin's book has some impedance nomographs. However, there are a
> =3D
>
> number of free calculators around some better than others. Google
> for=3D20
> impedance calculator and take your pick.=3D20
>
> Good luck.
>
> Steve.
>
>
>
> Randolph Voorhies wrote:
> > Hi Everyone,
> > I'm brand new not just to this list, but to the world of PCB
> product=3D
> ion in general. I'm currently a masters student at USC in robotics and
> h=3D
> ave been producing some relatively simple PCBs for the past few months
> us=3D
> ing Eagle. I'm now working on a fairly complex project that involves
> crea=3D
> ting a custom baseboard for an XTX module (xtx-standard.org). My formal
> =3D
> training is in digital electronics, computer architecture, and computer
> s=3D
> cience, so when it comes to signal integrity I'm pretty much at a loss.
> =3D
> Does anyone know where to find specific routing guidlines for
> differentia=3D
> l pair lines such as firewire and ethernet? I have read all sorts of
> conf=3D
> licting advice (differential impedence is crucial vs. differential
> impede=3D
> nce couldn't matter less, etc..), and have had a very difficult time
> find=3D
> ing exact numbers for trace spacing/individual trace impedence and so
> on.=3D
> If anyone with experience has any advice that has worked for them it
> wou=3D
> ld be very very very much appreciated.
> >
> > Thanks so much,
> > Rand Voorhies
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