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

Since many tools count length along a path between the centers of the
end points, putting a 1 mil match constraint is useless you plan to
manually check how each endpoint is routed.  You can hide a few mils
inside the pad at each end....

Sorry,
Aubrey

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of steve weir
Sent: Thursday, May 31, 2007 12:25 PM
To: leeritchey@xxxxxxxxxxxxx
Cc: Ravinder.Ajmani@xxxxxxxxxxxxxx; wdowsley@xxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: IEEE1394/Ethernet Routing Guidelines

Lee, Ravinder, et-al,

Sure you can't get anything remotely close to 1mil matched true
electrical length on the PCB itself, never mind what the package does. =20
The variations in eR over the signal path see to that.  But telling the
tool to effectively exactly match the physical length, simply reduces an
additional error term.  At least on paper that is a good thing, IF it is
free.  From the standpoint of entering constraints in the tool it might
seem to be.  But there are some potential hidden gotcha's, like the
coupling between adjacent segments of meanders, and excessive
real-estate for meanders.=20

Best Regards,



Steve.


Lee Ritchey wrote:
> Requiring length matching to 1 mil is excessively restrictive and not=20
> attainable due to unknown lengths in IC packages and on die lengths. =20
> 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=20
> the real length matching on the PCB is not the claimed 1 mil.  That's=20
> just what the tool reports.
>
>
>  =20
>> [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
>>    =20
> understand
>  =20
>> the need to match the length of differential pairs to within 1 mil. =20
>> 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,=20
>> 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=20
>> spacing/individual trace impedence and so on."
>> =20
>>   And now I don't find in any of the suggestions (or in Mr.Voorhies=20
>> note) anything about copper weight, dielectric constant at what=20
>> frequency,
>>    =20
> trace
>  =20
>> width, distance above or between reference planes, length matching,
>>    =20
> number
>  =20
>> of vias across how many layers, crosstalk coupling factor and it's=20
>> effect on even/odd mode impedance, pre-emphasis to compensate for=20
>> longer lengths at higher frequencies, various "cute tricks" to=20
>> emulate differential signals (one I really like, nearly eliminates=20
>> most of the above), differential drivers or complementary drivers or=20
>> current steering
>>    =20
> drivers,
>  =20
>> differential to common mode conversion factor, terminations,=20
>> 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=20
>> references are like preaching to the choir, we nod our collective=20
>> heads in agreement that those are indeed good sources and Mr.=20
>> Voorhies says to himself, Hey
>>    =20
> I
>  =20
>> can catch lightning in a jar, but I still don't WTF they just said,=20
>> thus the reason for his request.  Everyday we work with digit heads,=20
>> okay, digital guys, who connect to the dots and then look to the SI=20
>> and EMC
>>    =20
> guys
>  =20
>> to make the board work.  ps. we work well together cuz the only dots=20
>> 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=20
>> lengths to within 1 mil. (my layout guys can do that in about 30=20
>> seconds all day, another "cute trick"), and all added parts are=20
>> absolute mirrow symmetric (the parts might need vias).
>> =20
>> =20
>> =20
>> =20
>> =20
>> =20
>> =20
>> Sam.Charles@xxxxxxxx wrote:
>>   I second that.
>>
>>
>>
>>
>>
>> "Loyer, Jeff"=20
>>
>> 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=20
>> information. A pleasant change from the recent postings on the forum.
>>
>> Thanks Steve,
>>
>> Jeff Loyer
>>
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx=20
>> [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=20
>> to y=3D3D
>> ou:
>>
>> Signal Integrity Simplified, Bogatin
>> High Speed Signal Propagation, Johnson and Graham Digital Techniques=20
>> 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=20
>> will =3D3D
>>
>> need to go to www.speedingedge.com. Your alternative is to rely on=20
>> PHY=3D3D20 manufacturer guidelines.
>>
>> The basic guide that I can offer you is this:
>>
>> * Loosely coupled pairs are easier to manage than tightly=20
>> coupled=3D3D20 pairs. If you don't specifically need tight coupling =
for

>> density=3D3D20 reasons, you will be less likely to go wrong with
loosely coupled pairs.
>> =3D3D
>>
>>
>> * The crude approximation for a loosely coupled pair is Zodd =3D3D3D=20
>> 2*Zeven.=3D3D =3D3D20 Ie two loosely coupled 50 ohm traces will have=20
>> about a 100ohm odd mode=3D3D20 impedance.=3D3D20
>>
>> * Maintaining a uniform impedance and matched electrical length will=20
>> do=3D3D20 much to get you were you need to go. If you are a tiny bit=20
>> sloppy, it=3D3D20 shouldn't matter for a student project. Your =
project=20
>> doesn't have =3D to=3D3D20 pass FCC part B, and it is unlikely that =
you=20
>> have to support =3D maximal=3D3D20 length links.
>>
>> Eric Bogatin's book has some impedance nomographs. However, there are

>> a =3D3D
>>
>> number of free calculators around some better than others. Google=20
>> for=3D3D20 impedance calculator and take your pick.=3D3D20
>>
>> Good luck.
>>
>> Steve.
>>
>>
>>
>> Randolph Voorhies wrote:
>>    =20
>>> Hi Everyone,
>>> I'm brand new not just to this list, but to the world of PCB
>>>      =20
>> product=3D3D
>> ion in general. I'm currently a masters student at USC in robotics=20
>> and h=3D3D ave been producing some relatively simple PCBs for the =
past=20
>> few months us=3D3D ing Eagle. I'm now working on a fairly complex=20
>> project that involves crea=3D3D ting a custom baseboard for an XTX=20
>> module (xtx-standard.org). My formal =3D3D training is in digital=20
>> electronics, computer architecture, and computer s=3D3D cience, so =
when

>> it comes to signal integrity I'm pretty much at a loss.
>> =3D3D
>> Does anyone know where to find specific routing guidlines for=20
>> differentia=3D3D l pair lines such as firewire and ethernet? I have=20
>> read all sorts of conf=3D3D licting advice (differential impedence is =

>> crucial vs. differential impede=3D3D nce couldn't matter less, =
etc..),=20
>> and have had a very difficult time find=3D3D ing exact numbers for=20
>> trace spacing/individual trace impedence and so on.=3D3D If anyone =
with

>> experience has any advice that has worked for them it wou=3D3D ld be=20
>> very very very much appreciated.
>>    =20
>>> Thanks so much,
>>> Rand Voorhies
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