[SI-LIST] Re: SATA board to board connector question

  • From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
  • To: "olaney@xxxxxxxx" <olaney@xxxxxxxx>, jeff.loyer@xxxxxxxxx
  • Date: Tue, 28 Aug 2007 08:39:39 -0700

The assumption that conductor loss is a big problem with SATA needs to be
questioned.  True, there is less conductor loss with surface microstrip
than with stripline.  The question is whether or not it is significant
enough to outweight other considerations.  A big part of our jobs as
engineers is to know the difference between visible and significant.  In
this case, the copper loss is not the significant consideration.  The other
things that have been mentioned are more significant.

Lee


> [Original Message]
> From: <olaney@xxxxxxxx>
> To: <jeff.loyer@xxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Date: 8/27/2007 4:24:50 PM
> Subject: [SI-LIST] Re: SATA board to board connector question
>
> But that's backward from what I said.  All else being equal, microstrip
> has the edge.  Here's one citation: 
> Microstrip versus balanced stripline
> Masse, D.   Pucel, R.A.   Hartwig, C.P.   
> This paper appears in: Proceedings of the IEEE
> Publication Date: Aug. 1968
> Volume: 56,  Issue: 8
> On page(s): 1359- 1360
> Abstract
> Microstrip is found to exhibit lower loss than balanced strip-line of a
> comparable physical structure. This property is traced to conduction loss
> theoretically and is confirmed experimentally.
> Here's another:
> //www.freelists.org/archives/si-list/12-2005/msg00155.html
> Orin
>
> On Mon, 27 Aug 2007 14:07:31 -0700 "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
> writes:
> Thanks for your kind words regarding the article.  I?d point out that my
> results were based on the effects not only of Nelco N4000-SI (with a
> corresponding Er = 3.4, Df = 0.007), but also of more common (cheap) FR4
> materials with Er = 4.2 and Df as high as 0.017.
>  
> The only assertion I was questioning was that microstrip is more lossy
> than stripline ? do you have any data supporting this?
> Jeff Loyer 
>
>
>
>
>
> From: olaney@xxxxxxxx [mailto:olaney@xxxxxxxx] 
> Sent: Monday, August 27, 2007 1:33 PM
> To: Loyer, Jeff
> Cc: si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: SATA board to board connector question
>  
> No question, there are many decent materials on the market.  They fab
> like FR4, and often are called FR4 for marketing purposes.  FR4 is a
> concept, not a product spec per se, so the wise designer will do the
> homework and specify a vendor part # for consistent high performance
> results.  The fact remains that there is nasty FR4 and there is nice FR4.
>  If nothing but FR4 is called out, and your board house buys at lowest
> cost, you are rolling the dice regarding what you'll get back.  The idea
> that you can run umpty Gb/s over something called FR4 is gratifying, but
> incomplete.  The drop in dielectric constant with rising frequency for
> the nasty stuff is well known, as is the rise in the loss tangent, for
> classic, nasty FR4.  Digital signals are forgiving, and when combined
> with equalization, extensive simulation and other tricks, can be made to
> do amazing things on "FR4", or polyimide, or oak leaves laminated with
> racoon poop if need be.  But if it was easy, the demand for SI
> consultants would be rather lower than it is.  The very need for care and
> expertise underscores that the effect of the laminate is nontrivial.  
>  
> Having rescued clients by scrapping the tricks and simply using a decent
> microwave capable material for critical circuit portions, I stand behind
> my observations.  I also totally believe the results in your article.  Of
> course, the material upon which the article is based is "Nelco N4000-13
> SI, a readily available (though relatively expensive, compared to FR-4)
> material."  Good stuff.  I like the article.  Your advice concerning the
> transmission path implementation is right on: "Proper PCB stackup and
> trace geometry design are key elements in the fight to lower losses".  So
> is a short path.  The best advice for a SATA connection is simple: use
> the right connector and put the phy chip next to it.  That way, even
> those using the nasty type of FR4 (whose name is Legion, for they are
> many) have a fighting chance of success, without need to do double back
> flips to get there.  
>  
> Regards,
>             Orin
>  
> On Mon, 27 Aug 2007 09:57:16 -0700 "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
> writes:
> > Hi Orin,
> > Do you have some specific data on the difference in performance 
> > between
> > microstrip and stripline?  While I agree with your first premise 
> > (that
> > stripline introduces the parasitics of the via to get down to the
> > layer), my studies (see url below) have indicated equal loss between 
> > the
> > 2 mediums (stripline vs. microstrip), with the caveat that 
> > microstrip
> > often allows wider traces with resultant lower copper loss.  But,
> > impedance control on the inner layers (with no plating) is usually
> > better, and crosstalk is reduced.
> > 
> > I've often heard similar arguments to your "snorkeling" metaphor, 
> > but
> > haven't seen data supporting it.
> > 
> > This leads me to favor lower layers of stripline for long lengths 
> > of
> > high-speed signals, when I have a choice (a rare luxury).
> > 
> > Perhaps you've had some other experience or have data to share?
> > 
> > url to my study on microstrip vs. stripline loss:
> > http://pcdandm.com/cms/content/view/2572/95/
> > 
> > Note to all: there have been many postings on "Microstrip vs. 
> > Stripline"
> > on this forum, worthy of perusing if you're interested.
> > 
> > Disclaimer:
> > The content of this message is my personal opinion only and although 
> > I
> > am an employee of Intel, the statements I make here in no way 
> > represent
> > Intel's position on the issue, nor am I authorized to speak on 
> > behalf of
> > Intel on this matter.
> > 
> > Jeff Loyer
> > 
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx 
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > On Behalf Of olaney@xxxxxxxx
> > Sent: Monday, August 27, 2007 8:56 AM
> > To: nareshd@xxxxxxxxxxxxx
> > Cc: goister@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: SATA board to board connector question
> > 
> > Usually I prefer microstrip over stripline for signals as fast as 
> > SATA. 
> > The reasoning boils down to two considerations.  One is that 
> > transitions
> > between buried stripline and other layers brings its own set of
> > problems,
> > and the other is that in a world where (censored) insist on using 
> > FR4 in
> > the microwave region, the less contact a high speed signal has with 
> > the
> > stuff, the better.  Air is a reasonable microwave dielectric.  The 
> > FR in
> > FR4 means "fire retardent", which is not a microwave concept.
> > Microstrip
> > steps in the poop on only one side of the trace, where stripline is 
> > like
> > snorkling in it.  This makes a difference in how much signal 
> > quality
> > gets
> > eaten per unit length.
> > 
> > Don't get me wrong.  Stripline is excellent microwave practice when
> > proper dielectrics are used and connectorization is competently
> > implemented.  But, that level of attention seems reserved for a
> > different
> > world filled with directional couplers, 90 degree hybrids, and 
> > other
> > exotic animals never seen on this forum.
> > 
> > Orin
> > 
>
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