[SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs

I've never seen a demonstration that this mode conversion is a problem. 
I'd be interested in seeing such a case.  In many dozens of designs at this
data rate and higher, I've seen no failures from this amount of mismatch.

I hate to design out of fear something not proven to me.


> [Original Message]
> From: steve weir <weirsi@xxxxxxxxxx>
> To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
> Cc: Peterson, James F (EHCOE) <james.f.peterson@xxxxxxxxxxxxx>;
<si-list@xxxxxxxxxxxxx>
> Date: 10/20/2009 7:43:38 AM
> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs
>
> Lee, If the driver is really slow or the path very long that might be 
> OK.  For a clean driver on modest length links, 50ps skew can run 50% or 
> more of Tr/Tf.  I don't like that much mode conversion.
>
> Best Regards,
>
>
> Steve.
> Lee Ritchey wrote:
> > Actually, I was being conservative at 100 mils.  A mismatch of 300 mils
> > total is acceptable throughout the total path.  300 mils is roughly 50
> > pSec.out of a 320 pSec bit interval.
> >
> >
> >   
> >> [Original Message]
> >> From: Peterson, James F (EHCOE) <james.f.peterson@xxxxxxxxxxxxx>
> >> To: <si-list@xxxxxxxxxxxxx>
> >> Date: 10/20/2009 6:09:07 AM
> >> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs
> >>
> >> One comment on length matching at 100 mils : a lot of times the board
> >> we're looking at is only one section of the interface. There is often
> >> two more boards involved (a backplane and endpoint). When we say 100
> >> mils matched lengths, are we saying total matched length or at each
> >> board (so total could be 300 mils in that case)? The thread below says
> >> "matched at the receiver", which implies total, so the 100 mils should
> >> be budgeted through 3 boards.
> >>
> >> Regards,
> >> Jim Peterson
> >> Honeywell
> >>
> >> -----Original Message-----
> >> From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> >> On Behalf Of steve weir
> >> Sent: Tuesday, October 20, 2009 1:13 AM
> >> To: icer world
> >> Cc: Lee Ritchey; Paul Hurst; si-list@xxxxxxxxxxxxx; chundi srikanth;
> >> Lambert Simonovich
> >> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs
> >>
> >> 1) Matching much closer than 1/4 Tr/Tf offers little value by itself.  
> >> 100mils translates to about 16ps skew.  That corresponds to a 64ns
Tr/Tf
> >>
> >> for a 3.125G link which is a little to the outside, but it is not 
> >> horrible.  I would rephrase Lee's advice as:  Don't take any 
> >> extraordinary measures to match closer than 1/4 Tr/Tf as seen at the 
> >> receiver.  The longer the link, the slower Tr/Tf will be.  There is a 
> >> good treatment on this in Eric Bogatin's "Signal Integrity Simplified".
> >>
> >> If you get very tight matching truly for free, then fine.  But
obsessing
> >>
> >> about mechanical match to tight tolerances does not improve the design.
> >>
> >> It can in fact harm it if the matching is done with dense serpentines 
> >> which introduce their own dispersion and timing skew that is not 
> >> properly accounted in some tools. 
> >>
> >> 2) Continuous return path is very important.  Diff tolerates
obscenities
> >>
> >> like crossing moats, but at undesirable costs.  The best answer is
don't
> >>
> >> interrupt the return path.
> >>
> >> 3) Surface ground guards are more often unintended resonators than of 
> >> specific value.  See if the cross-talk can be satisfied with spacing.  
> >> If it can't, then consider alternatives.
> >>
> >> 4) I am not clear on what you are trying to recommend.  Are you
talking 
> >> about termination at both ends of the link, or even and odd mode 
> >> terminations?
> >>
> >> 5) An appropriate stitch density helps with EMC and signal integrity.  
> >> Follow Bruce Archambeault's hierarchy on layer assignments and your
life
> >>
> >> will be good:
> >> i. Route on one layer that faces a contiguous plane.
> >> ii. Switch between layers on either side of the same contiguous plane.
> >> iii. Switch between layers that reference planes on the same DC 
> >> potential that are adequately stitched together.  This is often 
> >> misunderstood as needing to assign a return stitch via near each 
> >> transition.  The point is to raise the resonant frequency of the 
> >> structure sufficiently so that it won't be a problem to the signal. 
> >> iv. If you must switch between layers that are stitched with bypass
caps
> >>
> >> and planar capacitance.  Be wary of PDN resonances in the signal 
> >> frequency range.  These will tend to occur at much lower frequency
than 
> >> cavity resonances.
> >>
> >> Steve.
> >>
> >> icer world wrote:
> >>     
> >>> 1) length matching must be consided  seriously and mismatch should be
> >>>       
> >> below 100mils;
> >>     
> >>> 2) generally speaking,the differential pairs impedance must keep
> >>>       
> >> 100ohm and   the ground return path must not be choped ;
> >>     
> >>> 3) the ground guarding trace should keep two times of differential
> >>>       
> >> trace width away from the differantial pairs,which can not influences
> >> the       differential pairs impedance and avoid crosstalk issues;
> >>     
> >>> 4) using serial and parallel matching resistors simultaneously   for
> >>>       
> >> debug ;
> >>     
> >>> 5) changing layers is not expected,but if necessary ,you should place
> >>>       
> >> a groung via near the signal via ;
> >>     
> >>> the above is just an advice ,and you'd better do a simulatiom to
> >>>       
> >> decide the rules of matching resistors and crosstalk;
> >>     
> >>>
> >>> ________________________________
> >>> From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
> >>> To: Paul Hurst <paul@xxxxxxxxxxxxxxxxxxxxxx>; si-list@xxxxxxxxxxxxx
> >>> Cc: chundi srikanth <chundis@xxxxxxxxx>; Lambert Simonovich
> >>>       
> >> <bertsimonovich@xxxxxxxxxx>
> >>     
> >>> Sent: Mon, October 19, 2009 11:46:36 PM
> >>> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs
> >>>
> >>> Length matching does not need to be tighter than 100 mils.
> >>>
> >>> It is not necessary to back drill vias used to connect component pins
> >>>       
> >> to
> >>     
> >>> traces or to change layers.  The 0.6 pF or so capacitance does not
> >>> adversely affect the signals at this data rate.
> >>>
> >>> It is not necessary to put "ground" vias next to routing vias as long
> >>>       
> >> as
> >>     
> >>> the PDS is well designed."
> >>>
> >>> "Guard"  traces have no value.  Proper spacing to avoid crosstalk
> >>>       
> >> does. 
> >>     
> >>> The "rule of thumb" for spacing given, while crude, is not far off.
> >>>
> >>> Lee Ritchey
> >>>
> >>>
> >>>   
> >>>       
> >>>> [Original Message]
> >>>> From: Paul Hurst <paul@xxxxxxxxxxxxxxxxxxxxxx>
> >>>> To: <si-list@xxxxxxxxxxxxx>
> >>>> Cc: chundi srikanth <chundis@xxxxxxxxx>; Lambert Simonovich
> >>>>     
> >>>>         
> >>> <bertsimonovich@xxxxxxxxxx>
> >>>   
> >>>       
> >>>> Date: 10/18/2009 5:45:08 PM
> >>>> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs
> >>>>
> >>>> Srikanth,
> >>>>
> >>>> Additional to Bert's comments:-
> >>>>
> >>>> 1) Match the lengths of the pairs exactly
> >>>> 2) Use a solid ground return
> >>>> 3) Avoid stubs by design or by using stub-drilling
> >>>> 4) If you have vias in the path use a small drill and a large
> >>>>         
> >> anti-pad and
> >>     
> >>>> place a ground via next to each signal via
> >>>>
> >>>> Regards
> >>>>
> >>>> Paul
> >>>>
> >>>> -----Original Message-----
> >>>> From: si-list-bounce@xxxxxxxxxxxxx
> >>>>         
> >> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> >>     
> >>>>     
> >>>>         
> >>> On
> >>>   
> >>>       
> >>>> Behalf Of Lambert Simonovich
> >>>> Sent: 16 October 2009 21:57
> >>>> To: 'chundi srikanth'; si-list@xxxxxxxxxxxxx
> >>>> Subject: [SI-LIST] Re: Routing guidelines for 3.12Gbps LVDS pairs
> >>>>
> >>>>
> >>>> Srikanth,
> >>>>
> >>>> A rough rule of thumb is greater than 3 times trace width separation
> >>>>     
> >>>>         
> >>> between
> >>>   
> >>>       
> >>>> pairs. Using a 2D field solver will ultimately give you the crosstalk
> >>>> coupling factor for the exact geometry in your stackup, and dictate
> >>>>         
> >> the
> >>     
> >>>> routing rules you need to follow to satisfy your noise budget. 
> >>>>
> >>>> Adding GND guarding will more often than not present more issues than
> >>>>         
> >> it
> >>     
> >>>> solves. You should stitch these guard traces at regular intervals
> >>>> approximately 1/10 of the wavelength of the highest frequency
> >>>>         
> >> component of
> >>     
> >>>> the aggressor signal to avoid it from resonating and coupling noise
> >>>>         
> >> back
> >>     
> >>>> onto other adjacent traces. This further reduces routing density of
> >>>>         
> >> the
> >>     
> >>>> board. By the time you factor in the additional space of one trace
> >>>>         
> >> width
> >>     
> >>>> between the guard trace and Diff pair, plus the additional via
> >>>>         
> >> stitching,
> >>     
> >>>> you will find you are already at 3 times separation between pairs and
> >>>>         
> >> you
> >>     
> >>>> would gain back more real estate.
> >>>>
> >>>> Regards,
> >>>>
> >>>> Bert Simonovich
> >>>>
> >>>>
> >>>>
> >>>>     
> >>>>
> >>>> -----Original Message-----
> >>>> From: si-list-bounce@xxxxxxxxxxxxx
> >>>>         
> >> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> >>     
> >>>>     
> >>>>         
> >>> On
> >>>   
> >>>       
> >>>> Behalf Of chundi srikanth
> >>>> Sent: October-16-09 12:40 PM
> >>>> To: si-list@xxxxxxxxxxxxx
> >>>> Subject: [SI-LIST] Routing guidelines for 3.12Gbps LVDS pairs
> >>>>
> >>>> Hi Techies,
> >>>> We have a 12-layer board in which we have lot of differential LVDS
> >>>>         
> >> pairs
> >>     
> >>>> operating at several hundreds MHz. And we have SERDES signals
> >>>>     
> >>>>         
> >>> (differential
> >>>   
> >>>       
> >>>> TX & RX pairs) operating at CPRI rate-4 i.e., 3Gbps. So can you just
> >>>>         
> >> share
> >>     
> >>>> me some inputs on exactly what are the guidelines to be followed
> >>>>         
> >> while
> >>     
> >>>> routing these signals. And is GND gaurding between the differential
> >>>>         
> >> pairs
> >>     
> >>>> improve the SI?Please share or refer me any documents in which i can
> >>>>         
> >> get
> >>     
> >>>> Good information on High-Speed design guidelines.
> >>>>
> >>>> Thanks
> >>>>
> >>>> With Best Regards
> >>>> Srikanth
> >>>>
> >>>>
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> >> -- 
> >> Steve Weir
> >> IPBLOX, LLC
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