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 > >> > >> > >> ------------------------------------------------------------------ > >> To unsubscribe from si-list: > >> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > >> > >> or to administer your membership from a web page, go to: > >> //www.freelists.org/webpage/si-list > >> > >> For help: > >> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >> > >> > >> List technical documents are available at: > >> http://www.si-list.net > >> > >> List archives are viewable at: > >> //www.freelists.org/archives/si-list > >> or at our remote archives: > >> http://groups.yahoo.com/group/si-list/messages > >> Old (prior to June 6, 2001) list archives are viewable at: > >> http://www.qsl.net/wb6tpu > >> > >> > >> ------------------------------------------------------------------ > >> To unsubscribe from si-list: > >> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > >> > >> or to administer your membership from a web page, go to: > >> //www.freelists.org/webpage/si-list > >> > >> For help: > >> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >> > >> > >> List technical documents are available at: > >> http://www.si-list.net > >> > >> List archives are viewable at: > >> //www.freelists.org/archives/si-list > >> or at our remote archives: > >> http://groups.yahoo.com/group/si-list/messages > >> Old (prior to June 6, 2001) list archives are viewable at: > >> http://www.qsl.net/wb6tpu > >> > >> > >> > >> > >> > >> > >> > >> ------------------------------------------------------------------ > >> To unsubscribe from si-list: > >> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > >> > >> or to administer your membership from a web page, go to: > >> //www.freelists.org/webpage/si-list > >> > >> For help: > >> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >> > >> > >> List technical documents are available at: > >> http://www.si-list.net > >> > >> List archives are viewable at: > >> //www.freelists.org/archives/si-list > >> or at our remote archives: > >> http://groups.yahoo.com/group/si-list/messages > >> Old (prior to June 6, 2001) list archives are viewable at: > >> http://www.qsl.net/wb6tpu > >> > >> > > > > > > ------------------------------------------------------------------ > > To unsubscribe from si-list: > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > > > or to administer your membership from a web page, go to: > > //www.freelists.org/webpage/si-list > > > > For help: > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > > > > List technical documents are available at: > > http://www.si-list.net > > > > List archives are viewable at: > > //www.freelists.org/archives/si-list > > or at our remote archives: > > http://groups.yahoo.com/group/si-list/messages > > Old (prior to June 6, 2001) list archives are viewable at: > > http://www.qsl.net/wb6tpu > > > > > > > > ------------------------------------------------------------------ > > To unsubscribe from si-list: > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > > > or to administer your membership from a web page, go to: > > //www.freelists.org/webpage/si-list > > > > For help: > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > > > > List technical documents are available at: > > http://www.si-list.net > > > > List archives are viewable at: > > //www.freelists.org/archives/si-list > > or at our remote archives: > > http://groups.yahoo.com/group/si-list/messages > > Old (prior to June 6, 2001) list archives are viewable at: > > http://www.qsl.net/wb6tpu > > > > > > > > > > > -- > Steve Weir > IPBLOX, LLC > 150 N. 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