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 > >>>> > >>>> > >>>> ------------------------------------------------------------------ > >>>> 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. Center St. #211 > >> Reno, NV 89501 > >> (866) 675-4630 Business > >> (707) 780-1951 Fax > >> http://www.ipblox.com > >> A Teraspeed Consulting Group LLC Partner > >> > >> Teraspeed Consulting Group > >> 121 North River Drive > >> Narragansett, RI 02882 > >> (401) 284-1827 Business > >> (401) 284-1840 Fax > >> > >> Teraspeed Labs > >> 13610 SW Harness Lane > >> Beaverton, OR 97008 > >> (503) 430-1065 Business > >> (503) 430-1285 Fax > >> A Teraspeed Consulting Group LLC Partner > >> > >> http://www.teraspeed.com > >> > >> This e-mail contains proprietary and confidential intellectual property > >> of IPBLOX LLC > >> ------------------------------------------------------------------------ > >> ------------------------------ > >> Teraspeed(R) is the registered service mark of Teraspeed Consulting > >> Group LLC > >> > >> ------------------------------------------------------------------ > >> 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. Center St. #211 > Reno, NV 89501 > (866) 675-4630 Business > (707) 780-1951 Fax > http://www.ipblox.com > A Teraspeed Consulting Group LLC Partner > > Teraspeed Consulting Group > 121 North River Drive > Narragansett, RI 02882 > (401) 284-1827 Business > (401) 284-1840 Fax > > Teraspeed Labs > 13610 SW Harness Lane > Beaverton, OR 97008 > (503) 430-1065 Business > (503) 430-1285 Fax > A Teraspeed Consulting Group LLC Partner > > http://www.teraspeed.com > > This e-mail contains proprietary and confidential intellectual property of IPBLOX LLC > ---------------------------------------------------------------------------- -------------------------- > Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC > > ------------------------------------------------------------------ > 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