Bill, The original question is a general one without details and the 133MHz speed is not very high. The suggested stackup keep things simple. Even both of the original two stackups can be made to work, with extra effort. By referencing all signals to continuous ground planes, there is no need to keep track of return signal path for any signal assuming all the ground planes are stitched together nicely. This is not true if the signals are referenced to power planes. Dual strip line crosstalk: I have advised increasing the distance between the signal layers and/or power planes to adjust for the board thickness. By following orthogonal routing on adjacent signal layers, crosstalk is minimized. You can also reduce the crosstalk by decreasing the dielectric thickness between signal layer and ground plane. The signals are coupled more strongly to the reference plane than to other signal layer. Power planes: The power planes may look adjacent, but they are more strongly coupled to the adjacent ground planes than to each other. Also, most of the current devices need multiple power inputs and by localization of power pours, the coupling between them can be made far less compared to the coupling to the ground. Those G/P pairs can be thin dielectric materials if required. I have used the suggested stackup on many different designs for PCIe, DDR2, SAS, SATAI/II, Gigabit Ethernet, FC and never had to worry about EMI/EMC, Noise and crosstalk. The stackup alone will not solve all issues, but is a starting point for good design. I do not see any limitation in the proposed stackup. The devices I have used needed multiple power supplies and the best place for decoupling caps had always been back of the BGA in my case. I could carve out multiple local power pours on the power planes and if required even on signal layers. -Prasad ________________________________ From: wjcsongr@xxxxxxxxxxxxxxxxxxx [mailto:wjcsongr@xxxxxxxxxxxxxxxxxxx] Sent: Monday, July 28, 2008 1:40 PM To: Mangipudi, Prasad Cc: 'SI LIST'; si-list-bounce@xxxxxxxxxxxxx; 'sunil bharadwaz' Subject: Re: [SI-LIST] Re: 12 Layer stack All, Another interesting topic to get the list fired up with discussion. Prasad, please explain why you think referencing all signals to GND is a good thing? And why coupling power planes of different noise levels in the middle of this mess is a good thing? Our Intel friends can certainly point to the now famous paper about this GND referencing stuff. I believe we will discover that as always, rules of thumb must be applied with care and not willy nilly everywhere. I'm very interested in the comments on this one. I would probably vote to minimize dual stripline wiring to minimize xtalk. With the stack below, you'll still have 4 good easily controlled wiring layers, 2 on top and bottom for fan out. And you can stick a hunk of that buried capacitance stuff in the middle. 'P' in the stack is plane. I don't care, except in the middle, if it's power or GND. With any stack discussed so far, you really only get 4 good layers. S-G-S-P-S-G-P-S-G-S-G-S. If that dual strip doesn't bother you, go for A. Also, doesn't the final answer depend on how many power planes you will need in your design? If you're a 10W board, for example, maybe one power layer is sufficient. There's that Ohm's law thing again: - ) Regards, Bill William Csongradi Senior Electrical Engineer Rockwell Collins Heads Down Display Center 319-295-7884 Mailing Address Rockwell Collins 400 Collins Road NE MS 105-167 Cedar Rapids, Iowa 52498-0001 "Mangipudi, Prasad" <Prasad_Mangipudi@xxxxxxxxxxx> Sent by: si-list-bounce@xxxxxxxxxxxxx 07/28/2008 03:19 PM To 'sunil bharadwaz' <sunil_bharadwaz@xxxxxxxxx>, 'SI LIST' <si-list@xxxxxxxxxxxxx> cc Subject [SI-LIST] Re: 12 Layer stack Sunil, Neither. I would use S/G/S/S/G/P/P/G/S/S/G/S to simplify the design. All signals reference to ground. Required board thickness is achieved by increasing the dielectric between power planes and/or signal planes. -Prasad -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of sunil bharadwaz Sent: Monday, July 28, 2008 11:08 AM To: SI LIST Subject: [SI-LIST] 12 Layer stack Hi , I have following two stack up's Stack 'A' 1)Signal 2)PWR 3)GND 4)Signal 5)PWR 6)Signal 7)Signal 8)Gnd 9)Signal 10)PWR 11)GND 12)Signal Stack 'B' 1)Signal 2)GND 3)Signal 4)Signal 5)Ground 6)Power 7)Power 8)GND 9)Signal 10)Signal 11)PWR 12)Signal Intent is to use one of these stacks for an FPGA based high speed Design (Max 133 Mhz). Can i know which one is preferable. regards Sunil.B ------------------------------------------------------------------ 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