Justin, This is a linear system. The capacitor (and it's associated transition pads, vias and traces) have exactly the same characteristics no matter where it is placed. Your problem has absolutely nothing to do with the fact that this is a capacitor. It is just another discontinuity in the transmission channel that must be modeled and simulated to ensure good signal fidelity at the receiver. The capacitor value will impact the low frequency cut off. The size of the capacitor body, location of the plates in the body, size of the pads, length of the vias, and trace layer, will determine the high frequency performance. The length of traces leading to and from the capacitor will determine the frequency of any half-wave resonances formed by the capacitor discontinuities, the IC package and the backplane connector, no matter whether it is placed on the Tx or Rx side. Model and simulate each of these. regards, Scott -- Scott McMorrow Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 (401) 284-1827 Business (401) 284-1840 Fax http://www.teraspeed.com Teraspeed® is the r Smith, Justin D. wrote: > I'm working on a 6.25Gbps channel SI issue with a constraint that is not > ideal. We have a legacy issue which requires that we put decoupling caps on > the "transmit" side of a transmission line instead of the receive side (where > we'd like them). This must be done to maintain "backwards compatibility" with > MODULE 2 that does not have decoupling on the receiver side. > > That said, we have this type of setup: > > ================================================================== > # MODULE 1 BACKPLANE MODULE 2 # > # XMT DCPL BP. BP. RCVR # > # |\ CAP Conn Trace Conn |\ # > # | \_____||__________[ ]________________[ ]______________| \ # > # | / || [ ] [ ] | / # > # |/ |/ # > =============================Fig 1================================ > (hope that cheesy ascii graphic comes through) > > Some questions arise that I would value the SI-LIST opinion on: > > 1)Where to put the decoupling cap with the constraint that we have to put it > on MODULE 1, which is the XMT side of the overall channel? > > a. Near to the transmitter (possibly greater reflected power into the > transmitter?) > b. Near the BP connector (as close to the RCV as we can? makes the > discontinuity more capacitive - but "lumps" issue with connector > discontinuity). > c. In the middle (distribute the discontinuities along the path - > possibly results in multiple reflections?). > > 2) Knowing that the backplane trace is very short (~1.5") does that effect > how you'd advise? > > 3) Parameters that I can work with are the length of the trace from FPGA to > connector on the module, location of the cap, and cap value/size. > > We have Hyperlynx as a SI tool. Thoughts? Anybody ever dealt with an issue > like this before? > > -justin > ============================================================ > The information contained in this message may be privileged > and confidential and protected from disclosure. If the reader > of this message is not the intended recipient, or an employee > or agent responsible for delivering this message to the > intended recipient, you are hereby notified that any reproduction, > dissemination or distribution of this communication is strictly > prohibited. If you have received this communication in error, > please notify us immediately by replying to the message and > deleting it from your computer. Thank you. Tellabs > ============================================================ > ------------------------------------------------------------------ > 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 > > > > egistered 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