Putting the caps at the receiver side has one other little advantage, and thats component density. Due to the low signal amplitude at the receiver, the coupling between capacitors is lower, thus you can place them closer together and still have the same x-talk level as if they were at the transmitter side. BR Gert ---------------------------------------- Absender ist HARTING Electronics GmbH & Co. KG; Sitz der Gesellschaft: Espelkamp; Registergericht: Bad Oeynhausen; Register-Nr.: HRA 5596; persönlich haftende Gesellschafterin: HARTING Electronics Management GmbH; Sitz der Komplementär-GmbH: Espelkamp; Registergericht der Komplementär-GmbH: Bad Oeynhausen; Register-Nr. der Komplementär-GmbH: HRB 8808; Geschäftsführer: Edgar-Peter Duening, Torsten Ratzmann, Dr. Alexander Rost -----Ursprüngliche Nachricht----- Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im Auftrag von Scott McMorrow Gesendet: Montag, 24. September 2012 17:04 An: emcesd@xxxxxxx Cc: si-list@xxxxxxxxxxxxx; Rose, Michael Betreff: [SI-LIST] Re: More discussion on AC coupling capacitor "Does placing the AC capacitor near the receiver really help SI?" Let's dispel this "fact" once again. In a linear interconnect that is well-matched, it does not matter where a DC blocking capacitor is placed. What matters are potential reflective resonances that occur between mismatched structures on the interconnect. Dies, packages, connectors, vias, and capacitors tend to be the major impedance discontinuities in an interconnect. It does not matter if dispersion happens before or after a discontinuity, or on both sides of it. The only thing that matters is keeping the discontinuities as small as possible. This can be done with DC blocking capacitors by performing optimization of the transition in and out of the capacitor with electromagnetic modeling tools. It is quite possible to keep 0402 capacitors "flat" out to 10 GHz and 0201 capacitors "flat" out to 20 GHz with good designs. "Which is the better place for placing the decoupling capacitor, near the transmitter or near the receiver?" If the capacitor transition design is not well-matched, then "best" placement depends upon how it interacts with other structures with impedance discontinuities. This might be packages, drivers, receivers, or connectors. It just depends which is the worst, how close the capacitor is to the structure, and the amount of interconnect loss in between them. Loss is good, since it will de-Q resonances and improve return loss in the region of the discontinuities by 2 times the interconnect loss between them. For this reason, I like loss line and switch cards, and prefer to use low loss practices in backplanes. Thus the best place to place an DC blocking capacitor is as far away as possible from other discontinuities. Oftentimes the worst discontinuity is the receiver, due to excessive capacitance. Placing a capacitor near the receiver can sometimes be counterproductive. Finally, what is the correct value ... it depends on the low frequency limit of your serial encoding pattern. The capacitor must be large enough to reduce low frequency wander and jitter problems. 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 registered service mark of Teraspeed Consulting Group LLC On Mon, Sep 24, 2012 at 3:05 AM, Tesla <emcesd@xxxxxxx> wrote: > Hi, Experts > > In the last thread,Michael said "Placing them at the RX side generally > helps to minimize impact of the impedance discontinuity given the > degradation of edge rate from dispersion" > Does place the AC capacitor near the receiver really help SI? > Which is the better place for placing the decoupling capacitor, near > the transmitter or near the receiver? > How to choose the value of the decoupling capacitor? it is based on > the data rate, code style or other things? can we calculate it manually? > > Thanks. > > Tesla. > ------------------------------------------------------------------ > 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 forum is accessible at: > http://tech.groups.yahoo.com/group/si-list > > List archives are viewable at: > //www.freelists.org/archives/si-list > > 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu