Hello, sorry for beeing late on this one .. I have asked as similar question as well when having seen a Source + load termination on a PCIe clock buffer from ICS/IDT. Here the information I got from IDT: The 9DB106 outputs are switched constant current sources. The output cannot sink current. The 49.9 ohm resistors are needed to sink current and pull the signal lines low. Double termination is not normally recommended. The reference designs you see using the 9DB106 should only have termination at the source. In some cases it needs to be at the load. For example some PCIe devices have internal 50 ohm termination to ground. In this case it is okay - just remove the termination at the source if it is a motherboard down design. The problem is add-in in boards that use a PCIe device with internal termination to ground. The ASIC vendor needs to correct their designs to be compatible with the PCI-SIG PCIe standard if it is being used on an add-in board. So my interpretation of this answer was, that the main reason for the source termination is the HSCL constant current output buffer. Without any defined sink current capability a reflection might not be terminated, but reflected. But I'm not sure if my interpretation of the answer was correct ... But based on this i think when asking this question it is requried to define the interface that is used .. Hermann EKH - EyeKnowHow Hermann Ruckerbauer www.EyeKnowHow.de Hermann.Ruckerbauer@xxxxxxxxxxxxx Veilchenstrasse 1 94554 Moos Tel.: +49 (0)9938 / 902 083 Mobile: +49 (0)176 / 787 787 77 Fax: +49 (0)3212 / 121 9008 schrieb Rajan Hansa: > Experts, > > Can anyone explain that why in some designs we see source as well as load > terminations, I mean termination at both side of traces ? If ringing is a > issue then only source termination should be sufficient to control it and we > can use a receiver with very high input impedance i.e. no need to match > input impedance of receiver with transmission line characteristics > impedance. > > Rajan > > > ------------------------------------------------------------------ > 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 > > 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 Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu