Howard Johnson has a nice analysis on dual series termination: http://www.sigcon.com/Pubs/news/2_20.htm Best Regards Charles Grasso Compliance Engineer Echostar Communications (w) 303-706-5467 (c) 303-204-2974 (t) 3032042974@xxxxxxxxx (e) charles.grasso@xxxxxxxxxxxx (e2) chasgrasso@xxxxxxxxx -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of steve weir Sent: Friday, August 12, 2011 6:47 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Why Termination at Both End ? Hermann, IDT is recommending against termination on both ends because it will attenuate the signal swing unacceptably. This is the same problem that plagued rail-to-rail signal CMOS and TTL/LVTTL standards. The price that one pays for using those standards is much tighter impedance control requirements on the PCB and the terminators. With a 49.9 Ohm sink resistor, the current source looks like a 49.9 Ohm source. If the resistor is placed at the far end, then the device looks like a very high impedance with an end termination of 49.9 Ohms. In that case any reflection from far-end mismatch such as caused by parasitic capacitance or trace impedance error travels back up to the clock buffer and bounces right off of it. Steve. On 8/12/2011 4:46 AM, Hermann Ruckerbauer wrote: > 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 > > > -- Steve Weir IPBLOX, LLC 150 N. Center St. #211 Reno, NV 89501 www.ipblox.com (775) 299-4236 Business (866) 675-4630 Toll-free (707) 780-1951 Fax ------------------------------------------------------------------ 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