I also think it is 130ohm differential. But on the subject of making >100 ohm trace and 50 ohm traces on the same layer, I've known people who will cut the reference plane underneath the >100ohm microstrip to force it to reference the next power/gnd plane beneath and thereby creating a super high impedance. I wouldn't do it personally but people have shipped product like that. -----Original Message----- From: steve weir [mailto:weirsp@xxxxxxxxxx] Sent: Wednesday, June 02, 2004 4:14 PM To: msharpes@xxxxxxxxxx; prasadsa@xxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: 50 ohm and 130 ohm on the same board M, that is an interesting application model. I think the prevailing wisdom would have been to use a lower impedance driver and transmission line to minimize the discontinuity caused by load capacitances. What you did plays on paper. I would be wary of how accurately the real loads match the line, especially with tolerancing, but if it worked, that is what counts. Still 130 ohms seems like a value derived from a pair and not a single ended. I think Prasad is in India, and we won't see a response until tonight. Regards, Steve. At 05:00 PM 6/2/2004 -0600, msharpes@xxxxxxxxxx wrote: >Although 130 Ohms seems on the high side, through some experience I can >talk of designing higher impedance transmission lines (in the 100 Ohm >neighborhood) for a fairly large distributed load. So, the concept was >that the capacitive nature of each of the distributed loads served to >lower the characteristic along the line so much so in some cases that >impedances of 80-100 Ohms were desired for a 50 Ohm 'effective' >impedance. The trick to doing this is to space the loads in an equal >pitch with the pitch being something less than an inch for the >transmission line to 'look' like something around 50 Ohms. Now, this >isn't portable to extremely fast designs, but we have had success with >terminated transmission lines of this type with the right drive strength >and source impedance to move 2V/ns at each load in a 3.3V system. > ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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