Joe, The first question is whether you are actually driving the traces with a differential signal. If so, the single-ended impedance of the traces doesn't matter. You really need to specify the differential impedance of the pair(s). Hyperlynx Signal Integrity software does a great job of describing the physical constraints required to get a true differential impedance from a signal pair in a pwb. It also gives the single-ended impedance of the traces should you wish to terminate for both single-ended AND differential signaling. Summary: It is quite possible to specify a single-ended impedance for a differential pair and end up nowhere close to the desired differential impedance. Many PWB fabricators can do the job for you if you just identify the differential pairs and the desired differential impedance. Gary D. Peterson Sandia National Labs -----Original Message----- From: Hurst, Joe [mailto:[log in to unmask]] Sent: Thursday, March 18, 1999 6:21 AM To: [log in to unmask] Subject: [TN] Specifying Differential Impedance Hi I have some very important differential pairs on a PWB that must have a particular differential impedance. The problem is: how can I specify this on the drawing sent to the fab shop? Our usual method would be to call for a particular single-ended impedance, which seems to be beating around the bush. It work to a point; but I think it leaves too many variables that could change the differential impedance. Any thoughts on this? Joe Hurst PWB Designer Computing Devices Canada A General Dynamics Company [log in to unmask] 613-596-7816 ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################ ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################