Dear Joe, You might like to try Polar Instruments field solving impedance calculator. This will let you see how track seperation actually alters impedance on differential traces. You will find that the single end impedance of each trace will need to be higher than usual as when driven differentially the lines act as though a "virtual ground" or "ground wall" appears between the two lines thus lowering the effective impedance. Kind regards Andy Burkhardt www.polar.co.uk At 08:20 18/03/99 -0500, you wrote: >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 ################################################################