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I am using Expedition 9,4 so if it is similar you couled start like this.
1) Open CES Editor (Constraint manager) and Define your layer stackup whith thickness and youre Differenz Z0 Ohm
2) Create a new Net Class under "Net Classes" for your Diffpair thats were you ajust the Spacing and the Trace width of jour Diffpair. The Chart will show you wath Impedance youre Diff pair will end up with.
of cource in dependence of your Layer stackup "Er" Trace width and Spacing.
3) Go to "Constraint Classes" and Creat a Constraint Class which match your tolerance requirements.
4) Finaly Select your two Nets which shouled be a Diffpair, right klick one of the nets and select "Create Diffpair"
The Diffpair net will now apear on the top of your list of nets you can now Rename if you want for example "" LVDS_TX0"
With a other right click on the Diffpair you can assigne then to the Net class and Constraint class you created for the Diff pair.
If you are only drafting the schematic name the differential signals as _p for positive at the last and _n for negative signals so that you can assign all the differential pairs at a single stretch by using wild card option in CES
setup -differential pairs -use"*_p"& "_n"as wild cards the tool will automatically assign differential pairs
thanks and regards