In the past I was using a 100-Ohm diffential high speed cable from Samtec: https:// Now I need another solution because this cable is too rigid for a new application.
Always 2 neighboring wires for 1 LVDS pair and then one wire free connected to GND? Still routing in 100-Ohm differential and 100-Ohm termination resistors close to the FPGA? I don't think that it's necessary to have unused coax wires between differntial pairs.
Crosstalk of adjacent pins should be sufficient low, I'm quite sure there's a specification available from Samtec.
A differential I/O standard needs to specify common mode impedance if it implements single ended signalling as e.g. Otherwise it's up to the cable and PCB designer to chose a common mode impedance.
You can use separate 50 ohm wires for a differential signal (of course with closely matched length) and still apply differntial parallel termination at the receiver.
It's the transmission line impedance measured against ground when connecting both wires in parallel, it's another parameter that characterizes a differential transmission line besides differential impedance.
The lowest value is achieved with a differential pair comprosed of two uncoupled single ended lines as you intend.I took a quick look at the datasheet, and there's several pages explaining the matching requirements--too much for me to wade through in a few minutes, but maybe you can work it out with some careful reading.You need to use an impedance calculator tool to figure out the geometry for routing your differential pairs.Increasing the common mode impedance just reduces the possible common mode crosstalk.Sorry but what exactly do you mean with "common mode impedance"?I believe the 100-ohm differential equates to 50-ohm single ended because you can think of the two 100-ohm impedances as being in parallel resulting in an effective 50-ohm single-ended equivalence.