- #36
f95toli
Science Advisor
Gold Member
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Ephant said:Ok. Let's compute it. Why can't ordinary LEMO connectors survive 480Mbps?? In original USB male plug, is it impedance matched to the twisted wires?
Because most likely that won't result in the right impedance. The impedance when you use a specific connector will depend on the geometry (distance between pins etc) of the connector itself, the dielectric constant of the materials used, how the shield of the cable it attached, in what order the wires are attached etc....There are lots of factors; and even very small differences will result in changes in the impedance; and if the impedance if wrong the signal coming through the cable will be reflected.
There is absolutely no way to calculate this analytically and get even close; simulations of this type are always done using EM simulation software (although a generic FEM solver like COMSOL might also work ).
It is entirely true -as is you were told- that there is nothing "special" about the LEMO connector sold for USB 2.0. But the point is that that particular type happens to have geometry that result in in the right impedance; this obviously does not mean that ALL Lemo connectors will work since Lemo makes connectors of many different sizes and with different pin configurations .
if you want to have a "well document" example of this just compare SMA connectors of different impedance; the 75 ohm and 50 ohms versions look very similar (and I think you they even fit together if you use some force, NOT recommended); however the impedance differs by 50% due to small differences in things like the diameter of the inner pins. If you try to use a 75 ohm connector in a 50ohm environment it will work fine at low frequencies; but at higher frequencies you will get lots of reflections.
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