Verify the cable before the
attenuator (cable between function generator and attenuator) is isolated and not
touching the cable after attenuator, DUT, LP, and Ethernet Cable
Normally the cable before
the attenuator carries noise with huge amplitudes, easily disturbing
other signal lines
Verify the link partner’s RJ45
shield is connected to earth ground
Normally the long cable
is on the link partner side. There can be higher impedance between
shield and earth ground if the only connection between cable shield to
earth ground is within CDN. Therefore, providing a small impedance path
to ground helps with CI testing
If the RJ45 shield is not
able to connect to earth ground for the application, connect the RJ45
shield or connector ground directly to a metal or conducted case to
provide a better radiated path for EMI noise to flow and radiate to the
surrounding area.
Longer cable is preferred on the
link partner side to reduce the effect on link partner
Verify the cable type matches on
both sides of the CDN
Verify the CDN type matches with
the cable types used
Mismatching unshielded cable CDN with a shielded cable can potentially
increase reflective noise due to the discontinuous of ground in the
signal lines
Using unshielded cable CDN with shielded cable can also increase the
impedance of the return path between signal lines and earth ground,
introducing higher amplitude noise on the signal lines
Verify the CDN is connected to
the earth ground
Prevent any cable looping that
can increase crosstalk
Verify the cable is not touching
any noise source
Shielded cable is recommended.
(CAT 6 cable preferred)
Having a shielded cable
is better than non-shielded cable since the shielded cable provides a
path from MDI lines to ground inside the CDN for through capacitors
For unshielded cables,
all noise can go directly to the signal lines, with no noise coupling to
ground