Hello , a differential port is when the signal is reference to another port.
So as shown in the photo we have DP1 (differential port 1) and DP2 (differential port 2)
DP1 is VNA port 1 referenced to VNA port 2 .
DP2 is VNA port 3 referenced to VNA port 4.

in the video they put an attenuator on leg 4 why creates delay and attenution.
Few questions:
1.What is the meaning of the middle point X they did?
2.how theoriticcaly this attenuator creates a scew between the two differential ports?
3. why the difference between the two crosses creates?
Thanks.

https://www.rohde-schwarz.com/us/kn...l-transmission-part-4-of-4_251220-712194.html

 

The "X" represents the electrical midpoint where the rising and falling signals cross perfectly. Adding an attenuator to Leg 4 increases its electrical length. It also slows the signal down and creates an intra-pair skew.