Two-port vs four-port measurements¶
A differential pair is a 4-port device: positive and negative on the near end, positive and negative on the far end. A 2-port VNA can only see one port-pair at a time. That is why labs often take four S2P files (or one S4P) for a single cable or connector.
What a 2-port file contains¶
Each .s2p holds S11, S21, S12, S22 for one stimulus/response pair, with the unused DUT ports terminated however they were during that sweep (often 50 Ω loads, sometimes open, sometimes the VNA’s other port if you used a fixture).
Those four files are not a complete 4-port S-matrix. Cross terms that were never measured are missing. Transmission mixed-mode (SDD21, SCC21, …) from the four S21 traces is the usual, defensible use. Reflection mixed-mode (SDD11, …) from four S11 traces is an approximation — each S11 saw a different termination on the far-side pair.
When you need a true S4P¶
Prefer a 4-port VNA (or a 2-port plus a switch matrix that the vendor’s software reassembles) when you need:
- Accurate SDD11 / SDD22 (return loss of the differential pair)
- Mode conversion on reflection (SDC11, SCD11)
- Cascading the DUT with other 4-port networks
- Fixture removal that uses the full matrix
Four S2Ps as a workflow¶
If the bench is 2-port only:
- Measure the four port-pairs (commonly p13, p14, p23, p24).
- Store each as its own dataset with a shared serial and a
mixed_mode(orport_pair) parameter. - Confirm polarity mapping before converting.
- Publish mixed-mode transmission traces. Treat mixed-mode reflection as approximate unless you also have an S4P.