How to Extract S2P from S4P Files

Why Extract S2P from S4P

A 4-port Touchstone file (.s4p) contains S-parameters between all port combinations (S11, S21, S31, S41, S12, S22, S32, S42, etc.). For component-specific analysis (e.g., just the TX path of a duplexer), it's easier to extract the relevant 2-port file and work with it as a standard .s2p.

Port Pair Extraction Concept

  S4P file: 4×4 S-matrix containing all 16 S-parameters

  Extracting port pair (i, j) as S2P:
  New S11 = original Sii  (reflection at port i)
  New S21 = original Sji  (transmission from port i to port j)
  New S12 = original Sij  (transmission from port j to port i)
  New S22 = original Sjj  (reflection at port j)

  All measured with other ports terminated in Z₀ = 50 Ω

Duplexer Example: Extract All Three Paths

  S4P duplexer port convention:
  Port 1 = TX input, Port 2 = Antenna, Port 3 = RX output, Port 4 = Ground

  Extract TX → ANT (ports 1 and 2):
  → save as tx_ant.s2p
  → Use to: measure TX passband IL, TX port S11, ANT port S22

  Extract ANT → RX (ports 2 and 3):
  → save as ant_rx.s2p
  → Use to: measure RX passband IL, ANT port S11 in RX band, RX port S22

  Extract TX → RX isolation (ports 1 and 3):
  → save as tx_rx_isolation.s2p
  → Use to: measure TX-to-RX isolation in RX band (critical spec!)

Port Extraction in RF View

1. Open RF View → SNP Converter tab
2. Load the .s4p source file
3. Select "Port Extraction" operation
4. Choose source port and destination port (e.g., Port 1 and Port 2)
5. Tap "Extract" → new .s2p file saved to output folder
6. Repeat for other port pairs
7. Load all extracted .s2p files into S-Parameter Plot for analysis

What You Lose in Port Extraction

Extracting a 2-port from a 4-port assumes the non-measured ports are terminated in Z₀ = 50 Ω. This is exactly the condition under which the S-parameters were measured (standard VNA termination). The extracted .s2p is therefore accurate for normal 50 Ω system analysis.

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