How to Analyze a Cellular Duplexer with S-Parameters

What You're Measuring

A duplexer separates TX (uplink) and RX (downlink) signals on a shared antenna port in FDD cellular systems. Full characterization requires measuring: TX passband loss, RX passband loss, TX→RX isolation, and antenna port VSWR in both TX and RX bands.

Step 1: Load the File and Verify Port Convention

  Typical 3-port convention:
  Port 1 = TX Input     (e.g., 880–915 MHz for LTE Band 8 UL)
  Port 2 = Antenna      (shared port)
  Port 3 = RX Output    (e.g., 925–960 MHz for LTE Band 8 DL)

  Verify with manufacturer datasheet before analysis.
  If ports are swapped, use RF View Port Change to remap.

Step 2: Extract Port Pairs Using RF View SNP Converter

1. Open RF View → SNP Converter → Port Extraction
2. From .s3p: extract ports 1,2 → save as "tx_ant.s2p"
3. Extract ports 2,3 → save as "ant_rx.s2p"
4. Extract ports 1,3 → save as "tx_rx_isolation.s2p"

Step 3: TX Passband Analysis (tx_ant.s2p)

  Load tx_ant.s2p → S21 dB
  Read at TX band (e.g., 880–915 MHz):
    S21 = −1.8 dB → TX Insertion Loss = 1.8 dB ✓ (spec: <2 dB)

  Check S11 (TX port return loss) across TX band:
    S11 = −16 dB → RL = 16 dB ✓ (spec: >14 dB)

  Check S21 at RX band (rejection of TX signal at ANT):
    S21 at 925–960 MHz = −55 dB → TX→ANT rejection = 55 dB

Step 4: TX→RX Isolation (tx_rx_isolation.s2p)

  Load tx_rx_isolation.s2p → S21 dB
  Read isolation at RX band (925–960 MHz):
    S21 = −52 dB → TX→RX isolation = 52 dB ✓ (spec: >50 dB)

  At 30 dBm (1 W) TX power:
  Leakage to RX port = 30 − 52 = −22 dBm  (below LNA input P1dB)

Step 5: RX Passband and ANT Port VSWR

  Load ant_rx.s2p → S21 dB:
    S21 at 925–960 MHz = −1.9 dB → RX IL = 1.9 dB ✓

  Load tx_ant.s2p → S22 (ANT port in TX band):
    S22 = −15 dB → VSWR = 1.43:1 ✓

  Load ant_rx.s2p → S11 (ANT port in RX band):
    S11 = −14.5 dB → VSWR = 1.47:1 ✓

Related Topics

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