Why TX-RX Isolation Matters
In FDD cellular systems, the transmitter and receiver operate simultaneously on different frequencies. The duplexer must provide sufficient TX-to-RX isolation so that TX power leaking into the RX path doesn't desensitize or damage the LNA. Insufficient isolation causes 'blocking' — the TX leakage signal saturates the LNA, reducing receive sensitivity.
Required Isolation Calculation
Typical LTE handset example: TX power: +33 dBm at PA output LNA P1dB: −10 dBm (starts compressing above this) Required safety margin: 10 dB below P1dB Required TX power at LNA input: ≤ −10 − 10 = −20 dBm Duplexer isolation needed: 33 − (−20) = 53 dB minimum In practice: duplexers specify isolation >52–55 dB in RX band
Step-by-Step Isolation Verification
- Load duplexer .s3p file
- SNP Converter → Port Extraction → extract TX port (P1) to RX port (P3) → save as "tx_rx.s2p"
- Load tx_rx.s2p → S21 view
- Place single marker at RX band center frequency (e.g., 1843 MHz for LTE Band 3)
- Read S21 value (e.g., −54 dB → TX-to-RX isolation = 54 dB ✓)
- Sweep marker across full RX band → find worst-case (minimum) isolation frequency
- Compare against specification: must be >50–55 dB across entire RX band
Isolation Across Full RX Band Check
Use BW Marker with custom threshold = −50 dB (isolation spec): Find frequencies where S21 > −50 dB (isolation below spec!) These are "isolation holes" — potential blocking frequencies Example: LTE Band 3 duplexer isolation sweep (1805–1880 MHz): 1810 MHz: −52.3 dB ✓ 1843 MHz: −55.1 dB ✓ (best isolation at band center) 1875 MHz: −50.8 dB ✓ (marginal — just meets spec) All pass → duplexer qualified for Band 3 isolation
RF View Isolation Check: Extract TX→RX path from duplexer .s3p/.s4p, load tx_rx.s2p, and use markers to read isolation across the RX band. BW Marker with −50 dB threshold flags isolation holes. Free on Android.