Definition
Isolation = attenuation from a source port to an unwanted destination port Isolation (dB) = −20·log₁₀|Sᵢⱼ| [positive, larger = better] Example: TX port → RX port in a duplexer: S31 = −52 dB → Isolation = 52 dB This means: only 10^(−52/20) = 0.00251 of the TX amplitude reaches RX At 33 dBm TX power: RX port sees 33 − 52 = −19 dBm (safely below LNA P1dB)
Isolation Requirements by Device Type
| Device | Port Pair | Required Isolation |
|---|---|---|
| FDD duplexer (LTE) | TX → RX | >50–55 dB |
| RF switch (SPDT, OFF path) | IN → blocked OUT | >30–45 dB |
| Directional coupler | IN → isolated port | >25–35 dB (directivity) |
| Wilkinson divider | OUT1 → OUT2 | >20 dB (at center freq) |
| Amplifier (S12) | OUT → IN | >20–30 dB reverse isolation |
Measuring Isolation with RF View
- For duplexer: extract TX→RX port pair (.s3p → Port 1+3 extraction)
- Load extracted .s2p → select S21 dB
- Place marker at RX band center frequency
- Read S21 value: e.g., −52 dB → TX-RX isolation = 52 dB ✓
- Sweep marker across full RX band → find worst-case (minimum isolation)
Isolation vs Rejection vs Attenuation
These terms are related but used in different contexts:
- Isolation: Between two specific ports in a multi-port device (duplexer, divider)
- Rejection: Filter attenuation at stopband frequencies (single device, same port pair)
- Attenuation: Generic reduction in signal level, could be either mechanism
RF View Isolation Measurement: Extract port pairs from multi-port files, load isolation path .s2p, use single marker for spot-check or BW Marker to find minimum isolation frequency. Free on Android.