Reciprocity Definition
Reciprocal network: Sᵢⱼ = Sⱼᵢ for all port pairs [S] = [S]ᵀ (symmetric matrix) Physical basis: Lorentz reciprocity theorem Applies to all passive networks (no magnetic bias) Does NOT apply to: active devices, magnetized ferrites
Identifying Reciprocity from S-Parameters
Load .s2p in RF View: Overlay S21 and S12 on same chart Reciprocal (SAW filter, cable, attenuator): S21 = S12 at all frequencies → traces coincide exactly Non-reciprocal (amplifier): S21 = +18 dB (forward gain) S12 = −22 dB (reverse isolation) S21 − S12 = 40 dB → not reciprocal
Practical Implications
| Device | Reciprocal? | Implication |
|---|---|---|
| SAW/BAW filter | Yes | Can use either port as input or output |
| LNA | No (S21≫S12) | Direction matters! Input→output only |
| RF cable | Yes | Works in either direction identically |
| Ferrite isolator | No (intentional) | Signal passes one way, blocked reverse |
| RF switch (ON state) | Yes (passive) | Bidirectional signal path |
RF View Reciprocity Check: Overlay S21 and S12 in S-Parameter Plot. Identical traces confirm reciprocity. Any difference immediately flags active or magnetized device. Free on Android.