Balun S-Parameter Requirements
A balun (3-port: Port 1=unbalanced/SE, Port 2=balanced+, Port 3=balanced−) must provide equal amplitude and exactly 180° phase difference at its balanced outputs. Any imbalance degrades common-mode rejection and introduces differential-to-common-mode conversion.
Critical Balun Specifications
| Parameter | Specification | Measurement |
|---|---|---|
| Insertion Loss (S21) | <0.5 dB | |S21| in dB at operating freq |
| Amplitude Balance | |S21|−|S31| < ±0.5 dB | Overlay S21 and S31 in RF View |
| Phase Balance | Phase(S31)−Phase(S21) = 180° ±5° | Delta marker on phase plot |
| Input Return Loss (S11) | >15 dB | BW Marker on S11 |
| Output Isolation (S23) | >20 dB | Single marker on S23 |
Analysis Procedure in RF View
- Load balun .s3p file → SNP Converter → Port Extraction
- Extract S21 path (Port 1→2): save as bal_out1.s2p
- Extract S31 path (Port 1→3): save as bal_out2.s2p
- Load both .s2p files in S-Parameter Plot
- S21 overlay: both traces should coincide (amplitude balance ✓)
- Phase view: both traces should be 180° apart (phase balance)
- Delta marker: measures Phase(S31)−Phase(S21) at operating frequency
Common Balun Defects
| Defect | S-Parameter Symptom |
|---|---|
| Poor amplitude balance | S21 ≠ S31 in magnitude by >0.5 dB |
| Poor phase balance | Phase(S31)−Phase(S21) ≠ 180° by >5° |
| High insertion loss | S21 < −1 dB (lossy balun core material) |
| Narrow bandwidth | Balance specs only met over small frequency range |
RF View Balun Analysis: Extract S21 and S31 paths, overlay them for amplitude check, use phase view with delta marker for 180° phase balance verification. Free on Android.