90-Degree Hybrid Coupler Overview
A 90-degree hybrid (branch-line coupler) splits input power equally between two output ports with a 90-degree phase difference between them. Essential for IQ modulators, balanced amplifiers, and antenna diversity systems.
Ideal 90-Degree Hybrid S-Parameters
Port 1 = Input, Port 2 = 0° output, Port 3 = 90° output, Port 4 = Isolated S21 = S31 = −3 dB = 0.707 (equal power split) Phase(S31) − Phase(S21) = −90° exactly at center frequency S41 = 0 (perfect isolation) S11 = S22 = S33 = S44 = 0 (all ports matched) In practice (measured): S21 = S31 = −3.0 to −3.5 dB Phase difference = −90° ± 5° across usable bandwidth S41 < −20 dB isolation S11 < −20 dB at center frequency
Analysis in RF View
- Load hybrid .s4p file → extract port pairs
- Main outputs: verify S21 = S31 = −3.0 ±0.3 dB (power balance)
- Check phase: Phase(S31) − Phase(S21) = −90° ± 5° (quadrature accuracy)
- S41 isolation: must be <−20 dB for good balanced operation
- Bandwidth of 90° ± 5° specification: typically 15–20% for branch-line
Branch-Line vs Lange Coupler
| Characteristic | Branch-Line | Lange Coupler |
|---|---|---|
| Fractional bandwidth | 15–20% | 40–60% |
| Size | Large (λ/4 × λ/4) | Compact (interdigitated) |
| Frequency range | 1–30 GHz | 1–40+ GHz |
| Phase accuracy | ±5° | ±2–3° |
RF View Hybrid Analysis: Load hybrid .s4p, extract S21, S31 paths, overlay their S21 dB traces (should coincide) and phase traces (90° offset). Phase delta marker quantifies quadrature error. Free on Android.