When Conversion Is Needed
Many RF devices have differential (balanced) ports: dipole antennas, balanced mixers, push-pull amplifiers, differential ADC inputs. These must be interfaced to single-ended 50Ω coaxial systems (VNA, test equipment, PCB microstrip) through a balun.
Types of Differential-to-Single-Ended Conversion
| Method | BW | IL | Best For |
|---|---|---|---|
| Marchand balun (coupled TL) | 3:1 to 10:1 | <0.5 dB | Microwave, broadband |
| Wound transformer balun | Very wide (DC) | <0.2 dB | HF/VHF to 500 MHz |
| LTCC integrated balun | Narrow (20–30%) | 0.5–1.5 dB | Mobile chip integration |
| Rat-race hybrid (180°) | ~20% | 3 dB (splits power) | Microwave balanced mixer |
Measuring Balun Conversion Performance
Load balun .s3p: Port 1 = SE input (50Ω coaxial) Port 2 = Balanced output+ (100Ω) Port 3 = Balanced output− (100Ω) Extract port pairs → two .s2p files: SE→Bal+: s21_bal_pos.s2p SE→Bal−: s21_bal_neg.s2p Amplitude balance check: Load both in RF View → overlay S21 magnitude → should coincide ±0.5 dB Phase balance check: Switch to Phase view → delta marker between both traces Should read ΔPhase = −180° ± 5° (differential output)
Common-Mode Rejection Ratio (CMRR)
CMRR = S_differential / S_common_mode [in dB] Good balun: CMRR > 20 dB across operating bandwidth This means: differential signal 20+ dB above common-mode leakage Measured using mixed-mode S-parameters: CMRR = Sdd21 − Scc21 [in dB] (requires 4-port VNA measurement or special balun test fixture)
RF View Balun Analysis: Load .s3p, extract SE→Bal+ and SE→Bal− as separate .s2p files, overlay in RF View for amplitude/phase balance check. Phase delta marker quantifies 180° balance error. Free on Android.