Why Calibration Is Essential
A raw VNA measurement includes systematic errors: directivity error (coupler leakage), source match error (non-ideal port impedance), and reflection/transmission tracking errors. Calibration characterizes these errors and removes them from all subsequent measurements.
12-Term Error Model
For a 2-port VNA, 12 systematic error terms: Per port: Ed (directivity), Er (reflection tracking), Es (source match) Transmission: Et_fwd, Et_rev (forward/reverse tracking) Isolation: leakage terms (usually small at RF) Calibration solves for all 12 error terms using known standards.
SOLT Procedure
- Short on Port 1 → characterizes reflection tracking (Er) and source match (Es)
- Open on Port 1 → additional Er/Es information (different phase)
- Load on Port 1 → characterizes directivity (Ed)
- Short, Open, Load on Port 2 → port 2 error terms
- Thru connection → transmission tracking (Et), isolation
Calibration Quality Verification
| Check | Expected Result | Problem If Wrong |
|---|---|---|
| 50Ω Load after cal | S11 < −40 dB | Dirty connector or bad Load standard |
| Short after cal | S11 = 0 dB ±0.1 dB | Reflection tracking error |
| Thru after cal | S21 = 0 dB ±0.05 dB | Transmission tracking error |
| S11 ripple vs freq | <±0.2 dB | Cable flexure between cal and measurement |
RF View VNA Companion: After VNA calibration and measurement, export .s2p and load in RF View for full analysis: Smith chart, matching design, simulation, Monte Carlo. Free on Android.