Why Fixture Correction Matters
Any PCB or waveguide fixture between the VNA calibration plane and the DUT pads adds parasitic effects — connector loss, launch inductance, stub capacitance, and transmission line delay. Fixture correction mathematically removes these to reveal the true DUT response.
Correction Method Selection Guide
| Method | Required Structures | Accuracy | Complexity |
|---|---|---|---|
| Port extension | None (just electrical delay value) | Low (ignores parasitic) | Trivial |
| Short-open de-embedding | Open + Short dummy structures | Good for lumped parasitics | Medium |
| ABCD cascade | Characterized fixture S2P | Good for distributed fixture | Medium |
| TRL calibration | Thru + Reflect + Line on PCB | Best (moves cal plane to DUT) | High |
Port Extension (Simplest)
Adds electrical delay to remove cable/connector phase shift: S11_corrected(f) = S11_raw × exp(−j4πfτ) τ = electrical delay = physical length / v_p For 10mm SMA to DUT on RO4003C (vp=0.60c): τ = 0.010 / (0.60 × 3×10⁸) = 55.6 ps Accuracy: removes phase shift but NOT amplitude effects Good for: initial rough de-embedding of cable delay
RF View: Load fixture-corrected .s2p files for accurate analysis. Compare raw vs corrected measurements to verify the de-embedding removed the fixture contribution. Free on Android.