How do I de-embed fixture and cable effects from my S-parameter measurements?
De-embedding and Fixture Removal
Fixture de-embedding is essential when the DUT cannot be directly connected to the VNA ports, which is the case for: PCB-mounted components, on-wafer devices (MMICs), packaged components in test fixtures, and non-coaxial devices requiring transitions.
| Parameter | SOLT Cal | TRL Cal | eCal |
|---|---|---|---|
| Accuracy | Good | Excellent | Good-very good |
| Standards Needed | 4 (S,O,L,T) | 3 (T,R,L) | 1 (module) |
| Bandwidth | Broadband | Band-limited | Broadband |
| Setup Time | 5-10 min | 10-20 min | 1-2 min |
| Best For | Coaxial, general | On-wafer, waveguide | Production, speed |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
When should I use TRL instead of de-embedding?
TRL calibration is preferred when: (1) You can fabricate the calibration standards on the same substrate as the DUT (PCB or MMIC wafer). (2) You need the highest possible accuracy (TRL does not rely on lumped standards like SOLT, which have imperfect models at high frequencies). (3) You are characterizing many DUTs on the same substrate (calibrate once, measure many). De-embedding (e.g., 2x-Thru) is preferred when: (1) You cannot fabricate calibration standards on the DUT substrate. (2) You are measuring packaged devices in a test fixture. (3) You want a quick method with minimal additional standards (just one 2x-Thru measurement).
How do I validate that de-embedding worked correctly?
Measure a known reference device using the same fixture and de-embedding procedure. Compare the de-embedded result to: (1) The manufacturer datasheet specifications. (2) A measurement of the same device in a different fixture (independent validation). (3) Simulation results (if a validated model exists). Common validation devices: a precision attenuator (known S21 and S11), a short length of transmission line (known impedance and loss), or a calibration kit verification standard. Also check for physical reasonableness: (a) A passive device should have |S21| ≤ 1 (0 dB) at all frequencies. (b) The S-parameters should be smooth (no sudden jumps or spikes). (c) Reciprocity should hold: S21 ≈ S12 for a passive device. Violations of these checks indicate de-embedding errors.
Can I de-embed using port extension only?
Port extension adds a lossless delay, which is equivalent to physically moving the reference plane by a known distance. It corrects only for the phase shift of the fixture. Port extension is adequate when: the fixture is short (< lambda/4), the fixture loss is negligible (< 0.1 dB), and the fixture is well-matched (RL > 25 dB). For lossy or mismatched fixtures: port extension introduces calibration errors because it does not account for fixture loss or internal reflections. Use full S-parameter de-embedding or TRL calibration instead. A quick check: perform port extension and then measure a SHORT at the DUT reference plane. If the return loss is > 0.1 dB or the phase is not exactly 180° ± 2°: the fixture has loss or mismatch that port extension cannot correct.