What is the error introduced by impedance mismatch between the DUT and the power sensor?
Mismatch Error in Power Measurement
Mismatch is the dominant error source in RF power measurement, especially at mmWave frequencies where connector return loss is worse than at lower frequencies.
| 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
Frequently Asked Questions
Is mismatch error random or systematic?
Mismatch error is systematic (it is deterministic if the reflection coefficients are known). However: in practice, the phase of the mismatch is often unknown (it changes with cable length, connector torque, and temperature). When the phase is unknown: the error is treated as a bounded systematic uncertainty (the magnitude is known but the sign is unknown). This is different from random noise (which has a Gaussian distribution). In uncertainty analysis: mismatch is typically included as a U-shaped distribution (the error is more likely to be near the extremes than near zero).
How does mismatch affect my link budget?
In a communication link: mismatch between components causes reflected power that does not reach the receiver. The mismatch loss: ML = -10 × log10(1 - |Γ|²) dB. For Γ = 0.1 (20 dB RL): ML = 0.04 dB (negligible). For Γ = 0.2 (14 dB RL): ML = 0.18 dB. For Γ = 0.3 (10.5 dB RL): ML = 0.41 dB. For Γ = 0.5 (6 dB RL): ML = 1.25 dB. In a link budget: mismatch loss is included as a negative term (loss). For each interface with imperfect matching: the total mismatch loss is the sum of the individual mismatch losses. At mmWave: connector transitions often have Γ = 0.1-0.2, contributing 0.1-0.5 dB of mismatch loss per transition. A system with 4 transitions may lose 0.4-2 dB total.
What is effective efficiency in a power sensor?
Effective efficiency (η_e) combines cable/mount loss and mismatch into a single sensor characteristic: η_e = (power absorbed by the sensing element) / (power available from the source). It differs from the calibration factor (CF) by including the mismatch: CF = η_e × M. For practical use: the CF provided by the sensor manufacturer incorporates η_e at a defined reference impedance (50 Ω). When the source impedance differs from 50 Ω: the mismatch factor M changes but η_e remains constant. So: applying both CF and mismatch correction separately is the correct approach.