How do I perform a source pull measurement to optimize the noise figure of an LNA?
Source-Pull Noise Figure Optimization
Source-pull measurement is the definitive technique for optimizing and characterizing the noise performance of an LNA. The noise parameters extracted from source-pull data are used by circuit designers to design the optimal input matching network for minimum noise figure.
| 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
Why can't I just measure NF at 50 ohms?
The noise figure measured at 50 ohms (NF_50) is not the minimum achievable noise figure. The LNA's optimum source impedance (Gamma_opt) is typically not 50 ohms. If Gamma_opt is far from 50 ohms: NF_50 can be significantly higher than NF_min (by 0.5-2 dB). Source-pull reveals the true NF_min and Gamma_opt, allowing the designer to design an input matching network that transforms the antenna impedance to Gamma_opt, achieving the best possible noise performance.
How many impedance states do I need?
For accurate noise parameter extraction: minimum 10-20 states distributed across the Smith chart, with good coverage near Gamma_opt. For high-quality noise circle plots: 50-200 states. The states should cover a range of magnitudes and phases centered on the expected Gamma_opt region. Most automated source-pull systems sweep 100-300 states in 10-30 minutes.
Can I do source-pull on an on-wafer device?
Yes. On-wafer source-pull uses the same tuner connected through wafer probes. The tuner's impedance range at the DUT reference plane is reduced by the probe loss (each dB of probe loss reduces the effective tuner coverage). At mmW frequencies: integrated on-wafer tuners or pre-matching networks on the probe substrate can extend the effective impedance range. On-wafer noise source-pull is standard practice for MMIC LNA design.