How do I measure the AM-AM and AM-PM distortion of a power amplifier?
Power Amplifier AM-AM and AM-PM Characterization
AM-AM and AM-PM distortion are the primary characterizations of a power amplifier's static nonlinearity. They completely describe the amplifier's instantaneous (memoryless) nonlinear transfer function and are the basis for behavioral modeling and digital pre-distortion.
| 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 does AM-PM matter?
AM-PM conversion causes spectral regrowth (adjacent channel leakage) and EVM degradation in modulated signals, similar to AM-AM compression. For some modulation formats (e.g., FM, GMSK): AM-PM is the dominant source of distortion because the signal has constant amplitude but varies in phase. For QAM signals: AM-PM rotates the constellation points as the signal amplitude fluctuates, creating errors. DPD must correct both AM-AM and AM-PM to achieve the best linearity.
What AM-PM specification is typical?
For linear applications (LTE, 5G base stations): AM-PM < 2-3 degrees over the operating power range. For saturated applications (radar, FM): AM-PM of 5-15 degrees is common and may not be a concern if the signal is constant-envelope. For DPD-linearized PAs: the uncorrected AM-PM can be 5-10 degrees, but the DPD corrects it to < 1 degree at the system output.
Do AM-AM and AM-PM change with frequency?
Yes. The gain compression and phase shift vary across the PA's operating bandwidth. A PA that is well-matched at band center may have different compression characteristics at band edges (due to the frequency-dependent matching network response). For wideband PAs (> 10% bandwidth): AM-AM and AM-PM should be measured at multiple frequencies across the band to capture the frequency-dependent behavior. DPD algorithms must model this frequency dependence for wideband correction.