How do I characterize the broadband linearity of an amplifier using modulated stimulus signals?
Modulated Signal Characterization of PA Linearity
Modulated signal testing has largely replaced two-tone testing as the primary linearity characterization method for modern PAs because two-tone tests do not capture bandwidth-dependent memory effects, PAPR-related clipping, and the statistical distribution of distortion that actually determines system performance.
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
- 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
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
Why not just use two-tone tests?
Two-tone tests measure IMD3 and OIP3, which are useful for narrowband linearity and cascade analysis. However, they do not capture: memory effects (two tones have zero bandwidth), PAPR-related compression (two tones have only 3 dB PAPR vs. 10-12 dB for OFDM), spectral regrowth shape (two tones produce discrete IMD products, not the continuous spectral regrowth of real signals), and EVM (two tones cannot be demodulated as a data signal). Modulated signal testing reveals all these effects and directly measures the performance metrics (ACLR, EVM) specified by wireless standards.
Which modulated signal should I use for characterization?
Use the actual signal format for the intended application: 5G NR for base station PAs, Wi-Fi 6E for WLAN PAs, DVB-S2X for satellite PAs. If the PA is multi-standard, test with the most demanding signal (highest bandwidth, highest modulation order). For general characterization (before the application is finalized), a 100 MHz OFDM signal with 256-QAM and approximately 12 dB PAPR provides a comprehensive test that exercises most PA nonlinearities.
What equipment do I need for modulated signal testing?
A vector signal generator (VSG, e.g., Keysight E8267D, R&S SMW200A) for modulated stimulus, a spectrum analyzer or VSA (e.g., Keysight N9040B, R&S FSW) for ACLR and EVM measurement, a power supply and bias tee for the PA, and attenuators/couplers for signal conditioning. For I/Q waveform capture and behavioral model extraction: a high-speed digitizer or the VSA's waveform capture mode, plus MATLAB or the instrument's analysis software. Total equipment cost: $100K-500K for a complete setup.