What causes AM to PM conversion in an amplifier and how does it affect system performance?
AM-to-PM Conversion
AM-to-PM conversion is often overlooked compared to AM/AM compression, but it can be the dominant source of EVM degradation in some PA topologies.
| 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
Is AM/PM more important than AM/AM?
For most applications: AM/AM dominates. But for phase-sensitive signals (8-PSK in GSM/EDGE, constant-envelope modulations with phase information): AM/PM can be the binding constraint. For wideband signals (OFDM): both AM/AM and AM/PM contribute to EVM. The relative importance depends on the modulation format and the PA technology.
How do I measure AM/PM?
Method 1: VNA with power sweep: measure S21 phase vs input power. The phase change per dB is the AM/PM coefficient. Method 2: VSA measurement: transmit a known modulated signal, measure the output constellation, and extract the AM/PM characteristic from the angle vs amplitude relationship. Method 3: two-tone test: the phase asymmetry between the upper and lower IM3 products is related to the AM/PM.
Does AM/PM affect radar systems?
Yes. In pulse-Doppler radar: AM/PM in the transmit PA modulates the pulse phase based on the pulse amplitude (which varies across the pulse due to the pulse shape). This creates spurious spectral lines in the Doppler filter output, which can mask weak target returns. The AM/PM requirement for radar PAs: typically < 1°/dB across the pulse dynamic range.