AM-PM Conversion
Understanding AM-PM Conversion
AM-PM conversion is one of the most challenging distortion mechanisms because it creates correlated amplitude and phase errors. Even an amplifier with low AM-AM compression can have significant AM-PM, degrading modulation quality.
AM-PM Characteristics
- Small signal: AM-PM is near zero. Phase is independent of amplitude.
- Approaching compression: Phase begins to shift (typically negative = phase lagging).
- In compression: AM-PM is large. 3-10 degrees/dB typical near P1dB.
- GaN vs GaAs: GaN typically has lower AM-PM than GaAs at the same relative power level.
Impact on EVM
For 64-QAM, AM-PM of 3 degrees contributes about 1.5% EVM. For 256-QAM (EVM requirement < 3.5%), AM-PM must be well below 2 degrees. DPD corrects both AM-AM and AM-PM distortion.
Frequently Asked Questions
What is AM-PM conversion?
AM-PM is the phase change of an amplifier output as input amplitude varies. Near compression, the phase shifts with amplitude. It is measured in degrees/dB and contributes to EVM degradation in digital communications.
Why is AM-PM hard to correct?
AM-PM creates correlated phase errors that depend on instantaneous amplitude. While DPD can correct AM-PM, it requires accurate phase measurement and modeling. Memory effects (where AM-PM depends on signal history) make correction even more challenging.
How does AM-PM affect different modulations?
Constant-envelope modulations (analog FM, GMSK) are immune to AM-PM. High-order QAM and OFDM are very sensitive because both amplitude and phase carry information. 256-QAM requires AM-PM below ~1-2 degrees for acceptable EVM.