What is the relationship between ACLR and EVM in characterizing power amplifier linearity?
ACLR vs. EVM for PA Characterization
Both ACLR and EVM are mandatory specifications in 3GPP (LTE, 5G NR), IEEE 802.11 (Wi-Fi), and other wireless standards. Understanding their relationship helps designers optimize the PA operating point for the most efficient use of linearity budget.
| 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
Frequently Asked Questions
Which metric is more important, ACLR or EVM?
It depends on the modulation order. For low-order modulation (QPSK): ACLR is the binding constraint because QPSK has very relaxed EVM requirements (17.5%) but ACLR must still meet the -45 dBc spectrum mask. For high-order modulation (256-QAM): EVM is often the binding constraint because the constellation points are very close together and even small distortion causes errors. In practice, both must be met simultaneously, and the PA design must satisfy the more constraining specification.
How does DPD affect ACLR and EVM differently?
DPD typically improves ACLR by 15-25 dB (from -30 to -50 dBc) but improves EVM by only 5-15 dB (from -22 to -32 dB). This is because ACLR improvement requires only correcting the PA's AM-AM and AM-PM distortion, while EVM is also affected by noise, phase noise, I/Q imbalance, and other impairments that DPD cannot correct. After DPD, the residual EVM is dominated by these non-PA sources.
Can I measure ACLR and EVM simultaneously?
Yes. A vector signal analyzer (VSA) or high-performance spectrum analyzer with demodulation capability can measure both ACLR (from the power spectrum) and EVM (from the demodulated constellation) simultaneously on the same captured signal. This is the standard measurement approach for 5G NR base station testing. Keysight, R&S, and Anritsu all offer instruments with combined ACLR/EVM measurement capability.