What is the effect of amplifier nonlinearity on adjacent channel leakage ratio?
ACLR and PA Nonlinearity
ACLR is the primary spectral emission metric for modern wireless transmitters, directly impacting the coexistence of adjacent-channel users.
| 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
Frequently Asked Questions
How do I measure ACLR?
Use a spectrum analyzer or VSA: set the channel bandwidth to match the signal (e.g., 20 MHz for LTE). Measure the integrated power in the desired channel. Measure the integrated power in the adjacent channel (offset by the channel spacing). ACLR = P_desired - P_adjacent (in dB). Ensure the spectrum analyzer dynamic range exceeds the ACLR requirement (use low noise floor settings and appropriate attenuation).
Can I improve ACLR without DPD?
Yes, through: better PA topology (Doherty, envelope tracking: these improve efficiency at back-off without degrading linearity), higher OIP3 PA (allows operating closer to P1dB while meeting ACLR), output filtering (a bandpass filter after the PA removes out-of-band energy, improving ACLR at the expense of filter insertion loss), and crest factor reduction (CFR: reduces the signal PAPR, allowing higher average power without compression).
What about EVM and ACLR simultaneously?
Both EVM and ACLR degrade with compression, but at different rates. ACLR typically becomes the binding constraint before EVM for lower-order modulations (QPSK, 16-QAM). For higher-order modulations (64-QAM, 256-QAM): EVM may be the binding constraint. The PA designer must meet both simultaneously: operating point selection balances EVM and ACLR.