Crest Factor
Understanding Crest Factor
Crest factor determines how much headroom a power amplifier needs to handle a signal without clipping. Higher crest factor = more backoff required = lower efficiency. This is the fundamental challenge of amplifying modern modulated signals.
Crest Factor Examples
| Signal | Crest Factor (dB) |
|---|---|
| CW (sine wave) | 3.0 |
| Two-tone | 6.0 |
| QPSK (filtered) | 3.5-5.0 |
| CDMA | 8-10 |
| LTE OFDM | 8-12 |
| 5G NR OFDM | 9-12 |
| Gaussian noise | 10-14 |
Frequently Asked Questions
What is crest factor?
Crest factor is the peak-to-RMS amplitude ratio. CF^2 = PAPR. A sine wave: 3 dB. OFDM: 10-12 dB. Higher crest factor requires more PA headroom and reduces efficiency.
How does crest factor affect PA design?
The PA must be linear up to the peak power level (CF above average). For OFDM with 10 dB CF, a PA rated at 40 dBm peak is operating at 30 dBm average. PA efficiency at 30 dBm is much lower than at 40 dBm.
Can crest factor be reduced?
CFR (Crest Factor Reduction) algorithms clip or scale signal peaks to reduce PAPR by 2-4 dB. This allows the PA to operate closer to compression. Some EVM degradation results. CFR + DPD is the standard approach for efficient OFDM amplification.