What is digital predistortion and how does it improve the linearity of a power amplifier?
Digital Predistortion
DPD bandwidth expansion: if the transmitted signal occupies 100 MHz, the DPD must process 300-500 MHz to capture the third- and fifth-order intermodulation products. This requires: DAC sample rate > 1 GSPS, observation receiver ADC bandwidth > 500 MHz, and FPGA/ASIC processing at the expanded sample rate. The computational cost of DPD increases with the model order and memory depth. A GMP model with 7th-order nonlinearity and 5 memory taps requires approximately 100-200 complex multiplications per sample.
| Parameter | Pipeline ADC | SAR ADC | Sigma-Delta ADC |
|---|---|---|---|
| Sample Rate | 100 MS/s - 10 GS/s | 1-100 MS/s | 10 kS/s - 50 MS/s |
| Resolution | 8-14 bits | 10-20 bits | 16-24 bits |
| Latency | Several clock cycles | 1 conversion cycle | Many cycles (decimation) |
| Power | High | Low-moderate | Low |
| Typical RF Use | Direct sampling, DPD | Control, monitoring | Audio, baseband |
- 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 much efficiency improvement?
Without DPD: a class AB PA must be backed off 6-10 dB from saturation for an OFDM signal (PAR = 8-12 dB), achieving only 5-15% efficiency. With DPD: the same PA can operate 3-5 dB closer to saturation while meeting ACLR requirements, improving efficiency to 20-35%. Doherty PA + DPD: 35-50% efficiency.
What feedback bandwidth do I need?
The observation receiver must capture up to the 5th-order distortion products: BW_obs = 5 × BW_signal. For a 100 MHz signal: 500 MHz observation bandwidth. Some DPD algorithms can work with 3× bandwidth for 3rd-order-only correction. Higher bandwidth = better linearization but more complex hardware.