Pulse-Density Modulation for RF Transmitter Applications
Jason Thaine Stauth and Seth R. Sanders
Conventional wireless transmitters use linear (class-A, AB) power amplifiers to perform amplitude modulation for high-data rate standards such as 802.11a/g/n that require high spectral efficiency. In these applications, the power amplifier (PA) typically operates with average efficiency in the range of 5% due to strict linearity requirements [1-4]. This work is focused on new transmitter architectures that use pulse-density modulation to perform linear amplitude modulation of the RF carrier with a nonlinear power amplifier (PA). Advantages of this approach include high linearity for wideband standards, high efficiency across the range of output power, and a simplified pure-digital implementation resulting in small die area. Our approach uses deterministic (programmed) pulse density modulation operating at the RF carrier frequency, combined with baseband ∆ modulation operating at baseband frequencies. The multi-stage approach shapes quantization noise away from the signal band allowing effective reconstruction of high peak-average power ratio (PAPR) waveforms with minimal digital processing power. The switching PA achieves output power levels comparable to WLAN or Bluetooth with average efficiency approaching 20% for the entire transmitter including power consumption for the PA, PA driver, and digital processing circuitry.
Figure 1: Die photo of pulse-density modulated transmitter
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