Electrical Engineering
      and Computer Sciences

Electrical Engineering and Computer Sciences


UC Berkeley


2008 Research Summary

Pulse-Density Modulation for RF Transmitter Applications

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Jason Thaine Stauth and Seth R. Sanders

Conventional wireless transmitters use linear (class-A, AB) power amplifiers to perform amplitude modulation for high-datarate 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
Figure 1: Die photo of pulse-density modulated transmitter

A. Jerng and C. G. Sodini, "A Wideband Delta-Sigma Digital-RF Modulator for High Data Rate Transmitters," IEEE Journal of Solid State Circuits, Vol. 42, August 2007, pp. 1710-1722.
F. Wang, D. Kimball, D. Y. Lie, P. Asbeck, and L. E. Larson, "A Monolithic High-Efficiency 2.4 GHz 20 dBm SiGe BiCMOS Envelope-Tracking OFDM Power Amplifier," IEEE Journal of Solid State Circuits, Vol. 42, June 2007, pp. 1271-1281.
J. T. Stauth and S. R. Sanders, "Optimum Biasing for Parallel Hybrid Switching-Linear Regulators," IEEE Transactions on Power Electronics, Vol. 22, September 2007, pp. 1978-1985.
J. T. Stauth and S. R. Sanders, "Power Supply Rejection for Radio Frequency Amplifiers: Theory and Measurements," IEEE Transactions on Microwave Theory and Techniques, Vol. 55, October 2007.