A vector-locked loop for power amplifier linearization

This paper describes a power amplifier linearization technique called the vector-locked loop (VLL), where the amplitude- and phase-modulated output RF signal is composed of two constant-envelope phase-modulated signals. This enables the use of non-linear high-efficient PAs in the chain. The theoretical background and limitations behind the VLL concept are discussed in the paper. A prototype has been designed for the 2.11-2.17-GHz band using off-the-shelf components. Experimental results show that the concept itself is functional and complex modulations can be reconstructed. The real-time feedback operation and the utilized components limit the achievable linearization bandwidth to less than 1 MHz and allow relatively shallow modulations only, such as OQPSK or PI/4-DQPSK. Problems related to the realization can be solved by full-custom component design if modest bandwidths are targeted.