Data Acquisition for Digital-Controlled Adaptive Analog Predistortion of a Power Amplifier

Data acquisition for low cost digital-controlled adaptive analog predistortion of a power amplifier is discussed. The predistorted amplifier has a RF input that is a UMTS signal comprising one or two adjacent carriers. The predistortion corrects for memoryless distortion. The data acquisition system measures the complex gain of the predistorted amplifier as a function of the input envelope, creating tables of statistical moments from which AM-AM and AM-PM curves and other information useful to an adaptive coefficient estimator can be extracted. A gain detector compares the RF input and output signals directly measuring the differential log-magnitude (DLM) and phase. A second detector measures the input envelope. The challenges associated with a low cost design include using a microcontroller with a slow ADC and limited memory, time alignment of the RF input and output signals at the gain detector, and time alignment of the sampled DLM, phase, and input envelope signals. The detector signals are sampled at a sub-Nyquist rate, significantly below the bandwidth of the input signal, exploiting the memoryless characterization of the amplifier distortion. Separate sample and hold (S/H) circuits are used for each detector signal, allowing a single ADC to be multiplexed. Offset timing between the S/H's is used to compensate for delay differences between the detectors. To reduce the sensitivity to the memoryless approximation and time misalignment at the gain detector, the derivative of the input envelope is also measured and samples associated with high values are rejected. This selective sampling process is shown to reduce the variance within each of the bins of the DLM and phase tables