Overview

From APPLIED MICROWAVE & WIRELESS, VOL. 13, NO. 3, MARCH 2001

Active linearization, a critical technology for modern communications systems, describes the automatic reduction of intermodulation (IM) products generated by high-power linear amplifiers. In almost all cases, linearization is more desirable than backing off a Class A amplifier, which can significantly effect efficiency.

The first part of this article focused on DSP-based digital predistortion, i.e., the adaptation mechanism is based on assessing the difference between the desired modulation and the power amplifier s output, and then using this as the basis of a gain and phase adjustment to the signal before the amplifier. The second part of this article introduces the concept of adaptive RF predistortion and offers a detailed look at a design example that employs an adaptive work-function-based predistortion technique.

Adaptive RF predistortion, a process that employs an adaptive work function to optimize out-of-band IM performance, is another valuable technique. RF predistorters can handle larger bandwidths than DSP-based digital predistorters and are able to adjust continuously for changes in the power amplifier s characteristics due to temperature, channel switching, power supply variation or transistor degradation. The RF predistorter functions independently of the modulation scheme, just like digital predistorters.

RF Predistortion

Predistortion, the most common linearization technique, inserts a nonlinear module between the input signal and the power amplifier. This module generates IMD products that are precisely anti-phase with the IMD products from the power amplifier, which reduces...