Introduction

WCDMA and other liner modulation schemes allow higher data rates and multiple wireless connections per carrier, but cause a high peak-to-average ratio of the carrier signals. Consequently, the amplifier now has to be largely backed off to meet the limits for adjacent-channel leakage. Since the PA's (power amplifier's) efficiency decreases the more the PA is backed off, linearization techniques are applied to combine a maximum efficiency with a minimum of IM (intermodulation). This application note details different techniques for tuning the MAX2009/MAX2010 analog predistorters while optimizing the ICs' performance.

Basic description

Linear modulation schemes like WCDMA allow higher data rates and multiple wireless connections per carrier, but they also introduce a high peak-to-average ratio of the carrier signals. Unlike constant envelope-modulation schemes where PAs (power amplifiers) can be driven into compression, the amplifier now has to be largely backed off to meet the limits for adjacent-channel leakage. Since PA efficiency decreases the more the PA is backed off, linearization techniques are applied to combine a maximum efficiency with a minimum of IM (intermodulation).

Well-known linearization technologies, e.g., feed forward (FFW) and digital predistortion (DPD) are expensive and need considerable space. Consequently, a method is needed that works with few components and is easy to handle.

The MAX2009/MAX2010 analog RF predistorters, when compared to FFW or DPD, need very few external components, are easy to adjust, and give a considerable amount of linearization.

The MAX2009/MAX2010 rely on AM-AM and AM-PM curve correction at the RF frequencies to improve IM3 and ACPR performance. Internally, the chip measures the signal power and distorts the phase and gain predistortion as a function of the current signal's amplitude. Although AM-AM and AM-PM correction rely on memory-less circuits, Class AB amplifiers can still benefit from the negative distortion created by the Maxim® parts and significantly improve performance.

As with all linearization techniques, a good signal-clipping algorithm which reduces the peak-to-average ratio of the signals before the PA (without exceeding the EMV limits) is helpful for analog predistortion. Using the MAX2009/MAX2010 together with proper signal clipping is a good combination.

Optimal Performance of MAX2009/MAX2010 RF Predistorters by tuning