APPLICATION NOTE 5923

Linearizing Microwave Transmission Power Amplifiers: Using SC1894 RFPAL to Improve ACP, EVM , and Efficiency


Abstract: Digital Microwave Transmission systems require low Bit Error Rates (BER) and must not transmit excessive Adjacent Channel Power (ACP) which would interfere with other wireless services. With respect to transmitter signal fidelity, BER is directly affected by the modulated waveform Error Vector Magnitude (EVM). Reducing the modulated waveform EVM is a primary goal in all transmitter design and the final stage Power Amplifier (PA) can degrade the EVM when operating near maximum power, creating intermodulation products (IMD). These IMD products are the dominant components of ACP. A traditional solution to avoid the IMD problem is to back off (reduce) the power until the IMD distortion is at an acceptable level. Using backoff results in an inefficient transmitter design and, depending on the waveform, one may not achieve the desired linearity specification regardless of how far the PA is backed off.This document is an introduction to linearizing a microwave transmission system PA using the SC1894 RFPAL. It includes basic system design considerations to implement a linearized microwave amplifier and the effects of Automatic Transmit Power Control (ATPC) on RFPAL performance.