This document is an introduction to linearizing a microwave transmission-system PA using Maxim's SC1894 RF PA Linearizer (RFPAL) from 4GHz to 80GHz. It includes basic system design considerations to implement a linearized microwave amplifier and the effects of automatic transmit power control (ATPC) on the SC1894 performance.
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 cannot achieve the desired linearity specification regardless of how far the PA is backed off.
Maxim's SC1894 can improve the PA linearity and minimize EVM, IMD, and ACP levels without sacrificing the PA output power. Furthermore, predistortion linearization allows a PA to operate at significantly higher efficiency compared to another PA operating in backoff with similar output power levels. In the case of GaN devices that can be inherently nonlinear, predistortion is often the only available method to achieve the required EVM and ACP levels. By enabling operation at maximum PA output power while minimizing EVM and ACP, the SC1894 devices provide the system with lower BER, better efficiency, and lower system and operating costs.