A Comprehensive Analysis And Simulation Of IMD In RF Power Amplifiers

With the rapid development of wireless communication technology, human in order to make more efficient use of frequency spectrum and achieve higher data transmission rate, widely use linear modulation technology, such as QPSK and QAM and so on. These linear modulation signal with changing envelop, through non-linear RF power amplifier will produce intermodulation distortion, causing serious inter-symbol interference and adjacent-channel interference. The high linear power amplifier has to be used to guarantee communication quality. Unfortunately high linear power amplifier usually is inefficient. For solving this problem, we need to understand the non-linear characteristics of RF power amplifier, and not simply add external circuit to sacrifice efficiency for linear. On the other side, in order to guarantee the quality of communication, these are stringent figures of non-linear distortion in various wireless systems. Quantitative description of these figures is also very important to guide nonlinear system design.This paper firstly present the nonlinear analysis basis of RF power amplifier, and detailed discussion the basic concepts ,theory and technology of nonlinear analysis, as well as summarize and compare various nonlinear technique. In order to introduce the harmonic balance technique, we use ADS simulation as an example. Then we introduce the intermodulation distortion characterization technology, and analysis some nonlinear characteristic excited with a one tone or multi-tone, such as AM-AM, harmonic distortion, intermodulation, adjacent-channel power ratio. Secondly, we study the input voltage/output current's transfer function of MOSFET power amplifier, and adopt Taylor series and piecewise linear function combing method, with which we derive third-order intermodulation distortion's analytical expression of two-tone signal exciting. In order to verify its correctness, we compare its results with normal third-order power series analysis results and ADS simulation results. Lastly based on spectrum density formula of CDMA signal passing through third-order power series model of power amplifier, we derive simplified relationship between the intermodulation distortion ration on tone test and the ACPR in CDMA signal test, and prove the validity with practical measurement data and experimental data derived using ADS.