Adaptive RF Cartesian Predistorter Based on the Low Frequency Even Order IM Terms Youngoo Yang, Young Yun Woo, and Bumman Kim Department of Electronic and Electrical Engineering and Microwave Application Research Center, POSTECH, Korea Abstract — A new adaptive predistortion method of high power class AB amplifier is presented. The 2 nd order harmonics(f2-f1) is generated in the properly filtered drain bias circuits of small amplifier with proper drive level to have relatively high 2 nd order and low 4 th order IM components. The 4 th order component(2f2-2f1) is generated by multiplying the 2 nd order component. These low frequency even harmonics modulates the main signal in I/Q channel form to generate the 3 rd and 5 th IM components. This PD is very compact and has low insertion loss and capability to independently control the amplitudes and phases of the 3 rd and 5 th IM components, which are very important to achieve higher performance without having local minima in the adaptive control. A 2.15GHz high power amplifier has been linearized for the 4.5MHz broad band CDMA signal. The results prove that this adaptive predistorter could achieve a drastic improvement of ACP and has excellent adaptivity. I. INTRODUCTION Predistortion technique has been very popular for the SCPA(Single Carrier Power Amplifier) of the base station and relay of mobile communication systems since it has relatively simple topology, good performance, and low cost [1]-[4]. For the base station of the 3G systems, broad band high power amplifiers with highly linear characteristics are demanded [5], and predistorters focusing on the cancellation of the 3 rd order distortion term or having narrow band characteristics would not be suitable for the 3G systems. In addition to the broad band and linear characteristics, the adaptive control of predistorters is very important because of their open loop structures. Adaptive control should track long term variations of the amplifier due to the temperature variation, device drift, and etc. as well as short term variations caused by AM-AM and AM-PM to maintain the optimum nonlinearity cancellation. In this paper, we present a new adaptive predistortion method using Cartesian type diode modulator to generate IM3 using the low frequency 2 nd IM component(f2-f1 for two-tone case) and IM5 using 4 th order component(2f2-2f1). It’s well known feature in Volterra series analysis that the 3 rd order IM components(2f2-f1, 2f1-f2) can be generated by modulating the fundamental components(f2, f1) and 2 nd order component(f2- f1) and the 5 th order IM terms(3f2-2f1, 3f1-2f2) by modulating the fundamental components and 4 th order component(2f2-2f1) [6]-[8]. This predistorter has the freedom to control the amplitudes and phases of the IM3 and IM5 independently, which means that it can position the IM3 and IM5 in any point of the Cartesian coordinate by adjusting only 4 parameters: gains of the in-phase and quadrature low frequency 2 nd and 4 th order IM components. DSP is used to adaptively control the long term gain and phase variations of the overall RF path and to optimize the ACP cancellation. We have linearized a 2.15GHz class AB high power amplifier with 120W PEP. The two-tone linearization results and adaptive control results of the cancellation of ACP will be presented. A broad band CDMA signal with a chip rate of 4.096Mcps and peak to average ratio of 11dB is applied. The results will be compared with both no predistorter and no adaptive control cases, respectively. II. SYSTEM DESCRIPTION Fig. 1 shows the system schematic diagram including adaptation circuits. The adaptation circuits are aimed at the compensation of both the long term fluctuations of gain and phase due to the temperature, aging, average input power level, and etc. and the short term fluctuations due to the gain(AM-AM) and phase(AM-PM) variation by canceling the IM3 and IM5 components. 0-7803-6540-2/01/$10.00 (C) 2001 IEEE