Microwave Power Amplifier Linearization Feedforward Technology

With the development of WCDMA-leaded 3 Generation mobile communication technology, the technology of linear modulation has been applied more and more widely, but there comes serious intermodulation when the envelope-varied modulation signal is amplified by non-linear RF power amplifier. So more critical requirements have been put forward for linear index of the PA in the base-station transmitter, if it is used in broadband modulation, multicarrier condition, and meanwhile to decrease the adjacent channel power rejection(ACPR) in order to get lower the Bit-Error Rate(BER).The pre-distortion circuit topology is simpler than the feed-forward one, but it can't meet the requirement of the linearity of the PA in transmitter; but the feed-forward system can achieve the performance of closed-loop system, and broadband characteristic, stability behavior. The application of feed-forward system is widespread in based-station and other communication equipment.First of all, the non-linear characteristic of PA has been analysed in this paper, include: amplitude nonlinear, phase nonlinear, and other nonlinear index. Furthermore, the comparative analysis on the linearization techniques has been set, such as power back, negertive feedback, pre-distortion, feed-foreward, and the technology of combination with the adaptive algorithm. And then, based on the theoretical study, the paper makes an overall scheme proof, according to the standards of 3GPP. Lastly, the index allocation, circuit structure, and components needed, have been established.According to the target index, MW4IC2230 and MRF6S21140H are selected to cascad to realize main PA, the Agilent ADS simulation was used to design the input/output match network, in order to improve the small signal gain, return loss, P1dB, and intermodulation part, and assess the needed index of the passive component in the system----such as assistant amplifier, error amplifier, divider, directional coupler, 3dB branch-line bridge, attenuator, phase shifter and etc. Then use ADS to testify the cascade circuit module. The simulation results indicate that: the inermodulated distortion of main amplifier has been restrained well, and proves that the needed index can be satisfied by this scheme.The Agilent ADS simulation confirmed that the scheme is able to have good linear performance under the double-tune input, the third-order intermodulation had remarkable improvement. The entire forward feed power amplifier system has been realized, and the debugging and test have also been made.The test result indicated that feed-forward power amplifier double-tune test (12.5W output), the third-order intermodulation coefficient is smaller than -44dBc, may make 21dB improvement on third-order intermodulation of the main power amplifier's coefficient, and the estimated ACPR is higher than -47dBc. The design of this paper achieves good performance, theoretically satisfies 3GPP standards to the WCDMA based-station transmitter.