Spectrum Modeling of RF Amplifiers Based on Intercept Points with Measurement Improvements

Abstract

Orthogonal frequency-division multiplexing has been developed into a popular modulation scheme for wireless communication systems, used in applications such as LTE and 5G. In wireless communication system, spectrum regrowth caused by RF amplifiers will generate distortions to both passband in use and adjacent channels such that the transmission quality will be degraded. The study of this dissertation aims to predict the spectrum regrowth for OFDM based signals at the output of a RF amplifier due to the nonlinearity. Based on Taylor polynomial coefficients, a power spectrum expression for amplified OFDM signals in terms of intercept points up to nth-order is derived. This model is useful to RF engineers in choosing the testing RF amplifiers with appropriate specifications, such as intercept points and gain, to meet the requirements of wireless standards. The proposed model is confirmed by the measurement results. For predicting spectrum regrowth accurately with the proposed method, the key parameter, intercept points, need to be correctly determined. IP3 is an important figure of merit for describing the nonlinearity of RF amplifiers. It is available from the datasheet and calculated from the interception between the line of the output power at fundamental frequencies versus the input power and the line of the power of the 3rd-order intermodulation product conventionally measured from a two-tone test. However, the measured output power at fundamental frequency is not only from the fundamental tones, but also affected by 3rd-order intermodulation products. As the input power further increases, the higher-order intermodulation becomes visible, which might affect the measurements at the frequencies of fundamental tones and of lower-order intermodulation products. Thus, a similar issue arises for the intercept point measurements beyond 3rd-order. In this dissertation, starting with a two-tone test, the impacts from 3rdorder IM products to fundamental tones are analyzed. The measured IP3 is thus corrected by removing the 3rd-order IM impacts at fundamental frequencies. The measurement improvements on intercept points of different orders using higher-order IM measurements are then derived. As higher-order IM products are usually far less than 3rd-order IM products, higher-order intercept points are of less importance. Thus, in the experimental measurements, the IP3 comparison between the result of the conventional method and that of the corrected method correction is explained and analyzed, which verifies the method is necessary. By using the improved IP3 from measurements, the corresponding adjacent channel power ratio can be calculated accurately, which is useful to RF amplifier design