Research On RF Linear Power Amplifier

As one of the strongest nonlinear devices, power amplifiers affect the communication systems directly, whose nonlinear distortions result in spectrum expansion and adjacent channel interference, then worsen the bit error rate. Furthermore because of the distortion, system data transmission rate is declined resulting in the drop of system capacity, or the system channel frequency space is expanded resulting in the drop of spectrum utilization. The linearity techniques are the only measures to overcome the effects of PA nonlinear, which are techniques using auxiliary circuits to eliminate those distortions produced by PA nonlinear in order to maintain the system capacity and avoid those interferences. The design of a linear PA includes three aspects, quantitative analysis of the PA nonlinear effects on communication system in order to educe the necessary PA nonlinear index of system; the PA linear scheme and its realization; PA auxiliary circuits design including balun ,filter etc. How to describe quantitatively the PA nonlinear effects on communication system and how to design a linear PA are becoming the research hotspots. This paper consists of three research works, PA nonlinear analysis; feedback predistorsion linear PA design; PA auxiliary circuit-microstrip balun and DGS microstrip filter-design. The main contributions are shown below: 1)puts forward a estimate method of PA nonlinear distortion effects on CDMA BER and the necessary power backup value of PAs. It is well known that PA nonlinear distortions worsen the communication system capacity, but the relation of the PA nonlinear index and communication performance index is ambiguity. The PA nonlinear index for a communication system is put forward, but its origin is not clear. Based on the basic model of CDMA and the power series model of PA, a quantitative analysis method of PA nonlinear effects on CDMA system BER is put forward. Two-tone test is used to analyze the PA nonlinear characteristics, including IM3 and IP3. But the experiment and analysis results show that IM3 and IP3 cannot describe the PA nonlinear effects on CDMA system and the results of spectrum regeneration and its adjacent channel interference. ACPR can characterize the effects of PA nonlinear, and is a common index of CDMA system, but its relation with PA performance parameters (E.g. 1dB compress point power and IP3) is scarcity. Based on the basic model of CDMA and the power series model of PA, a formula to compute the ACPR of CDMA system by means of the PA power gain and 1dB compress point power or IP3 and the formula to compute the PA power backup value in order to meet the require of system are deduced,. 2)Puts forward and analyzes feedback predistortion linearization . Feedforward linearization technique has high nonlinear improvement but is complex and difficult to realize. Feedback technique is a closed loop system and has adaptive capacity, but the requirements of system delay time and gain are quite rigorous. From the point of linearization, feedback technique improves the nonlinear in cost of gain decline. Predistortion technique has moderate nonlinear improvement with simple structure, it is a key point how to get the predistortion signal characterized the PA nonlinear. A feedback predistortion PA linearization method is brought forward, which uses the difference signal of PA input signal and its output signal as the predistortion signal characterized the PA nonlinear well, fed back to PA input port. The feasibility of feedback predistortion technique and its improvement are analyzed by theoretical analysis,circuit simulations and experimental tests. Experimental results show that the feedback predistortion technique has simple structure and it is easy to produce and control the predistortion signal. A PA using Motorola MHL9838 device is fabricated and a two–tone test with 30dBm /tone is proceeded, as the insert loss is set to -8dB, the IM3 is improved by about 5dBm; as the insert loss is set to -2dB, the IM3 is improved by about 11dBm. The adaptive algorithm is discussed, and then an adaptive feedback predistortion scheme based on LMS algorithm is put forward. 3)Puts forward double-coupled line balun. Balun is a key component of push-pull PA, whose balance characteristics will affect PA's performance. To realize better balance performance, the transmission line shall have very high even-mode impedance, namely, the line width and its space shall be small which will result in large insert loss. So non-coplanar tight coupled transmission line shall be used. Based on the quasi-TEM transmission line, a microstrip balun with double coupled-line structure is put forward; an admittance formula is derived, which can determine the microstrip parameters quickly. This balun and its admittance formula are tested by ADS simulations and experiments. Furthermore, a method to realize such balun using common microstrip is discussed, viz. multi-coupled-line balun. Experiments show that a microstrip line with 89 ? odd-mode impedance and 42? even-mode impedance can realize better performances of 0.3dB amplitude unbalance,3o phase unbalance and more than 20% bandwidth. At last, a technique adding lumped capacitances at the ends of the coupled line to improve directivity of microstrip couplers is discussed, which can improve balun's characteristics. 4)Investigates the equivalent circuit of DGS microstrip and its utilization in filter design. Filter is one of the absolutely necessarily auxiliary circuits of PA, it is worthy to investigate how to design filter using its microstrip transmission line. DGS microstrip has band-block characteristics. The frequency characteristics of the microstrip line with DGS and the influences of dumbbell shape and the material permittivity are analyzed by meansof 3-D field analyses, its equivalent circuit and the method of equivalent parameters extraction are analyzed by means of circuit analyses. Then filter design using DGS microstrip is discussed. ADS simulations and experimental tests verify its validation and its usefulness in filter design.