欠采样在基带预失真功率放大器线性化的应用

从理论证实了欠采样在基带预失真中的可行性,将欠采样用在基带预失真系统上,降低了功率放大器失真信号的采样率而完整地保留了信号中的非线性信息.最后,利用欠采样理论构建了基带预失真功率放大器系统,得到了19 dB三阶交调失真的改善,与正常采样下(5倍输入信号Nyquist采样率)的三阶交调失真的改善度相比相当,从而降低了系统的复杂度及成本.     

防破音D类音频功放的设计与应用

随着多媒体便携设备的普及,音频功放已经成为音频部分的标准配置,D类功放以其高品质高效的特点得到了越来越广泛的应用。在便携产品中,音频功放由于输入音乐信号过大或者电源电压过低,会产生削顶失真。采用防破音技术,可以通过自动增益调节技术来提供一个完美的解决方案。文章介绍了常见的防破音技术,提出了一种改进的AGC(自动增益控制)技术在D类功放中的设计与应用。改进的AGC技术通过对PWM输出的采样来判断失真程度,依据失真程度用防破音电路产生的PWM波形来自动调节运放增益,实现最大功率的无失真输出。     

An adaptive predistorter for a power amplifier based on adjacentchannel emissions [mobile communications]

A slowly adapting predistorter is presented. The approach is to minimize the transmitter output power in spectral regions occupied only by intermodulation (IM) products. In this way, only a spot power measurement is required. This technique relies on the principle that the power amplifier's characteristics vary slowly with time. By monitoring the out-of-band power one can obtain an estimate for the distortion introduced by the power amplifier. Adaptation is accomplished by iterative adjustment of the predistorter parameters to minimize the IM power. For a polynomial predistorter, the authors analytically demonstrate that the IM power is a quadratic function of the coefficients. A variety of algorithms therefore apply. The authors present an analog static predistortion linearization circuit that uses the envelope of the baseband signal to generate the nonlinear functional used in predistorting the input signal. The improvement obtained with an amplitude-modulated input signal was 15 dB in the third- and 5 dB in the fifth-order intermodulation products. The IM improvement could be maintained with the use of a robust direct search algorithm     

Theoretical and experimental study of amplifier linearization basedon harmonic and baseband signal injection technique

This paper presents a novel linearization scheme for RF amplifiers based on simultaneous harmonic and baseband signal injection. In this method, second-order frequency components generated by predistortion circuits are fed to the input of the main amplifier to mix with the fundamental signal for third-order intermodulation distortion (IMD) cancellation. A general and rigorous analytical formulation of baseband, harmonic, and the proposed injection techniques is presented, and from these derived expressions, the optimum conditions for IMD suppression are developed. The result also reveals the practical limitation of the proposed method subject to gain and phase error associated with the RF and baseband circuitry. For comparison purposes, an amplifying system is constructed for the experimental investigation of second-order signal injection approach. Both two-tone and digitally modulated waveforms are employed in these measurements     

3G系统中AGC的FPGA设计实现

详细介绍了数字AGC的基本原理,3G接收机需要采用AGC电路处理输入的无线信号,从而给无线环路中的可变增益放大器或数控衰减器提供外部控制信号,使得无线链路输出基本恒定且与输入信号电平无关的信号给基带部分处理,同时在很宽的范围内保持线性。RF输入电压经IF放大后,检波器检测出该电压的包络。该包络电压经AGC处理后,产生增益可变器件的控制电压,从而减小IF的输入和增益。     

Linearisation of RF PA by even-order nonlinear baseband signal processed in digital domain

This paper represents, for the first time, the linearisation method of RF power amplifiers, which is based on using the second- and fourth-order nonlinear signals shaped by digital signal processing in a baseband domain. The linearisation signals are generated by the second- and fourth-order nonlinear modifications of the baseband signal in the proposed manners. The composite second- and fourth-order linearisation signals modulate carrier second harmonics, and the modulated signals are then driven to the gate and drain of the amplifier transistor. The linearisation effects of the proposed method are evaluated on a single stage power amplifier for the simulated QAM and OFDM digitally modulated signals at diverse input power levels up to the 1 dB compression point. Moreover, the FPGA implementation of a system for the generation and processing of the fundamental signal, and the second-order linearisation signals is presented in the paper, and the proposed technique is verified for the generated QAM signal. In addition, the linearisation of the two-way asymmetrical Doherty amplifier is performed in the experiment by using software defined radio platforms for synthesising the fundamental signal and the second-order linearisation signals.     

Theoretical Analysis of Intermodulation Distortion of Reflection-Type IMPATT Amplifiers

The basic equations for a reflection-type IMPATT amplifier are used to derive expressions for the output when the amplifier is driven by a multifrequency input signal. The third-order intermodulation distortion is expressed and graphically presented for various diode, circuit, and signal parameters. The results provide a guideline for designing amplifiers with minimum intermodulation distortion or prescribed distortion level.     

Third- and fifth-order baseband component injection for linearization of the power amplifier in a cellular phone

This paper proposes a new predistortion technique to improve the adjacent channel power ratio of the power amplifier (PA) in a cellular phone. This technique injects the third- and fifth-order distortion components in the baseband block, and eliminates the fundamental component from the injected signals in order to operate effectively up to near power saturation. We show this technique can raise the distortion-compensation limit, through mathematical analysis, and confirm it with simulation. We also examine the distortion compensation performance of the PA using a wide-band code-division multiple-access uplink signal and a high-speed downlink packet access signal, and the possibility of applying a new low-voltage high-capacity battery.     

A 5-GHz 20-dBm Power Amplifier With Digitally Assisted AM-PM Correction in a 90-nm CMOS Process

This paper presents an integrated CMOS power amplifier and a technique for correcting AM-PM distortion in power amplifiers. The linearization technique uses a varactor as part of a tuned circuit to introduce a phase shift that counteracts the AM-PM distortion of the power amplifier. The varactor is controlled by the amplitude of the IQ baseband data in a feedforward fashion. The technique has been demonstrated in a 5-GHz class-AB CMOS power amplifier designed for WLAN applications and implemented in a 90-nm CMOS process. The power amplifier delivers 16 dBm of average power while transmitting at 54 Mb/s (64 QAM). The proposed linearization technique is shown to improve the efficiency of the power amplifier by a factor of 2.8.     

An Approximation of Volterra Series Using Delay Envelopes, Applied to Digital Predistortion of RF Power Amplifiers With Memory Effects

In this letter, a new model for digital predistortion (DPD) of radio frequency power amplifiers for wide-band applications is proposed. The model is based on a spline approximation of Volterra series by considering second-order cross-terms. The advantage of the spline cross-term model is a reduction in the number of model parameters. We further reduce the model order by operating on delayed envelope samples, rather than the complex baseband samples. A block of wide-band code-division multiple access signal is sent through a 90 W class-AB power amplifier, based on the freescale LDMOS output device, and the input/output baseband samples were used to identify the DPD parameters.     

Improvement of intermodulation distortion in microwave m.e.s.f.e.t. amplifiers using gate-bias compensation

A simple technique is described for improving the intermodulation distortion performance of microwave m.e.s.f.e.t. amplifiers. The technique utilises transistor gate-bias compensation controlled by the input signal level of the amplifier. For a constant output power of 0 dBm, an improvement in third-order intermodulation distortion product of up to 10 dB has been observed. The advantages and limitations of the technique are discussed.     

一种数字预失真器的实现方法

针对功率放大器引起的非线性失真问题,提出一种基于极坐标查找表的数字基带预失真系统,使用Altera DSP Builder Quartus II等工具软件完成该系统的设计、综合与适配。仿真及实验结果表明,该系统能较好的补偿功率放大器的非线性失真,且具有占用现场可编程门阵列（FPGA）硬件资源少,系统复杂度低的特点。     

高保真音频功放器设计

针对高品质音频的需求,设计了一种高保真音频放大器。该音频放大器主要由信号放大电路、功率放大电路和有源滤波电路组成,其中信号放大部分采用场效应管及运算放大器。其受外界的干扰性较小,稳定性较高,放大倍数可达到20～30倍,随需要而进行调节。功放部分采用LM4766型芯片,采用差动放大电路对信号实现音频信号的放大,失真小于百分之零点一。滤波电路采用了二阶有源低通滤波电路,对输入的音频信号进行处理。该设计的音频功率放大器具有失真度小、增益可调、体积小等特点,具有较高的实用价值。     

S类放大器交越失真的仿真研究

交越失真是由于晶体管的门坎电压而产生,减小或克服交越失真是人们一直关注的课题之一。本文采用Multisim9对一款S类音频功率放大电路进行仿真,给定不同的信号幅度和频率,用虚拟示波器观察其输入输出信号的波形,同时用虚拟仪表量测其幅值。结果表明S类电路没有完全消除纯乙类电路所存在的交越失真。     

Modeling nonlinear distortion of ultra wideband signals at X-band

Nonlinear distortion in the adjacent channels of an X-band amplifier driven by an ultra-wideband digitally-modulated carrier is analyzed. Statistical properties of the input signal with a complex power series-based behavioral model of the amplifier are used to calculate the output power spectrum. Comparisons are made between measured and predicted adjacent channel power rejection for the X-band monolithic microwave integrated circuit.     

A hybrid digital/RF envelope predistortion linearization system for power amplifiers

This paper presents an adaptive wide-band digitally controlled RF envelope predistortion linearization system for power amplifiers (PAs). A field-programmable gate-array-based lookup table is indexed by a digitized envelope power signal, and instantaneously adjusts the input signal amplitude and phase via an RF vector modulator to compensate for the AM-AM and AM-PM distortion. The advantages of this predistortion architecture over conventional baseband digital approaches are that a 20%-33% wider correction bandwidth is achievable at the same clock speeds, and linearization can be performed without the need for a digital baseband input signal. The timing match between the input RF signal and predistorting signal, which is one of the critical factors for performance, was investigated and adjusted to obtain optimum performance. Using three-carrier cdmaOne and wide-band multitone signals, the linearization performances for a 0.5-W GaAs heterostructure field-effect transistor, a 90-W peak-envelope-power (PEP) silicon LDMOS PA, and a 680-W PEP LDMOS PA were examined. In addition, the predistortion performance variation for different signals was studied in terms of signal envelope statistics, output powers, and PA power capacities.     

Carrier Distortion in Hysteretic Self-Oscillating Class-D Audio Power Amplifiers: Analysis and Optimization

An important distortion mechanism in hysteretic self-oscillating (SO) class-D (switch mode) power amplifiers-carrier distortion-is analyzed and an optimization method is proposed. This mechanism is an issue in any power amplifier application where a high degree of proportionality between input and output is required, such as in audio power amplifiers or xDSL drivers. From an average-mode point of view, carrier distortion is shown to be caused by nonlinear variation of the hysteretic comparator input average voltage with the output average voltage. This easily causes total harmonic distortion figures in excess of 0.1-0.2%, inadequate for high-quality audio applications. Carrier distortion is shown to be minimized when the feedback system is designed to provide a triangular carrier (sliding) signal at the input of a hysteretic comparator. The proposed optimization method is experimentally proven in an audio power amplifier leading to THD figures that are comparable to the state of the art. Experimental hardware is a hysteretic SO bandpass current-mode-controlled single-ended audio power amplifier capable of 45 W into 8 Omega or 80 W into 4Omega from a plusmn34 V supply with less than 0.03% THD from 100 Hz to 6.7 kHz. Carrier distortion is shown to account for this limitation in THD performance.     

Effects of baseband predistortion at the transmitter on the performance of a DEQPSK satellite modem

The paper presents the results of an extensive series of computer-simulation tests to determine the effects of bandlimiting, nonlinear distortion with and without predistortion, and adjacent channel interference (ACI), on the tolerance to additive white Gaussian noise of a digital satellite modem. The modem transmits a differentially encoded quaternary phase-shift keyed (DEQPSK) signal over a satellite link, where the high-power amplifier (HPA) at the earth-station transmitter may introduce AM—AM and AM—PM conversion effects into the DEQPSK signal, but the satellite transponder is linear. Four different bandwidths of the transmitted signal are tested, together with three different levels of output back-off in the HPA at the transmitter. The predistorter operates on the baseband signal at the input to the modulator in the transmitter and assumes a prior knowledge of the HPA characteristics. ACI can be introduced into the transmitted DEQPSK signal by the corresponding DEQPSK signals, which occupy the two immediately adjacent frequency bands. The latter signals are assumed to have the same transmission rate and bandwidths as the first signal. Various combinations of the different distortion and interference effects are studied, using an equivalent baseband model of the transmission system, and the arrangements selected are those achieving the most efficient use of bandwidth together with the best tolerance to additive white Gaussian noise. A surprisingly good performance is achieved.     

基于动态栅压功放的高效超宽带发射机结构

提出的超宽带射频发射机的结构具有良好的效率和线性度。设计和制作了一款宽带的动态栅压偏 置的功率放大器。动态栅压偏置可以获得优良的宽带性能。总发射机可以支持184.32 MHz 的调制带宽。基带信 号和相关偏置电压由宽带模拟基带处理单元生成。基带信号经过直接变频的调制方式到达功放输入端,偏置电压 经过差分运放以及延时电路到达功放栅极。实验结果表明,与传统A 类功放相比,动态栅压偏置可以增加系统的效 率和线性度。     

Second-order distortion of amplified intensity-modulated signalswith chirping in erbium-doped fiber

Second-order distortion produced in an erbium-doped fiber amplifier (EDFA) is investigated theoretically and experimentally. Rate equations for intermodulation products are led by the perturbation method. From numerical calculation, it is shown that optimum combinations of input power to EDFA and signal wavelength exist that give a distortion-free condition, Improvement of the composite second-order (CSO) distortion, which results from the cancellation of the laser distortion and the gain tilt induced distortion, was confirmed for VHF multichannel transmission. Furthermore, CSO distortion-free transmission in cascaded EDFA's has been demonstrated for a 42-channel AM-SCM signal by experimentally determining the optimum operating condition of the EDFA such that each amplifier generates no additive distortion