Baseband Modeling and Distortion Equalization of the Feedback FM Demodulator by Functional Methods

Exact and approximate baseband models for the feedback FM demodulator are derived. Exact and approximate distortion equalizers that may be used to remove distortion in the output of the demodulator are also presented.     

Error Analysis of a Quadrature Coherent Detector Processor

A simple method is presented for computing the output signal distortion resulting from relative phase and gain errors between the two quadrature detector channels of a baseband processing system. Signal-to-noise loss and sidelobe levels are computed for two types of waveforms to illustrate the utility of the method.     

机翼／外挂的次谐颤振响应

对机翼／外挂次谐颤振响应作近似分析。从机翼分离出的外挂被视为简谐激振力作用下的带间隙分段线性振子,研究这种振子的对称周期响应及其分叉。采用Ｐｏｉｎｃａｒｅ映射方法得出分叉条件,再用当量线化法导出分叉条件近似式,并用它来预示机翼／外挂颤振系统的次谐响应。算例表明这种近似分析是可行的。     

模拟中性大气边界层中污染物传输与扩散的动力相似讨论

文章从湍流雷诺数相等出发,推导出风洞中模拟中性大气边界层动力相似准则一充分发展的湍流中,运动相似的流动动力也相似。文章还讨论了模拟烟流抬升和扩散的相似准则,概述了国内外一些主要实验室应用的近似模拟技术。     

Multipath interferometry technique for airborne array calibration

A new technique is developed to compensate multiple-wavelength distortion in airborne antenna arrays. This approach exploits the phase information in microwave reflections from arbitrary terrain. To handle reflections incident over a broad angle, a range-Doppler preprocessor is used in each element channel to resolve wavefronts incident simultaneously from different directions. The phase information for each direction of arrival is compared between elements and processed by optimal estimators to determine the phase corrections needed to compensate the distortion. To develop the estimators, a statistical model of the complex baseband terrain reflections is developed. This is in turn used to generate conditional probability densities involving the range Doppler observations and the parameters to be estimated. These densities are subsequently used to develop minimum variance and maximum likelihood estimators. The new estimators use additional information that has not been exploited by previous techniques and therefore provide enhanced performance     

Error Measures for Normal Random Variables

Four commonly used measures of error are the root mean square error (RMSE), the geometric mean error (GME), the mean radial error (MRE), and the circular or spherical error probable (CEP, SEP). Exact or approximate closed form expressions are given. for each for zero mean, normal random variables in two and three dimensions. Numerical comparison of the measures suggests the relationship GME ? SEP(CEP) < MRE ? RMSE.     

Optimum Filters for Image Registration

Two-dimensional cross-correlation techniques are applied to the problem of image registration under the assumption of small geometric distortion. Optimum filter functions for use with arbitrary window functions are derived for two performance measures of interest: peak-to-sidelobe ratio, and mean- square registration error; the latter is examined in terms of the contribution due to distortion and the contribution due to noise. A generalized Lagrange multiplier approach is used to derive at least approximate ate solutions to the case where the images are modeled as random processes and to the case where the images are assumed to be deterministic.     

Detectability of Spread-Spectrum Signals

Methods of detecting spread-sprectrum signals without knowledge of the pseudorandom code used to generate the signal are described. Exact and approximate methods of calculating relationships among detection probability, false alarm rate, and signal-to-noise ratio are given for radiometers and for channelized pulse-detection systems. The detection performance of the radiometer is compared graphically with that of pulse-detection systems, for two different kinds of pulse detection decision rules. Detection performance as a function of certain signal parameters is shown to be very different for a pulse-detection system than for a radiometer, and this difference in behavior provides a basis for selecting signal parameters that minimize the probability of detection. The reasoning that underlies the selection process is explained, and the process is outlined for each of several signal parameters.     

Adaptive suppression of passive intermodulation in digital satellite transceivers

For the performance issues of satellite transceivers suffering passive intermodulation interference,a novel and effective digital suppression algorithm is presented in this paper.In con trast to analog approaches,digital passive intermodulation (PIM) suppression approaches can be easily reconfigured and therefore are highly attractive for future satellite communication systems.A simplified model of nonlinear distortion from passive microwave devices is established in consid eration of the memory effect.The multiple high-order PIM products falling into the receiving band can be described as a bilinear predictor function.A suppression algorithm based on a bilinear poly nomial decorrelated adaptive filter is proposed for baseband digital signal processing.In consideration of the time-varying characteristics of passive intermodulation,this algorithm can achieve the rapidness of online interference estimation and low complexity with less consumption of resources.Numerical simulation results show that the algorithm can effectively compensate the passive intermodulation interference,and achieve a high signal-to-interference ratio gain.     

Optimum Windows for Image Registration

Two-dimensional cross-correlation techniques are applied to the problem of image registration under the assumption of small geometric distortion. Optimum window functions are derived for two performance measures of interest: peak-to-sidelobe ratio and mean-square registration error. The latter is examined in terms of the contribution caused by noise and the contribution caused by geometric distortion. A generalized Lagrange multiplier approach is used to derive approximate solutions assuming random images. The case of Gaussian autocorrelation functions is examined in detail. Results of applying the theoretically derived window functions to real data are presented, showing significant improvement in correlator performance.     

软件无线电的可重构流处理器体系结构

 针对软件无线电中存在通用数字信号处理器（DSP）计算能力不足以及专用基带处理器缺乏扩展性的问题,提出了一种新的处理器：面向软件无线电的可重构流处理器及相应的专用指令集。仿真证明,该处理器的可重构设计具备指令集扩展能力,可以对多种无线通信标准甚至是新的通信标准提供支持;面向软件无线电的流体系结构和专用指令集保证了对多种高速无线基带信号的实时处理,运算能力是通用DSP的5~13倍。该处理器为软件无线电中的硬件设计难题提供了一种新的解决途径。     

Optimum Transponder Design for Pseudonoise-Coded Ranging Systems, Weak Signals

Optimization of the bandwidth of a turnaround transponder is carried out for weak signals at the transponder receiver. Optimum transponder bandwidth is found to be around 0.77-1, where T is the period of one element of the ranging code. Linear transponders are compared with transponders having baseband limiting. Phase modulation is assumed. It is shown that, for weak signals, linear transponders are preferable for a partitioning of the transponder transmitted power giving (carrier power)/(total power) > 0.43, while baseband limiters are preferred for (carrier power)/(total power) < 0.43.     

Clutter Spectra of Low PRF AMTI Pulse-Doppler Radar

Approximate expressions are derived for the video clutter spectra in the receiver of a low pulse repetition frequency (PRF), airborne moving target indicator (AMTI), pulse-Doppler radar for both step-scanning and continuous-scanning antennas. The receiver is assumed to process the received waveform with a clutter-tracking oscillator and a window function is employed to obtain short-term spectra. Except for the broadening effects of the window function, it is shown that the clutter spectrum can be simply related to the antenna voltage-gain pattern. It is further shown, in the scanning antenna case, that the combined spectral broadening due to platform motion and antenna scanning cannot be assumed to be the result of the convolution of the separate effects unless the antenna gain pattern has a Gaussian shape. The approximate clutter expressions are illustrated by examples and are shown to agree well with the results of computer calculations.     

频率选择表面机载雷达天线的研究

研究了降低机载雷达天线散射截面的频率选择表面(FSS)极化扭转板。根据Floquet定理利用模式电压概念建立矢量模式,对二维周期阵列的频率选择表面进行理论分析和计算。设计并加工了一种降低机载雷达天线散射载面的频率选择表面极化扭转板结构。计算和实验表明:频率选择表面的极化扭转板在雷达天线工作通带内保持与原极化扭转板的性能不变,而在工作通带外具有降低雷达散射载面的效果。降低雷达散射截面7～10dB。     

Acquisition Time of Tracking Loops Operating in the Frequency-Search Mode

This paper presents a theory for computing the signal acquisition time of a second-order tracing system operating in the frequency-search mode. In particular, a closed-form expression is obtained which relates the acquisition time to the relevant system parameters, to the initial loop detuning, and to the slope of the ramp that sweeps the voltage-controlled oscillator. Although the theory is only approximate and the resulting formula is exact only in the limit when the initial loop detuning ?F goes to infinity, it has been found that it gives quite accurate results as soon as ?F is some times greater than the equivalent noise bandwidth of the loop.     

机载斜视SAR改进ETF快速成像算法

 根据机载斜视SAR(Synthetic Aperture Radar)成像模式,详细推导了任意场景位置的准确传递函数,在此基础上,提出了一种改进的ETF（Exact Transfer Function）成像算法。并推导得到了算法使用中分块处理应该满足的边界条件。由于推导过程基于准确运动模型和参考距离模型,方位相位的计算不做近似截取而是做准确计算,使得该算法具有更高的相位精度。同时该算法可运行于现有的成像系统,兼容性好;对数据分块并行运算,解决了斜视情况下大距离徙动带来的问题并且提高了运算速度。最后,点目标仿真成像结果证明了算法的有效性。     

基带滤波调制技术深空测控应用性能分析

深空测控的下行链路包括测控和数传2种典型的传输信道,其中数传信道一般具有发射功率大和数据传输速率高的特点,其带外辐射容易对测控信道产生干扰。以典型月球探测器为例,分析比较了数传信道采用常规BPSK（二进制相移键控）调制和基带滤波BPSK调制时,对测控信道的带外辐射影响,并对基带滤波BPSK调制的误码率性能进行了仿真分析,相应结论对未来深空探测任务测控系统总体设计具有十分重要的参考意义。     

Considerations for the Short-Term Stability of Frequency Multipliers

The effects of frequency multiplication circuits on the short-term stability of an oscillator system are analyzed. The short-term stability of real components such as varactor multipliers are compared to the short-term stability of an ?ideal? frequency multiplier which maintains the stability of the basic oscillator. In addition, it is shown that positive feedback like that used in a synchronized oscillator, while useful for tracking, degrades short-term stability.     

On the fundamental acoustic mode in variable area,low Mach number nozzles

We consider the propagation of sound in nozzles of substantially varying cross-section, taking into account reflections from the tapering walls and non-uniform convection by the axially accelerated or decelerated mean flow. Exact solutions of the acoustic equations are obtained for the simplest case of the fundamental longitudinal mode in quasi-one-dimensional, low Mach number nozzle flow. The effects of non-uniform mean flow on sound are demonstrated by comparing horns with nozzles, and 15 properties (stated in italics and labelled P1, P2 ... P15 in the text) are proved and discussed, viz. concerning (1) relations between acoustic pressure and velocity; (2) equipartition or biasing of kinetic and compression energies; (3) conservation or evolution of wave action; (4) existence of elementary or special solutions and (5) cut-off frequencies, and amplitude and phase laws. We conclude with a simple formula for the approximate calculation of the acoustic fields; the formula can be made more accurate by using a correction factor for amplitude and phase, which is plotted against area ratio for propagation upstream or downstream in diffusers and convergent nozzles.     

Digital Oscillator Having Low Sensitivity and Roundoff Errors

A digital oscillator structure is proposed which has very low sensitivity and roundoff errors, particularly for low frequencies. The oscillator coefficient implemented is a normalized scaled value of the two's complement of the original coefficient, such that all bits represented are significant. The signal is scaled where appropriate to keep the overall transfer function unchanged. Multipliers used in the proposed structure have the same width as that in the direct form digital oscillator, although the performance is significantly improved.