用MB1504实现寻呼机测试仪中的频点设置

介绍在单片机AT89C52的控制下,用锁柏环频率合成芯片MB1504设置寻呼机测试仪中的发射频点,以实现寻呼机测试仪与寻呼机之间的空中接口。     

一种新的线性调频信号的瞬时频率估计方法*

基于HHT（HilbertHuang transformation）是一种能分解出信号的任何频率分量的主成分分析法以及线性调频信号（LFM）的瞬时频率是关于时间的直线,将LFM 信号先作HHT得到其瞬时频率,然后利用该瞬时频率中间部分的时频点作最小二乘（LSM）直线拟合。这种改进的LSM直线拟合方法不仅剔除了HHT固有的边界效应在瞬时频率斜线两端产生的高频谐波,从而有效地抑制了这种边界效应对瞬时频率估计的影响,而且直接得到了较为准确的LFM 信号的瞬时频率参数估计值。与通过改变HHT算法来减小HHT边界     

基于精英遗传算法的GSM网络频点优化设计

随着GSM网络规模不断扩大,需要更多的频率资源,而频率资源是有限的,因此,合理分配频率,提高频谱利用率非常重要。但是,频率分配是一NP完全问题。文中首先给出一个综合考虑频点信号作用的范围、距离,扇区的角度的数值化干扰评价模型,在此模型上采用精英遗传算法优化某市现有的GSM网络频率分配。实验结果表明,基于精英遗传算法的GSM网络频点优化设计有较好的实际应用效果,优化后的GSM网络效率的实际评价和频点分布的状况均优于实际情况和普通的遗传算法。     

弱时频正交性条件下的混合矩阵盲估计

针对语音信号的弱时频正交性,提出一种基于主分量分析的混合矩阵估计方法.在时频域中,允许每个时频点存在任意多个源信号,通过对每个时频点进行主分量分析,检测只有一个源信号存在的时频点,此类时频点最大特征值对应的特征向量即为混合向量的一个估计,因此对所有估计出的混合向量进行K均值聚类,将聚类中心作为混合矩阵的估计.实验仿真表明,提出的方法提高了混合矩阵的估计精度,特别适用于估计欠定情况下的混合矩阵.     

机载收发信机锁相频率合成器的设计与实现

针对无人机机载设备体积小、重量轻、功耗小等要求,基于芯片ADF4360-3为机载数据终端的发射机与接收机设计了一种可设置上百个可变频点的锁相频率合成器。重点介绍了频率合成器的控制电路设计、环路滤波器的设计和仿真,以及变频程序算法的实现,并给出了主要算法的程序代码;分析了上电锁定错误的原因并提出有效的解决方法。通过无人机飞行试验验证:所设计的锁相频率合成器,性能稳定,具有很强的实用性,可推广应用于其他无线电测控系统的收发信机中。     

四款2.0音箱新品中低端也有好声音

2.0音箱在很长一段时间之内处于不温不火的地位。特别是在2.1音箱大批出现，低价冲击市场后,2．0音箱甚至“失声”了一段时间。但从2007年的市场发展来看，2.0音箱再次活跃了起来。究其原因,主要是由于“分频点”的设置不同,2．0音箱对音乐的表现能力上天生就要优于2.1音箱；由于用户对音乐回放和音质要求的提升，使得2.0音箱的市场需求加大，再度火热起来。     

基于主成分分析的UWB信号的TOA估计算法

提出了利用主成分分析法估计信号在超宽带(UWB)系统中的波达时间（TOA）的算法。算法先对带有噪声的接收信号的相关矩阵进行主成分分析，求出信号特征值对应的特征向量，然后把接收信号分别投影到上述的各个信号特征向量上，求投影之和。通过设置复杂度和采样频率较低的阈值，比较了主成分分析法和MEP算法的时延估计精度。仿真结果表明，主成分分析法时延估计误差小于12cm的概率大于90%，而MEP算法的概率小于45%。     

雷达自适应频率控制系统干扰检测研究与实现

雷达自适应频率控制系统是针对有源压制性干扰的重要抗干扰措施.对该系统的总体设计、干扰检测算法、最优频率点选择和干扰检测电路等系统组成的关键技术进行介绍.该控制器以单片机和74LS161芯片为核心,综合运用信号检测与估计理论,通过频谱分析找出干扰最小的频点,然后控制发射机以该频点进行工作,达到有效对抗干扰的目的.     

S波段微带均衡器的设计与实现

设计了一种用于低噪声放大器后端的微带增益均衡器,通过对其结构进行理论分析、设计、仿真及加工调试,完全实现了所需的目标幅频曲线,改善了输出增益平坦度。根据测试结果,该微带均衡器工作于2 GHz-3 GHz频带内的幅度在2 GHz频点最大衰减3.5 dB,3 GHz频点最小衰减0.5dB,并随频率呈递增特性,且输入输出回波损耗超过14 dB。     

基于HMC833LP6GE频率源设计

本文介绍了一种杂散小、相位噪声好、频率调整步进小以及高稳定度的频率合成器的设计方法,选用了ADI公司的HMC833锁相环芯片进行频率源设计. 频率源在现代通信中是重要组成的部分.其模块的指标直接影响到整个系统的性能.许多人对高性能的频率源,做了深入研究,本文应用ADI公司的HMC833LP6GE芯片,设计出了高性能、高稳定度和高精度的本振源.     

A new music-empirical wavelet transform methodology for time–frequency analysis of noisy nonlinear and non-stationary signals

The goal of signal processing is to estimate the contained frequencies and extract subtle changes in the signals. In this paper, a new adaptive multiple signal classification-empirical wavelet transform (MUSIC-EWT) methodology is presented for accurate time–frequency representation of noisy non-stationary and nonlinear signals. It uses the MUSIC algorithm to estimate the contained frequencies in the signal and build the appropriate boundaries to create the wavelet filter bank. Then, the EWT decomposes the time-series signal into a set of frequency bands according to the estimated boundaries. Finally, the Hilbert transform is applied to observe the evolution of calculated frequency bands over time. The usefulness and effectiveness of the proposed methodology are validated using two simulated signals and an ECG signal obtained experimentally. The results demonstrate clearly that the proposed methodology is immune to noise and capable of estimating the optimal boundaries to isolate the frequencies from noise and estimate the main frequencies with high accuracy, especially the closely-spaced frequencies.     

Online call control in cellular networks revisited

Wireless communication networks based on frequency division multiplexing (FDM in short) play an important role in the field of communications, in which each request can be satisfied by assigning a frequency. To avoid interference, each assigned frequency must be different from the neighboring assigned frequencies. Since frequencies are scarce resources, the main problem in wireless networks is how to fully utilize the given bandwidth of frequencies. In this paper, we consider the online call control problem. Given a fixed bandwidth of frequencies and a sequence of communication requests arriving over time, each request must be either satisfied immediately after its arrival by assigning an available frequency, or rejected. The objective of the call control problem is to maximize the number of accepted requests. We study the asymptotic performance of this problem, i.e., the number of requests in the sequence and the bandwidth of frequencies are very large. In this paper, we give a 7/3-competitive algorithm, say CACO, for the call control problem in cellular networks, improving the previous 2.5-competitive result, and show that CACO is best possible among a class of HYBRID algorithms.     

Nonlinear transformation of term frequencies for term weighting in text categorization

In automatic text categorization, the influence of features on the decision is set by the term weights which are conventionally computed as the product of term frequency and collection frequency factors. The raw form of term frequencies or their logarithmic forms are generally used as the term frequency factor whereas the leading collection frequency factors take into account the document frequency of each term. In this study, it is firstly shown that the best-fitting form of the term frequency factor depends on the distribution of term frequency values in the dataset under concern. Taking this observation into account, a novel collection frequency factor is proposed which considers term frequencies. Five datasets are firstly tested to show that the distribution of term frequency values is task dependent. The proposed method is then proven to provide better F₁ scores compared to two recent approaches on majority of the datasets considered. It is confirmed that the use of term frequencies in the collection frequency factor is beneficial on tasks which does not involve highly repeated terms. It is also shown that the best F₁ scores are achieved on majority of the datasets when smaller number of features are considered.     

Design of Asymptotically Convergent Frequency Estimator Using Contraction Theory

The problem of estimating unknown frequencies of a sinusoidal signal simultaneously is a classical problem in signal and system theory. Many approaches and algorithms are proposed in literature to develop estimators for a measurable sinusoidal signal having multiple sinusoids with unknown amplitudes, frequencies and phases. In this work, an asymptotically convergent frequency estimator is given for estimation of n-unknown frequencies of a measurable sinusoidal signal. The contraction theory approach is adopted to show the asymptotic convergence of the proposed estimator in quite a simplified manner. Approach given here exploits the results of contraction theory related to semi-contracting systems. The nonlinear estimator based on dynamic system approach, guarantees global boundedness and convergence of the state and frequency estimation for all initial conditions and frequency values. It further ensures simultaneous globally convergent estimation of states and frequencies of a sinusoid involving multiple frequencies. Numerical simulations are presented for different combinations of frequencies to justify the claim.     

发动机总成悬置系统解耦方法研究

目前汽车发动机动力总成悬置系统设计的主要任务是选择悬置元件的刚度、位置和角度,使悬置系统自由振动模态频率避开发动机怠速激励力频率与车身自振频率,并尽量提高各模态振型的解耦程度,从而提高悬置系统隔振效果.悬置系统按预定频率严格解耦设计是使设计出的悬置系统模态频率完全等于按汽车设计频率规划预定的频率,并使各模态的振型严格解耦,即各向振动能量的解耦度等于1.本文从悬置系统的自由振动方程出发给出了对悬置系统按预定频率严格解耦设计的方程组,可以利用广义逆矩阵的理论求该方程组的解,亦可通过方程组构造函数进而求出该方程组的解,从而提供比当前的悬置系统模态优化设计更为简便高效的优化设计方法.相应的算例验证了本文提出的按预定频率严格解耦设计方程和求解方法的正确性.     

Dynamic simulation of thermopneumatic micropumps for biomedical applications

A model simulation of dynamic behavior of thermopneumatic micropump is presented. The model uses conservation of energy, mass, and momentum to predict the behavior of existing thermopneumatic micropumps. Applied to existing micropumps, simulation predicts trends similar to those reported experimentally. Dynamic simulation of effect of design parameters on performance of micropump is, then, carried out through the article. Results suggest that increasing operating frequencies results in higher volume flow rates until a critical frequency is reached. At higher frequencies volume flow rates decrease. Critical frequencies are dependent on damping. The higher the damping coefficient the lower the critical frequency becomes. For high frequency operation the performance of the micropump is dominated by both damping and heat capacity of micropump components. For low frequency operation the performance is dominated by heat losses from walls of air-chamber. The model provides general guidelines for building and operating the micropump.     

Frequency characteristics of coherent structures and their excitations in small aspect-ratio rectangular jets using large eddy simulation

Large eddy simulations of air jets from small aspect-ratio (AR) rectangular nozzles are performed with the dynamic subgrid-scale closure. Mean streamwise velocity profiles are in good agreement with experimental data. Results indicate that vortices originating from the longer side of the rectangular jet are dominant compared with that from the shorter side. Furthermore, entrainment is slight in the potential core, and significantly increases in the following vortex roll-up region. However, the jet entrains more with smaller AR. Power spectral density of the streamwise velocity indicates that the oscillations consist of a series of sub-harmonic frequencies, with the predominant frequencies reducing along the axial direction. Analysis shows that among multiple frequencies, there is a characteristic one at f = 0.22 which dominates the near field of the rectangular jet. The characteristic frequency is independent of velocity components, aspect ratios of the jet and locations. Based on this characteristic frequency, calculations with different forced frequencies imposed on the inlet nozzle are carried out. Results indicate that when the forced frequency is approximately equal to the characteristic frequency, development of the coherent structures is the most intense in the near field, and exhibits the strongest entrainment.     

Natural frequencies of vibration of cantilever sandwich beams

Theoretical and experimental studies were made in obtaining the natural frequencies of cantilever sandwich beams subjected to only gravity forces. The method of minimizing the total energy of the system was used for determining the frequencies. A vibration system made by Unholtz-Dickie was utilized to set the beam in vibration. Resonance occurred when the frequency of the shaker coincided with the natural frequency of the beam. The resonance frequencies were measured by transducers mounted at various locations on the beam. A total of sixteen beams of various lengths, thickness and core density were tested.

It was found that the natural frequency of a cantilever sandwich beam depends largely upon the thickness, length, core density and stiffness of the beam. In addition, the natural frequency has a nonlinear variation with the mode and for any particular mode, the value of the frequency increases as the length of the beam decreases.

Design factors were developed based upon the ratios of the theoretical frequencies of homogeneous beams having the same thicknesses and stiffnesses of that of sandwich beams and of the frequencies experimentally determined for similar sandwich beams.     

正弦波频率估计的修正Rife算法

分析了Rife算法的性能，指出当信号频率位于离散傅里叶变换（Discrete Fourier Transform，DFT）两个相邻量化频率点的中心区域时，Rife算法精度很高，其均方根误差接近克拉美-罗限（Cramer-Rao Lower Bound，CRLB），但当信号频率位于量化频率点附近时，Rife算法精度降低。本文提出了一种修正Rife（M-Rife）算法，通过对信号进行频移，使新信号的频率位于两个相邻量化频率点的中心区域，然后再利用Rife算法进行频率估计。仿真结果表明本算法性能不随被估计信号的频率分布而产生波动，整体性能优于牛顿迭代法（一次迭代）。接近二次迭代，在低信噪比条件下不存在发散问题，性能比牛顿迭代稳定。本算法易于硬件实现。