A K-Band Phase-Locked Transmitter-Receiver System for CW Balanced-Bridge Measurements

A major sensitivity limitation common to most microwave CW balanced-bridge systems must be attributed to bridge distortion noise, caused by residual FM in the transmitted signal. The described superheterodyne transceiver minimizes this limit for a narrowband K-band bridge system by utilizing precise, coherent IF detection. A phase-locked local oscillator provides means for an arbitrary, stable preselection of the in-phase or quadrature component with the help of a calibrated IF delay line. The transmitter is either 1) frequency-stabilized to a sample cavity by means of a high-gain AFC loop, or 2) phase-locked simultaneously to a K-band harmonic of a VHF quartz oscillator and to a tunable VHF oscillator (VFO). This yields flexibility in a wide range of applications, such as measuring small reflection coefficients, dielectric constants, or magnetic tensor susceptibilities (e. g., in ESR spectroscopy). Analytical expressions for phase and amplitude distortions are derived for a bridge containing one high-Q element. In the systems theory of operation, analytical formulas for the noise spectral densities and the loop errors are given, together with numerical examples. The additional receiver noise, due to residual FM and increased bridge power, is demonstrated by means of measured IF-noise spectra. A cavity-Q measurement with ±1 percent accuracy, using 5-?W incident bridge power, proves the system's capability for measurements of small reflection coefficients.     

Analytical study of binary differential impulse radio-ultra wide band over single-mode fibre systems using two receiver structures

A binary differential impulse radio-ultra wide band (IR-UWB) communication scheme over a singlemode optical fibre is examined. For a receiver structure, the conventional electrical receiver as well as an optical receiver structure, which is similar to the optical receiver used for digital, optically phase-modulated differential phase shift keying, are considered. The optical receiver can alleviate the IR-UWB receiver implementation challenges and it is studied for the first time in the context of IR-UWB. Considering various important noises, for example, phase noise, laser intensity noise, thermal noise and shot noise, analytical expressions for the error probability of the aforementioned receivers are derived. The mathematical models for optical components including laser diode and single-mode fibre, along with the analytical expressions for the receiver?s error probability, are used to evaluate the overall performance of an UWB communication system over a fibre transmission medium. Furthermore, the electrical receiver is compared with the optical receiver and it is shown that the performance of the optical receiver can be as good as that of the electrical receiver and even better. The impact of wireless channel fading, bias current of laser diode and the coherence time of laser diode on the UWB over fibre system performance is also examined.     

Average BER performance of FSO SIM-QAM systems in the presence of atmospheric turbulence and pointing errors

This paper presents the exact average bit error rate (BER) analysis of the free-space optical system employing subcarrier intensity modulation (SIM) with Gray-coded quadrature amplitude modulation (QAM). The intensity fluctuations of the received optical signal are caused by the path loss, atmospheric turbulence and pointing errors. The exact closed-form analytical expressions for the average BER are derived assuming the SIM-QAM with arbitrary constellation size in the presence of the Gamma–Gamma scintillation. The simple approximate average BER expressions are also provided, considering only the dominant term in the finite summations of obtained expressions. Derived expressions are reduced to the special case when optical signal transmission is affected only by the atmospheric turbulence. Numerical results are presented in order to illustrate usefulness of the derived expressions and also to give insights into the effects of different modulation, channel and receiver parameters on the average BER performance. The results show that the misalignment between the transmitter laser and receiver detector has the strong effect on the average BER value, especially in the range of the high values of the average electrical signal-to-noise ratio.     

Error estimates for frequency responses calculated from time-domainmeasurements

Estimates are derived for the errors in frequency responses calculated from measured step or impulse response data. Three error sources are considered-aliasing errors, errors due to random noise added to the amplitudes of the data, and errors due to random noise added to the time coordinates of the data. Two types of error estimates are derived. Pre-measurement estimates are based on easily determined parameters of the signal and noise; post-measurement estimates are based on the difference between two measurements of the same signal. The post-measurement error estimates apply to additional sources of error. The goal of the paper is to give easily used error estimates for easily implemented methods rather than to present the most sophisticated methods     

弱粘接结构中谐振频率与刚度比的数值研究

利用超声波垂直入射的方法,研究了粘接层界面具有弹簧模型的边界条件下,波的反射系数与谐振频率的关系。首先,利用粘接层界面的一次反射导出切向和法向刚度比表达式;其次,推导了粘接层界面存在多次反射和透射时刚度比、谐振频率及反射系数之间的关系,得到了弱粘接结构的反射系数随着不同刚度比的变化规律;最后,分析了弱粘接层厚度的变化对谐振频率的影响。理论分析和数值计算的结果表明:随着刚度比的增加,谐振频率向高频方向漂移;并且,随着粘接层厚度的增加,粘接结构将出现多阶谐振频率。     

Approximate Cramer-Rao bound on Doppler error in correlation-processing relatively narrowband noise radar

Limitations on accuracy of Doppler estimation in continuous-wave noise radar with correlation processing are studied. Second-order properties of output of the correlation receiver are evaluated and an approximate Cramer-Rao bound on errors of Doppler measurement is derived. The accuracy of Doppler measurements is found to be affected by the following factors: power spectral density of noise signal, frequency response of the low-pass filter in correlator, observation time, velocity of the target and signal to noise ratio. It is shown that the random nature of the transmitted signal induces additional fluctuations at the output of correlator which limit the accuracy even in the infinite signal to noise case. Qualitative extension of the results to a case covering multiple targets and clutter is made. It is argued that the performance will decrease and that increasing transmitted power may not provide significant improvement when clutter is present.     

Performance of a laser microsatellite network with an optical preamplifier

Laser satellite communication (LSC) uses free space as a propagation medium for various applications, such as intersatellite communication or satellite networking. An LSC system includes a laser transmitter and an optical receiver. For communication to occur, the line of sight of the transmitter and the receiver must be aligned. However, mechanical vibration and electronic noise in the control system reduce alignment between the transmitter laser beam and the receiver field of view (FOV), which results in pointing errors. The outcome of pointing errors is fading of the received signal, which leads to impaired link performance. An LSC system is considered in which the optical preamplifier is incorporated into the receiver, and a bit error probability (BEP) model is derived that takes into account the statistics of the pointing error as well as the optical amplifier and communication system parameters. The model and the numerical calculation results indicate that random pointing errors of sigma(chi)2G > 0.05 penalize communication performance dramatically for all combinations of optical amplifier gains and noise figures that were calculated.     

OFDM削波噪声迭代消除算法

高峰均功率比(PAPR)的OFDM信号通过功率放大器的时候会产生非线性干扰,同时降低了放大器的工作效率。传统削波算法可以降低信号的PAPR,但是会带来较大的频谱扩展。作为一种新的削波算法,误差削波可以在降低OFDM信号PAPR值的同时不带来任何频谱扩展。但是这种削波会给信号带来更大的带内干扰噪声。提出了一种OFDM削波噪声迭代估计和消除算法,它能有效的消除由于误差削波带来的噪声。新方法通过建立削波噪声模型,在接收端根据该噪声模型用迭代的方法重新产生削波噪声。仿真结果表明,使用削波噪声消除算法后,使得系统的误码率性能接近未削波信号的水平。     

The Direct Digital Measurement of Low-Pass Noise Power

A direct digital method of measuring the noise power in a low-pass random waveform is described. The input-output relationship and the sensitivity of the digital detector are determined. Direct digital measurement of noise power offers a number of advantages in the instrumentation of radiometers and other noise-measuring systems.     

小波变换电力谐波功率测量误差分析与抗泄漏算法研究

通过理论分析指出,在信号非同步采样条件下,基于小波变换的谐波功率测量方法存在着很大的带内泄漏.仿真表明,带内能量泄漏引起的测量误差可达5×10~(-2).针对该泄漏误差,推导了误差公式,并提出了一种基于整系数交迭窗函数的抗带内泄漏的小波变换谐波功率测量新算法.结果可知:抗带内泄漏功率测量新算法的相对误差小于1.2×10~(-4),测量准确度比传统的小波变换谐波功率测量算法提高了两个数量级.     

基于卡尔曼滤波器的频率源远程校准技术研究

针对GPS接收机接收下来的钟差数据带有很强的信号噪声,采用LabVIEW软件建立卡尔曼滤波模型,对离散钟差数据序列进行处理和预测,更加准确地反应频率源的性能。试验数据表明,选择参数合理,有效地降低了噪声造成的误差,从而提高了校准精度。     

GaAs MESFET and related processes

Since inception of GaAs MESFET in 1971, growth and processing technology of GaAs has matured to the extent that the analogue as well as digital IC production is persued at the industrial level. The ever increasing demand for higher frequency of operation, low noise figure and higher gain has led to newer device structures such as HEMT and HJBT based on GaAs and related compounds. Furthermore there exists exciting and proven capabilities in GaAs and related compounds to generate, detect and convert light into electrical signals. This has opened up vast field of opto-electronic devices and their integration with MESFET and other conventional devices. Basic building block of all these developmental activities still remains the GaAs MESFET, which have also been extensively used as low noise amplifiers, mixers, oscillators and high power amplifiers in descrete form. This paper reviews the design aspects, fabrication technology, d.c. and microwave characterization for both low noise and high power MESFET. Various technological advancements like via-hole for source grounding, air-bridge technology for low parasitic interconnects and polymide passivation, which have helped in further improvement in terms of higher frequency of operation, low noise and high power output are reviewed. Finally some representative results on the devices fabricated at CEERI are also presented.     

Closed-form inversion of the reflection ellipsometric function of a film-substrate system: absorbing-substrate optical constant

A closed-form inversion expression for obtaining the optical constant (complex refractive index) of the substrate of an absorbing-film-absorbing-substrate system from one reflection ellipsometry measurement is derived. If, in addition, the film thickness is to be determined, a second measurement at another angle of incidence may or may not be used. The derived formula does not introduce errors itself, and tolerates errors in input variables very well. Random and systematic errors in the measured ellipsometric parameters do not affect the value obtained for the optical constant of the substrate: it is always the true value to two decimal places. Two examples in ellipsometry and in the design of reflection-type optical devices, one each, are presented and discussed. In addition, experimental results for a commercially available wafer are also presented. Two other closed-form inversion expressions for obtaining the optical constant of the substrate from two and three measurements are also presented and briefly discussed.     

相关时延估计误差对测频误差的影响

声学多普勒测速技术采用宽带编码信号进行多普勒频移估计时会产生测频误差,该误差来源主要分为两种：（1）宽带信号频谱不对称;（2）相关时延存在估计误差。文章主要讨论相关时延估计误差对测频误差的影响,分析了其产生的原因以及该误差对测频误差的影响。忽略相关函数简化误差以及滤波后频点不对称,推导得到由相关时延估计误差导致的测频误差的解析式。进行了仿真和湖试数据分析,不同频移下的测频误差与理论测频误差一致,不同填充系数下的实际测速误差与理论测频误差变化趋势相同。该对比结果验证了相关时延估计误差会对测频误差产生影响,证明了由相关时延估计误差导致的测频误差的解析式可信。     

丝材含量对Co基玻璃包覆非晶丝复合膜微波噪声抑制特性的影响

制备了含有Co基玻璃包覆非晶丝的膜状噪声抑制材料,采用微带线测试系统测试了复合膜的S参数,并计算出功率损耗比,研究了丝材含量对复合膜噪声抑制特性的影响。结果显示,在较低频率(10MHz～3.5GHz),复合膜导致的反射随着丝材含量的增加而增大,但当丝材含量增加至60cm-2时,反射接近饱和。丝材含量高于30cm-2时,样品的S21曲线在测试频率范围内出现吸收峰,且峰值对应的频率随着丝材含量的增加逐渐减小。丝材含量为60cm-2时,复合膜的功率损耗比在2.7～8.5GHz范围内高于75%,具有较好的噪声抑制效果。     

Recursion relations for the impedance of periodic layers

Previous numerical work is extended by deriving simple analytic expressions for the impedance of periodic layers over a wide frequency range within the reflection stop band (not just the center Bragg frequency) for an arbitrary number of periods in the structure, for arbitrary layer thicknesses (not just quarter-wave layers), for sizable absorption, and for arbitrary sizes of the refractive index differences. When the number of periods in the structure is infinite, exact expressions for impedance, which are valid for all frequencies in the reflection stop band, are derived. For a finite number of periods in the structure, it is shown that the asymptotic approach of the impedance toward its value for an infinite structure has a decaying exponential dependence. It is shown that the characteristic number of periods in this decaying exponential dependence is determined by the condition number of the transverse field matrix. Simple analytic expressions for the phase shift throughout the reflection stop band are derived, as well as simple analytic expressions to show that a small fractional error in the VCSEL cavity mode frequency can still result from a large fractional error in the cavity thickness if the layers in the Bragg mirror have a small refractive index difference. These simple analytic expressions are useful for design.     

Network of sensors: acquisition probability

A network of sensors is considered one of the most attractive remote sensing technologies available at present. In the system under consideration a network of sensors and a remote base station communicate using optical wireless links. This is accomplished by a base station that acquires and identifies sensors using a unique subcarrier frequency. The sensors use an active retroreflector to communicate with the base station, which reduces the complexity, cost, and power consumption of the sensors. The base station employs an imaging receiver (detector matrix), in which signals arriving from different directions are detected by different pixels. The imaging receiver mitigates ambient light noise and interference between simultaneous uplink transmissions from different sensors, provided that the transmissions are imaged onto disjoint sets of pixels. We describe a scheme that allows simultaneous acquisition and identification of a sensor in a network by an imaging receiver. A probability model of erroneous acquisition of this scheme due to noise is derived. The model's results indicate that the matrix size, the signal, and the noise powers have the greatest influence in determining acquisition probability.     

A Noise Temperature Measurement System for Mismatched Noise Sources

A measurement system is described which allows us to determine the noise temperature of mismatched noise sources with unknown reflection coefficients. The system consists of a null-balancing radiometer extended by a circuit to compensate for the reduced noise power emitted by the mismatched one-port. In this circuit, no passive nonreciprocal components are necessary. Special consideration is given to measurement errors that are caused by the correlation of the input and output noise waves of the radiometer preamplifiers. Several methods to eliminate these errors are described. Furthermore, a circuit is given by which the measurement range is extended down to 0 K without the use of cold reference noise sources. Finally, a practical setup, operating in the 20-40-MHz range, is described together with corresponding measurement results.     

Limiting phase errors of passive wireless SAW sensing with differential measurement

This paper addresses a statistical analysis of the limiting estimate errors of the time delay (phase difference) between two reflectors of the passive wireless surface acoustic wave (SAW) sensor with single differential measurement. The estimation is provided in a sense of the maximum likelihood function at the ideal coherent receiver in the presence of Gaussian noise. Assuming the Gauss-shape interrogating radio frequency pulse, we prove that linear drifts in its amplitude and phase do not affect the distribution of the sensor phase (and phase difference) at the measurement point. Rigorous and approximate solutions for the mean and mean-square errors, along with the error probability for the estimate to exceed a threshold, are studied in detail. The plots are applied to evaluate the errors in a wide range of signal-to-noise ratios and thresholds. Practical recommendations for designers of the remote SAW sensor interrogating systems are also given.     

FRF-based structural parameters estimation using strain data: sensitivity equation,measurement and excitation selection

In this paper, a structural model updating technique is presented using frequency domain representation of incomplete strain data to identify the location and severity of structural damages. The most important challenges of model updating methods such as selection of excitation, measurement locations and interested frequency ranges are addressed. Distribution of strain energy in all structural elements and norms of the derived sensitivity equation are utilized for the selection of measurement and excitation locations. The important effect of different level of noise in measurement data and modelling error on the accuracy and robustness of the proposed method are investigated. A bowstring truss and a two-storey single-bay frame are used to evaluate the numerical ability of proposed method. The results prove the potentials of the proposed method in identifying the location and severity of damage in the presence of different errors.