Phase shift bandwidth and scan range in microstrip arrays by the element frequency tuning

In this paper, the far-field phase shift properties of microstrip patch antennas are investigated. It is shown that, similar to reflectarrays, the resonant nature of microstrip patches can be used to change the phase of the radiated field. This phase change can be caused by the dimensional change of the microstrip patch, or by a reactive loading of its cavity such as an aperture on its ground plane. However, the available phase shift is limited by the antenna impedance bandwidth. The problem is initially investigated for conventional patch antennas, determining the available phase shift range. It is then studied for a wideband E-slot microstrip antenna, showing a considerably larger phase shift range. Then, a micro-electro-mechanical (MEM) based ground plane membrane, activated by an electrode from below, is proposed to adaptively generate and control the required phase shifts. It provides a low loss, continuously variable phase shifter that can be used at high frequencies for beam scanning in small arrays.     

Method for design of allpass filters

A new method for allpass filter design is presented. The method leads to a solution for the phase that is equal to the desired phase for 2N+1 values of frequency (if N is the order of the allpass filter). These values can be chosen at will in the approximation interval. The resulting phase error therefore displays a ripple behaviour. The solutions obtained in this way could be used as an initial solution for an exchange algorithm in order to produce an equiripple solution     

Phase Compensation Receivers for Optical Communication

Phase compensation receivers are investigated in the context of optical communication, with particular emphasis on low visibility channels. It is shown that phase compensation can be viewed as a reduction of the number of spatial modes in the signal field so that an improvement in communication performance can be realized by a receiver which exploits the spatial coherence of the compressed signal mode. The phase compensation receiver structure separates nicely into configurations for measuring the phase of the signal field across the aperture and the required processors to obtain the phase compensation control from the measurements. The optimum phase control, which is chosen to spatially concentrate the signal power in the focal plane, is shown to be the minimum-mean-squared-error estimate of the aperture field phase, when the estimation error is "small enough." The phase observation-estimator structures are placed in an idealized phase compensated receiver. Minimum signal power requirements for adequate phase estimation performance are established. Communication performance, in terms of the parameters that describe the low visibility channel, is examined for both compenstated and uncompensated receivers. Depending on the system used, and for phase coherence times in the tens of milliseconds, phase compensation is most appealing for signaling rates from the middle kilobit to low megabit per second range; the resulting improvement in communication performance made possible by phase compensation is as much as 40-60 dB in optical power relative to an uncompensated receiver.     

Dual-phase continuous phase modulation for spread-spectrum multiple-access communication

A new class of direct-sequence spread-spectrum multiple-access (DS-SSMA) systems with continuous phase modulation (CPM) is defined. The signals are unique in that the spreading is done by adding an extra phase term to the information phase while maintaining phase continuity, constant envelope, and efficient bandwidth usage. The spreading phase is formed from the spreading code and is independent of the information phase, which allows despreading to be done separately before data detection, which, in turn, allows a simple CPM detector to be employed for data detection. The information phase is considered in the minimum-shift keying (MSK) format, and a serial-MSK-type spread-spectrum receiver is considered for performance analysis. Expressions for the signal-to-noise ratio, the power spectral density, and the probability of bit error are developed, along with methods for computing their values to an arbitrarily close approximation. Numerical results show that the proposed system is an attractive alternative to the conventional DS-SSMA systems.     

Choosing an American digital HDTV terrestrial broadcasting system

A public process has been in place in the United States for six years to establish an HDTV terrestrial broadcasting standard. The process, having moved through a planning phase, a competition phase, and an examination phase, has now entered a cooperation phase. Remarkable progress has been made-a testament to the process. During 1994 the American digital HDTV terrestrial broadcasting system will be tested, fully documented, and recommended to the FCC for adoption     

Phaseless bi-polar planar near-field measurements and diagnosticsof array antennas

Antenna near-field measurements typically require very accurate measurement of the near-field phase. There are applications where an accurate phase measurement may not be practically achievable. Phaseless measurements are beginning to emerge as an alternative microwave antenna measurements technique when phase cannot be directly measured. There are many important aspects for successful implementation of a phaseless measurement algorithm. This paper presents appropriate phaseless measurement requirements and a phase retrieval algorithm tailored for the bi-polar planar near-field antenna measurement technique. Two amplitude measurements and a squared amplitude optimal sampling interpolation method are integrated with an iterative Fourier procedure to first retrieve the phase information and then construct both the far-field pattern and diagnostic characteristics of the antenna under test. In order to critically examine the methodologies developed in this paper, phaseless measurement results for two different array antennas are presented and compared to results obtained when the near-field amplitude and phase are directly measured     

Use of microwave lenses in phase retrieval microwave holography of reflector antennas

The Misell (1973) phase retrieval algorithm for microwave holography requires the capability to axially move an antenna's subreflector or feed to obtain a defocused far-field magnitude pattern. A microwave lens may be used as an alternative method of defocusing an antenna. A metal plate lens has been designed and constructed for use in phase retrieval holography at the Georgia Tech Woodbury Research Facility. Its phase variation across the aperture was chosen to have a parabolic profile in order to imitate the effect of subreflector or feed translation. A two-dimensional (2-D) finite-difference time-domain simulation has been performed to characterize the lens. Measurements, using the measured aperture phase of the lens transfer function as a phase correction term in the Misell algorithm, show the results to be comparable with that of phase coherent holography. Other types of microwave lenses such as a Fresnel zone plate, a bed of circular waveguides, and a standard dielectric lens are also being studied.     

Goldstein枝切法对存在间断相位缺陷的解缠研究

Goldstein枝切法作为相位解缠中路径积分法的重要算法之一,其解缠结果易受到噪声或间断相位缺陷所引起的残差点影响。为了研究相位间断缺陷对解缠算法的影响,模拟了具有间断相位缺陷的数据,采用Goldstein枝切法进行了系统的解缠研究。重点研究了残差点对枝切线的搜索窗口半径大小的影响,并将解缠相位与真实相位进行了比较。结果表明,在单相位间断和双不相交的相位间断缺陷的情况下,Goldstein枝切法仍然具有较好的解缠效果;对于双交叉相位间断缺陷,Goldstein枝切法在这一局部区域无法得到正确的解缠结果;通过研究枝切线搜索窗口半径对解缠的影响,验证了存在“有效枝切线搜索窗口半径”的结论。此实验结果对于采用或联合采用Goldstein枝切法进行的相位解缠理论研究和应用具有参考价值。     

Development of a millimeter-wave ferrite-filled antisymmetrically biased rectangular waveguide phase shifter embedded in low-temperature cofired ceramic

The theory of a new ferrite-filled rectangular waveguide phase shifter is presented, showing that the device offers more than twice the maximum phase shift of a classical dual-slab ferrite-loaded waveguide phase shifter. The complete analytical field derivation of the new phase shifter is presented. The prototype 36-GHz phase shifter has been completely embedded in an uncharacterized experimental low-temperature cofired ferrite ceramic material, essentially becoming an integral part of a chip package. The fabricated prototype is only 3.5-cm long, 5-mm wide, and 1-mm high. Preliminary measurements of the nonoptimized prototype reveal a controllable, nonreciprocal phase shift of 52.8/spl deg/ at 36 GHz for a bias current of 500 mA, and an insertion loss of approximately 3.6 dB, including transition loss. This paper marks the first time that a ferrite waveguide phase shifter has been realized as a lightweight, compact, and rugged module that can be easily mass produced at low cost. Since it is embedded in a package, the phase shifter can be readily integrated with other active and passive system components that would ideally be contained within the same package.     

Design of IIR Notch Filter with Approximately Linear Phase

A method for design of linear phase notch IIR filters is presented in this paper. The filter is realized in the form of a?parallel connection of two all-pass sub-filters. In order to achieve the resulting filter with an approximately linear phase in the pass-band, the applied all-pass sub-filters should be also with a?linear phase. The desired amplitude characteristic of a?resulting filter is obtained by all-pass sub-filter phase approximation. It can be achieved by applying either a?pure delay or an IIR all-pass filter in one branch. If a?pure delay is positioned in one branch, equiripple phase of all-pass sub-filter from the other branch is guaranteed for an equiripple amplitude characteristic of the resulting filter. Efficiency of the proposed algorithm is demonstrated on a?few examples. The algorithm exhibits a?fast convergence and an easy determination of initial values. In the case where IIR all-pass sub-filters are placed in both branches, the resulting amplitude characteristic satisfies all the prescribed demands, but the amplitude characteristic is not equiripple even when both all-pass sub-filters have equiripple phase.     

Design of dual-reflector antennas with arbitrary phase and amplitude distributions

A synthesis method based on geometrical optics for designing a dual-reflector antenna system with an arbitrary phase and amplitude distribution in the aperture of the second reflector is presented. The first reflector may be illuminated by a pattern with an arbitrarily curved phase front. A pair of first-order ordinary nonlinear differential equations of the formdy/dx=f(x, y)are developed for the system. Questions concerning uniqueness, existence and bounds for the solutions can be answered. Calculations and numerical results for the design of a uniform amplitude and phase dual-reflector system are presented.     

Phase conjugator for active retrodirective array applications

A novel phase conjugator for active retrodirective array applications is presented. The circuit provides conversion gain as well as phase conjugating operation by using active devices, and results in a simple and compact design through the use of a common port for RF and IF signals. Measurement results indicate a 3.2 dB conversion gain with an RF-IF isolation of 20 dB. This phase conjugator can be used as a component in a retrodirective array when combined with an antenna     

Novel Phase Modulator Linearity Measurement

Phase modulator linearity is an important concern for high dynamic range phase modulated optic links. However, the determination of phase modulator linearity is difficult as it requires an optical phase demodulator that generally contains nonlinear distortion that tends to contaminate the measurements. In this letter, we present a simple and effective approach for phase modulator linearity measurement that can be made into a photonic integrated circuit and does not require a reference linear phase modulator. We also present the measurement results.     

A Dynamic Calibration Method for Biphase Phase-Shift-Keyed Modulators

A method is proposed whereby a (microwave) phase bridge can be constricted to simultaneously measure both the phase and amplitude balance of a biphase phase-shift-keyed (PSK) modulator. Given an initial narrow-band phase shifter that is capable of either continuous uncalibrated phase shifting or slow switching between calibrated fixed-90/spl deg/ phase shifts, the phase and amplitude balance of a second wide-band phase shifter (biphase modulator) can be determined. Furthermore, these measurements may be dynamically and simultaneously displayed in real time on a dual-trace oscilloscope. A significant feature of the method is the unique identification of either or both the phase and amplitude unbalance by means of the symmetric or asymmetric features that these unbalances induce in the display Pattern. Quantitative criteria for the sensitivity of the technique are presented.     

光谱相位相干技术测量飞秒脉冲的倍频误差

从理论上推导了第Ⅱ类相位匹配下宽带飞秒脉冲的二次谐波光场,分析输入飞秒脉冲的非共线相位匹配方式、脉冲带宽引起相位失配与群速失配对测量的影响。结果表明,为了消除飞秒脉冲的带宽影响,需要对测量记录的光强乘以一个调制因子;测量相位误差与非共线相位匹配的夹角和晶体长度成正比;相位失配与群速失配产生相位测量误差,且第Ⅱ类相位匹配方式下脉冲附加相位值较大;强度和相位误差需要在脉冲重建结果中补偿。     

Indirect torque control of switched reluctance motors using iterative learning control

This paper proposes the use of iterative learning control (ILC) in designing a torque controller for switched reluctance motors (SRMs). The demanded motor torque is first distributed among the phases using a torque-sharing function. Following that, the phase torque references are converted to phase current references by a torque-to-current converter and the inner current control loop tracks the phase current references. SRM torque is a highly nonlinear and coupled function of rotor position and phase current. Hence, the phase current references for a given demanded torque can not be obtained analytically. Assumption of linear magnetization characteristics results in an invertible torque function. However, the nominal phase current references obtained using this torque function will lead to some torque error as motor enters into magnetic saturation. For a constant demanded torque, the error in the phase current references will be periodic with rotor position. Hence, we propose to use ILC to add a compensation current to the nominal phase current references so that torque error is eliminated. Similarly, current tracking for the nonlinear and time-varying system is achieved by combining a simple P-type feedback controller with an ILC controller. The proposed scheme uses ILC to augment conventional feedback techniques and hence, has better dynamic performance than a scheme using only ILC. Experimental results of the proposed scheme for an 8/6 pole, 1-hp SRM show very good average as well as instantaneous torque control.     

Δk-radar equivalent of interferometric SAR's: a theoreticalstudy for determination of vegetation height

Theoretical aspects of estimating vegetation parameters from SAR interferometry are presented. In conventional applications of interferometric SAR (INSAR), the phase of the interferogram is used to retrieve the location of the scattering phase center of the target. Although the location of scattering phase center for point targets can be determined very accurately, for a distributed target such as a forest canopy this is not the case. For distributed targets the phase of the interferogram is a random variable which in general is a function of the system and target attributes. To relate the statistics of the interferogram phase to the target attributes, first an equivalence relationship between the two-antenna interferometer system and an equivalent Δk radar system is established. This equivalence relationship provides a general tool to related the frequency correlation function (FCF) of distributed targets, which can conveniently be obtained experimentally, analytically, or numerically, to the phase statistics of the interferogram. An analytical form for the p.d.f. of the interferogram phase is obtained in terms of two independent parameters: 1) ζ: mean phase and 2) α: degree of correlation. ζ is proportional to the scattering phase center and n is inversely proportional to the uncertainty with which ζ can be estimated. It is shown that α is directly related to the FCF of the distributed target which in turn is a function of scattering mechanisms and system parameters. It is also shown that for a uniform closed canopy the extinction and the physical height of the canopy top can be estimated very accurately. Some analytical and numerical simulations are demonstrated     

Visual Cryptography Based on an Interferometric Encryption Technique

This paper presents a new method for a visual cryptography scheme that uses phase masks and an interferometer. To encrypt a binary image, we divided it into an arbitrary number of slides and encrypted them using an XOR process with a random key or keys. The phase mask for each encrypted image was fabricated under the proposed phase‐assignment rule. For decryption, phase masks were placed on any path of the Mach‐Zehnder interferometer. Through optical experiments, we confirmed that a secret binary image that was sliced could be recovered by the proposed method.     

Performance Optimization of High Order QAM in Presence of Phase Noise and AWGN: Application to a Decision Directed Frequency Synchronization System

The paper presents an analysis of the impact of Local Oscillators (LO) phase noise on the performance of digital transmission systems. Using an efficient phase noise model, we study degradations induced by phase noise and Additive White Gaussian Noise (AWGN) on a carrier recovery system combined to high order Quadrature Amplitude Modulations (QAM). Simulation results show that an optimum trade-off between AWGN and phase noise robustness for the loop bandwidth can be determined. In order to compare simulated and analytical performance, a new general expression of the bit error probability is developped for square QAM in the presence of phase noise over Gaussian channel. Furthermore, novel decision areas related to Quadrature Amplitude Modulations are determined in order to improve the performance of the carrier recovery algorithm in the presence of phase noise and frequency offset.