Two Loop Adaptive Delta Modulation Systems

Two adaptive delta modulation (ADM) coders, one for A/D conversion of speech and the other for fiat spectrum signals, have been developed by incorporating a second adaptive feedback loop around the basic delta modulation (DM) coder. The dynamic range of the coder for integrated spectrum has been extended to more than 35 dB with a signal-to-noise ratio (SNR) > 28 dB, for a pulse-repetition frequency (PRF) of 32 kbits/s only, by using an adaptive circuit consisting of digital/analog slope detector and an FET expander. For flat spectrum signals, similar results with constant SNR over the band are obtained at a bit rate of 40 kbits/s. Thus the coders have performances equivalent to those of a 7-digit PCM, using only 32 and 40 kbits/s, respectively.     

Instrumentation for measurement of the short-term frequency stability of microwave sources

The short-term frequency stability of microwave sources employed as carrier frequency generators in coherent radar systems is often expressed in terms of the power spectrum of the microwave signal. It may also be expressed in terms of the equivalent modulation spectrum that would produce the microwave signal spectrum. This paper discusses apparatus capable of measuring the modulating signal spectrum corresponding to a sideband-to-carrier level of -115 dB/Hz at 1000 Hz and greater from the carrier frequency at X-band. A passive reference is employed, although use of an active reference source is considered. Two stability measuring instruments are discussed, one utilizing a reflection cavity resonator and the other a transmission cavity resonator. The latter unit provides a number of significant features; for example, the capability of direct calibration, direct measurement of the limiting sensitivity of the instrumentation, and measurement of intentional frequency modulation of the source under observation.     

Line tracking in nonstationary processes

Semistationary processes as defined by Priestley possess an evolutionary spectrum, the discrete component of which is used to define nonstationary processes possessing a modulated line spectrum, i.e. lines modulated by complex-valued functions of time which can be interpreted as amplitude and phase modulation. It is shown (1) that the derivative of an appropriately modified version of the phase modulation of a line exists, is continuous and can therefore be defined as an instantaneous frequency, (2) that the modulation functions can be estimated using complex demodulation. Using the estimated phase modulation, estimates of the instantaneous frequency are obtained by using a linear spline regression with unknown nodes, (3) that the estimate of a modulated line spectrum is unique up to an error ? depending on the possible time-frequency resolution, (4) that if the underlying process possesses a modulated line spectrum and if all the modulation functions are band-limited, then the process can be exactly reconstructed from its sampled version (i.q.m.).The successful application of the estimation procedure is shown by an example dealing with the problem of detecting of instabilities and transient behaviour of the internal clock of organisms which plays a role for instance, if organisms have to be adapted to new time conditions (intercontinental flights, shiftwork, seasonal changes, etc.).     

Frequency stabilisation of AlGaAs lasers to absorption spectrum ofrubidium using Zeeman effect

Describes an optical reference signal obtained by frequency locking a AlGaAs laser to the absorption spectrum of rubidium. The rubidium absorption spectrum is frequency-modulated by an AC magnetic field using the Zeeman effect. This technique results in an optical reference signal which is free from undesired modulation     

BOC调制导航信号关键技术研究

BOC调制作为一种导航信号体制,具有良好的抗多径和抗干扰性能,同时实现共用载波频率条件下的频谱分离。与BPSK调制信号相比,BOC调制信号具有多径误差小、码跟踪热噪声误差小的优点。通过对BOC调制技术和合路方式的深入分析,给出了各种BOC调制的自相关和功率谱函数,以及各种合路方式的分析比较。通过仿真验证了BOC信号的码跟踪性能和抗多径性能的优越性。     

垂直腔面发射激光器的内调制特性

面向芯片原子钟(Chip Scale Atomic Clock,CSAC)的垂直腔面发射激光器(Vertical Cavity Surface-Emitting Laser,VCSEL)通过微波调制产生具有特定光频差的相干激光,与原子作用后的跃迁谱线频率作为参考标准,最终可获取高精度的频率信号。因此,垂直腔面发射激光器在芯片原子钟系统中至关重要。介绍了VCSEL激光器的内调制原理,搭建了其内调制特性实验测试平台,开展了激光器对射频调制响应特性研究,记录了激光器边带信号随着注入电流和射频输出功率的变化情况,并分析了射频调制对激光器边带信号的影响特性以及Bogatov现象引起的边带不对称现象。实验结果显示:当射频信号频率为3.41734 GHz,注入电流为1.2 mA,射频输出功率为3.5 dBm时,可获得优化的高频调制光谱,为芯片原子钟提供优质的光源。     

基于谱估计的噪声调频信号检测方法研究

对干扰信号的频谱特征进行精确测量和分析是实施有效的电子反干扰(ECCM)的重要步骤.通过对噪声调频干扰信号的频谱特征分析,提出采用合适的功率谱估计方法来检测噪声调频信号的主要频谱特征参数.利用MATLAB/Simulink仿真软件分别建立了噪声调频信号源和谱估计方法的系统模型,研究了系统采用不同的谱估计方法和参数设置的情况下所能够达到的检测精度,并根据仿真结果分析比较了不同谱估计方法的性能.     

Analysis of delta PWM static AC-DC converters

An analysis and implementation of delta modulation (DM) technique in the control of AC-DC converters are presented in this paper. The DM technique offers advantages of easy implementation, continuous converter voltage control, and a direct control on the line harmonics. A simple model of the delta modulator is proposed to predict switching frequency, identify dominant harmonics, and determine switching points of the converter waveforms. Results of parametric variations of the modulator and performances of the single and three phase DM converters are reported. Experimental results of the single and three phase DM AC-DC converters validate the analytical results     

Comparison of different fast modulation techniques for FIR laser stabilization

In the first part of this paper the characteristics of frequency deviation and modulation bandwidth required for phase-locking optically pumped far infrared lasers to reference synthesized signals are examined. Then different fast modulation techniques suitable for this purpose are discussed pointing out their particular features. Experimental results concerning laser frequency modulation by Stark effect, injection locking and electrically variable reactances (e.g Schottky diodes, plasma cell) are reported and compared.     

Estimation of the Signal-to-Noise Ratio in High Information and Constant Factor Delta Modulation Systems

A "statistical average step size," γav, which is a function of the rms power of a band-limited white Gaussian input signal, is defined for high information and first order constant factor delta modulation systems. The signal-to-noise ratios (SNR) are then calculated for these instantaneously adaptive delta modulation systems by substituting γavfor the fixed step size in the formulas for quantization (i.e., granular) and slope-overload noise applicable to linear (single integration) delta modulation systems. The calculated SNRs are in close agreement to those obtained by computer simulations. As γavis difficult to calculate exactly, an empirical expression is found which is applicable to different encoding conditions. This empirically formulated γavcan be used to estimate SNR to an acceptable accuracy for most practical situations. A noise component not present in linear delta modulation is identified and called "overshoot noise" and the cross-correlation ρxebetween input and error signals is studied. The SNR and ρxefor linear delta modulation are presented as reference bases.     

A single-phase delta-modulated inverter for UPS applications

The performance characteristics of the rectangular wave delta modulation (RWDM) scheme for uninterruptible power supply (UPS) applications is investigated. Normalized characteristic curves that show the effect of various modulator parameters on the frequency spectrum of the inverter output voltage are obtained using discrete Fourier transform (DFT) and harmonic analysis techniques. The performance of a single-phase half-bridge inverter with an LC filter is discussed, and experimental results are provided to validate the predicted and simulated results. It is shown that the harmonic content of the inverter output waveform can be controlled through the control of the modulator parameters     

雷达发射机辐射发射测试研究

简要介绍了所测雷达的发射机组成结构和发射机辐射发射的测试方法。结合实装测试数据,详细分析了雷达在三种不同调制方式下频率综合器输出、固态放大器输出和速调管耦合输出的信号频谱特征,为建立雷达发射机电磁兼容预测模型提供有益的参考。     

宽带无线OFDM系统调制算法的仿真分析

为了根据信道条件的变化,动态地选择适当调制方式,以便最大限度地提高频谱利用率,文章采用正交频分复用（OFDM）的多载波技术,提供了在OFDM子载波上MQAM、MPSK等调制方式的仿真测试,得出了MQAM的容噪性能星座图和误比特率曲线,MPSK和MQAM误比特率随M取值的变化情况。这些分析结果为在第四代移动通信系统中,根据不同信噪比环境选择合适的调制方式提供了参考依据。     

Multidimensional complex signals with single-orthant spectra

An extension of the notion of the analytical signal to multidimensional signals is presented. The real and imaginary parts of this signal are a linear combination of the original signal and of its complete and partial Hilbert transforms. Its Fourier image exists only in one orthant of the multidimensional frequency space. The orthant is a half-axis in one dimension, a quadrant in two dimensions, an octant in three dimensions, etc. A multidimensional complex signal makes it possible to introduce the definitions of instantaneous amplitude, instantaneous phase, and partial instantaneous complex frequencies (partial derivatives of the instantaneous phase) and to formulate a modulation theory of a multidimensional harmonic carrier. The 2-D equivalent of 1-D single-sideband modulation is defined and called single quadrant modulation. It is shown that the multidimensional complex signal with a signal orthant spectrum may be defined as a convolution of the real signal with the multidimensional complex delta distribution     

Prime sequences: quasi-optimal sequences for OR channel code division multiplexing

A new family of multiplexing sequences suitable for low duty cycle pulse position modulation schemes and for frequency hopped stread spectrum systems is introduced. The sequences have maxima in their crosscorrelation functions of two or one.     

Power spectrum of directly modulated single-mode semiconductor lasers: Chirp-induced fine structure

The time-averaged power spectrum of a single-mode semiconductor laser under direct current modulation is calculated using the small-signal analysis of the stochastic rate equations. The general formalism includes the effect of frequency chirping owing to current modulation as well as the effect of phase diffusion related to spontaneous emission. Chirp-induced dynamic line broadening is shown to result from a superposition of the additional unresolved sidebands generated at multiples of the modulation frequency on both sides of the optical line. The effect of intensity modulation is to introduce an asymmetry such that the low-frequency sideband has a relatively higher amplitude. The theory explains reasonably well the reported experimental observations. At the same time it provides physical insight into the dynamic process of frequency chirping and its relation to the frequency modulation. In particular, the frequency and the decay rate of transient relaxation oscillations govern the interplay between simultaneously occurring intensity and frequency modulations.     

Delta modulation strategies for resonant link inverters

The suitability of delta modulation strategies for resonant link inverters is explored in detail. The control and spectral characteristics of various delta modulators are examined in an effort to develop a model for the same. The more fundamental question of comparing delta modulation systems with conventional pulse-width-modulated (PWM) systems, given similar devices, is addressed, and it shown that the former provides a viable high-performance alternative. As the resonant DC link topology allows at least an order of magnitude increase in the switching frequency, ΣΔM systems using a given family of devices are capable of better spectral response than a conventional PWM inverter     

Crosstalk in 1.5-μm InGaAsP optical amplifiers

A dynamical model for multichannel amplification by near-traveling-wave optical amplifiers is presented, and results on crosstalk induced by either amplitude modulation or frequency modulation are given. The mechanisms influencing the crosstalk most are the residual facet reflectivities and the detuning of the channels relative to the amplifier Fabry-Perot spectrum. Calculations of worst-case crosstalk levels are included. The model is verified experimentally for amplitude-modulated signals, and crosstalk levels up to -7 dB are reported. For frequency-modulated signals, estimated crosstalk is significantly lower and can be reduced by high quality facet coatings     

频率调制光谱稳频法的信号处理算法研究

频率调制光谱稳频法利用参考气体对激光吸收量来反馈控制激光波长稳定,反馈信号的提取通过调节参考信号与吸收分量同相的方式实现.为免除移相的麻烦,提出利用吸收分量和色散分量的合成信号作为反馈信号,通过仿真证明可以利用合成信号作为反馈信号.该合成信号的提取利用两正交参考信号与合成信号耦合的方式实现,该方式方便了信号的提取,免除了调相的麻烦.最后还提出一种判定激光波长偏移的方法.     

Rigorous assessment of small-signal analysis for linear anddispersive optical communication systems operating near zero-dispersionwavelength

This paper presents a rigorous small-signal theory for linear single-mode fibers taking into account the first- and second-order fiber dispersion. From this theory, exact small signal intensity modulation (IM) and frequency modulation (FM) fiber transfer functions are presented. Exact expressions of the intensity and frequency noise spectra at fiber output due to laser noise taking into account all Langevin noise terms are also derived. Accurate numerical simulations of the IM and FM fiber transfer functions, and intensity and frequency noise spectra after linear transmission along single-mode fiber are compared with theoretical predictions and very good agreement is achieved. In addition, the theoretical predictions are compared with other author's results and the discrepancies are thoroughly explained. A new expression for the transmission distance which can lead to further significant reduction of intensity noise spectrum in systems using single-mode lasers with reduced linewidth enhancement factor is presented. The theoretical and simulation results show that the magnitude of the small-signal IM and FM fiber transfer functions, and the intensity and frequency noise spectra after linear single-mode fiber transmission are not affected by second-order fiber dispersion. The theory indicates that second-order fiber dispersion solely introduces delay distortion in the IM and PM fiber transfer functions. So, with linear transmission second-order dispersion effects on the intensity and frequency noise have proved to be irrelevant even for very long broad-band fiber systems