Quadrature Sampling with High Dynamic Range

Many radio and sonar systems require signal outputs in complex low-pass form. To achieve this, it is possible to use uniform sampling of the bandpass signal, together with computation of the quadrature component by way of a Hilbert transform. The bandpass to low-pass translation is accomplished by undersampling. A hardware implementation is described which achieves 70 dB spurious-free dynamic range and a bandwidth of 30 kHz.     

Sampling Bandpass Signals

In a recent paper [1], Brown examined the sampling of a real finiteenergy bandpass signal having an (angular) bandwidth ? (in radians per second) at the theoretically minimum (average) rate of ?/? samples per second. Following Grace and Pitt's [2] quadrature sampling, a particular case of Kohlenberg's second-order sampling [3, 4], Brown has proved the feasibility of a separate interpolation of the in-phase and quadrature components of the signal when ?o = k?/2 (Brown's condition), where ?o is the center (angular) frequency of the signal and k is an arbitrary positive integer. Here the problem is reconsidered from a general point of view, introducing, under Brown's condition, an interpolation formula which includes that of Grace and Pitt and which extends a theorem of Populis [5-7]. We indicate the necessary and sufficient conditions to obtain a separate interpolation, offering closed formulas to obtain the interpolation functions. We also discuss the minimum oversampling rate needed whenBrown's condition is not verified.     

Output Signal-to-Noise Ratios for Amplitude-Modulated Noise-Like Lasers

A general expression of the output SNR of a photodetector is derived for a noise-like laser amplitude-modulated by a stationary Gaussian random modulating signal in the presence of a background light. The electric field Vx(t) of the noise-like laser is assumed to be a stationary narrowband Gaussian random process with zero mean. Two types of modulating signal are considered, the baseband and bandpass modulating signals. More specifically, the effects of the center frequency of the modulating signal, the modulating degree, the bandwidth ration of the noise-like laser to the modulating signal, the effective average quantum rate, and input CNR on output SNR are studied. The detection characteristics of the noise-like laser are also made clear by comparison with the case of a coherent laser.     

Probability of Error in a Bandlimited Quadriphase Communication System

The probability of error is computed in a quadriphase communication system, with a causal bandlimiting bandpass filter. Intersymbol interference in each channel and intersymbol interference from the neighboring channel is evaluated. The effect of nonoptimal sampling time on the probability of error is presented.     

The Detection Performance of the Siebert and Dicke-Fix CFAR Radar Detectors

The Siebert and the Dicke-fix CFAR radar detectors, used to maintain a constant false alarm rate (CFAR) in radar receivers under very similar circumstances, are considered. The Siebert detector represents the maximum-likelihood detection procedure for a signal in Gaussian noise of unknown power level, whereas the Dicke-fix makes use of a bandpass limiter to normalize the input and thus ensure a constant false alarm rate. The detection performance of the two detectors is determined and a comparison shows that over a wide range of parameters, the Dicke-fix introduces a loss which is approximately 1 B larger than for the Siebert detector.     

Output Signal-to-Noise Ratio for Amplitude and Intensity Modulated Laser

General expressions are obtained for output SNR for both amplitude- and intensity-modulated lasers, where bandlimited Gaussian noise has been chosen as a modulating signal, and in the presence of background light. Two types of modulating signals are considered: the baseband and bandpass modulating signals. Detailed calculations are made for output SNR when an ideal narrowband optical filter is used. The dependence of output SNR on several parameters, such as the center frequency of the modulating signal, the effective average quantum rate, and input SNR, are discussed. In addition, the difference in performance between amplitude and intensity modulation is discussed. The detection characteristics of the homodyne system are also considered.     

The Response of Hard-Limiting Bandpass Limiters to PM Signals

The signal-to-noise ratio of the output of a hard-limiting bandpass limiter to a PM signal is calculated by using the probability density function of the random phase variable. The signal-to-noise ratio transfer characteristics are plotted for comparison.     

Optimal and Suboptimal Perfomance of a PCM/FM Communication System

Theoretical results are presented on probability of error in the detection of binary FM signals for various frequency deviations with fixed bit rate, restricted predetection filter bandwidth, ideal limiterdiscriminator reception, and integrate-and-dump postdetection filter decision. The results show that suboptimal system performance can be achieved by choosing the largest possible value of deviation index which lies approximately at 0.7 + 2n, where n = 0, 1,...,k. k is determined by the reconstructability of signal after the bandpass filter.     

欠采样信号多采样频率处理方法

讨论了带通信号 A/D 转换中的欠采样问题,通过引入余数定理,给出了一种使用多采样频率的欠采样解决方案。以模数转换为例阐述欠采样问题的解决思路,给出了一些结论。     

An Analysis of the Performance of the Oscillating Limiter Driven by FM Signals Corrupted by Noise

The behavior of the oscillating limiter (OL) driven by FM signals is surveyed, and its performance with signal corrupted by noise is investigated. For high values of the carrier-to-noise ratio (CNR), if the frequency deviation of the signal is small in comparison with the locking range of the OL, it is calculated, and experimentally verified, that a system OL discriminator is equivalent to a system bandpass limiter discriminator followed by a linear network whose frequency response has been specified. When the frequency deviation is not so small, the baseband noise power increases with it; a formula is given that allows the calculation of this power when the signal is such that the circuit operates in quasistationary fashion. For low values of the CNR, a mathematical analysis presents unsurmountable difficulties. However, heuristic argumentation leads to an interpretation of the operation of the OL in the threshold region, which is substantiated by an experimental investigation. The results of this paper enable a comparative evaluation of a system OL discriminator and a system bandpass limiter discriminator, to which the former reduces when the feedback path in the OL is open.     

The Effect of Hard Limiting an Angle-Modulated Signal Plus Noise

The effect of hard limiting an angle-modulated signal plus narrow-band Gaussian noise is analyzed. Several examples are considered?sinusoidal angle modulation, Gaussian angle modulation, and biphase angle modulation. The general conclusion is that when a zonal band-pass filter is used, which rejects dc and second harmonics, an angle-modulated signal plus Gaussian noise provides the same output signal-to-noise ratio as shown by Davenport for a CW signal plus Gaussian noise. However, when a narrow bandpass filter is used, which has a bandwidth approximately equal to the input angle-modulated signal, an angle-modulated signal plus Gaussian noise has a better output signal-to-noise ratio than a CW signal plus Gaussian noise.     

Nonlinearity Model with Variable Knee Sharpness

Soft limiter models used in the past do not provide a convenient way of changing the knee sharpness while maintaining small signal linearity. A new model is described which does this. Both instantaneous characteristics and bandpass limiter characteristics are discussed. The model is also suitable for analyzing and simulating compressors, expanders, and detectors.     

The Calculation of Frequency-Modulation Distortion in Linear Networks

A numerical method is developed for the calculation of steady-state FM distortion in a linear passive network from the character of the FM signal input and the steady-state transfer characteristics of the network. The method of calculating FM distortion requires the solution of a finite set of linear equations which is accomplished readily by a digital computer. To illustrate the use of the method, the FM distortion introduced by a Chebyshev-response bandpass filter is calculated for a range of network and input signal parameters.     

Robust SVA method for every sampling rate condition

Linear apodization, or data weighting, is the traditional procedure to improve sidelobe levels in a finite sampled signal at the expense of resolution. New apodization methods, such as spatially variant apodization (SVA), apply nonlinear filtering to the signal in order to completely remove sidelobes without any loss of resolution. However, the results are strongly influenced by signal sampling rate. Some variations which improve results have been previously published, but sidelobe cancellation gets worse since sampling frequency is not settled at Nyquist (or a multiple). This paper presents a new and efficient technique based on SVA that drastically reduces sidelobe levels for every sampling rate condition. The algorithm is, essentially, a parameter optimization of a variant filter for each pixel of the image. A one-dimensional case and a two-dimensional generalization are presented, as well as some applications to target detection capability in a synthetic aperture radar (SAR) system.     

动态数据建模中时间序列采样间隔的研究

动态数据建模认为,平稳连续随机过程是白噪声激励相应的线性时不变连续时间系统的输出,为便于计算机处理,需要确定对连续随机过程进行采样的采样间隔,通常人们采用仙农采样定理来对待。文中阐述了对动态数据建模中时间序列采样的问题,用多项式插值的采样理论进行了研究,指出在动态数据建模中,若要使从连续时间随机过程到离散时间序列的统计特性得到保持,采样频率应大于系统最大极点自然频率的6倍。     

计数法测量采集速率的不确定度评价

介绍了用标准周期计数法评价线性测量系统采集速率时，测量结果的不确定度分析和评价过程；讨论了影响评价结果不确定度的几个主要来源；给出了减小采集速率评价不确定度的主要措施，通过一组实例，给出了采集速率不确定度分析和评价结果，该过程及结论可应用在对于计量标准进行相应指标的不确定度分析上，也可用于估计采集速率指标本身的不确定度。     

Probability Density Function at the Output of the Delay-Lock Discriminator

The first-order probability density at the output of the correlation network of the delay-lock discriminator is determined using the methods developed by Arthur, Kac, and Siegert. The analysis assumes that the input signal is stationary and Gaussian and that the discriminator is locked on. No restrictions are made on the bandpass filter characteristics in the channels.     

On detection using filter banks and higher order statistics

We suggest a method, based on the use of filter bank and higher order statistics (cumulants), for detection of transient signals. The method first uses a bandpass filter bank, which separates the spectrum of the observed signal into narrow frequency bands. Each subfilter of the filter bank is then followed by a cumulant estimator, and thereby suppressing colored noise. By selecting those subfilters that have large output energies, the filter bank can approximate the behavior of a matched filter. Moreover, no a priori information about the waveform of the signal is needed. The performance of the detector is evaluated by using a simulated signal as well as a measured signal. The presented detector is compared with the optimal matched filter detector.     

Adjacent Channel Interference in a Binary Bandpass Communication System

The effect of adjacent channel interference on the probability of error in a binary bandpass communication system with an integrating and dumping detector is investigated. Narrowband filters are assumed in the receiver of the main signal and transmitters of both main and interfering signals. Plots of the probability of error as a function of signal to noise ratio in the main channel or as a function of carrier frequency difference between the main and interfering signals are presented, assuming that the filters are of the Butterworth type. These figures are helpful in the selection of minimal frequency spacing of adjacent channels.     

Analysis of Squaring Circuit Mechanizations in Costas and Squaring Loops

Two different implementations of the carrier recovery loop for suppressed carrier binary phase-shift keyed (BPSK) signals are considered. In particular, the tracking performance of the squaring loop in which the squaring circuit is mechanized by a limiter/multiplier lier combination to produce the absolute value of the incoming signal, followed by appropriate bandpass filtering, is compared with that of the usual squaring loop whose squaring circuit is the timestwo wo multiplier which exhibits a square law characteristic.