A Semi-Empirical Approach to the PLL Threshold

This correspondence considers the response of the PLL near threshold to an input consisting of a modulated carrier and white, Gaussian noise. For high input signal-to-noise power ratios ?, the output noise power is Gaussian with a parabolic spectrum. As ? is decreased, the PLL tends to lose lock which gives rise to impulses or ?spikes? in the output with a resulting white power spectrum. The additional output noise due to these ?spikes? causes a threshold in the output signal-to-noise ratio. Unfortunately the loss of lock rate in the PLL depends on the modulation as well as the noise power. A semiempirical approximate expression for the loss of lock rate as a function of the noise and sinusoidal frequency modulation is presented and is used to determine the optimum design procedure for PLL's to demodulate FM signals of varying modulation indexes, ?.     

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.     

Adaptive Radar Beacon Forming

Propagation errors along paths between an array radar and a distribution of targets cause degradations in angle measurements and detection range. The overall objective of the research described in this paper was to analyze and demonstrate the use of conjugate reflections for compensating adverse effects of path errors. The effect of reflecting the conjugate of an incident wave is described mathematically and is demonstrated by computer simulation. Repeated conjugate reflections are shown to result in the formation of a single beam usually focussed on a target highlight. Echoes from this spatial reference, or "beacon" are shown to provide the means by which aperture phase errors may be effectively compensated. Results of radar simulations include two-way patterns computed for an example involving a distribution of three-point targets and half-wave-length Gaussian aperture errors. Without compensation a gain loss of 12 dB is computed; with error correction, based on echoes from an adaptively focussed beam, the two-way pattern is within a small fraction of a dB of the ideal pattern. The effect of noise on adaptive beacon forming was considered for a case involving one target. Repeated conjugate reflections improve signal-to-noise ratio as long as the effect of noise is less than the effect of aperture dephasing on the power reflected back to the target. An example is presented in which signal-to-noise ratio at the output of the receiver combining network is increased from 4 to 11.8 dB.     

Mismatched Filtering of Sonar Signals

A replica correlator (matched filter) is an optimum processor for a receiver employing a pulse of continuous wave (CW) signal in a white Gaussian noise background. In an active sonar, however, when the target of interest has low Doppler shift and is embedded in a high reverberation background, this is not so. High sidelobes of the correlator frequency response pass a significant portion of the signal contained in the mainlobe of the reverberation spectrum. In order to reduce the sidelobes of the correlator output spectrum and at the same time keep the increase in its 3 dB bandwidth to a small amount, we propose lengthening of the replica of the transmitted signal and weighting it by a Kaiser window. It is demonstrated that by extending the weighted replica by 50 percent compared with the transmitted signal, it is possible to reduce the sidelobe levels to at least 40 dB below the mainlobe peak, with the concomitant increase of the 3 dB band-width by less than 5 percent. The degradation in signal-to-noise ratio (SNR) performance for such a ?mismatched? filter receiver with respect to the matched filter is less than 1.5 dB.     

北斗导航接收机频率稳定度传递算法

为了提高北斗导航接收机的灵敏度,提升其弱信号跟踪能力,通常需要利用长时间的相干积分来提高环路信噪比。但是,当相干积分时间加长到一定程度时,环路性能反而有所下降,信噪比提升也不能达到理论值。针对由剩余频率误差和晶振误差引起的相干积分能量损失问题,主要研究了频率偏差对环路跟踪性能的影响,并提出了利用频率稳定度传递策略辅助弱信号跟踪的方法,解决了北斗导航接收机弱信号跟踪性能提升的问题,最大程度地改善了相干积分的效果,实现了对弱信号的跟踪。利用软件接收机平台对提出的频率稳定度传递算法进行验证,仿真结果表明该算法可使环路信噪比提升4dB ~5dB,充分说明了其可行性及有效性。     

Error Probabilities in PCM/FM with Phase-Lock Loop Discriminators

Previous literature has analyzed PCM/FM telemetry systems using idealized FM demodulators at the receiver. However, in many systems operating today, phase-lock loop demodulators are employed which depart noticeably from ideal operation as the loop falls ?out of lock.? In this paper an approximate expression for error probability is derived, which clearly indicates the effect of the loop on the previously published data using ideal discriminators. It is shown that the presence of the loop becomes apparent at input signal-to-noise ratios below approximately 3.5 dB, while the amount of error increase over the ideal case depends, to a large extent, upon the shape of IF filtering being used.     

Dynamics of the Phase-Locked Loop without a Limiter

The general (nth order) phase-locked loop is analyzed, of which the amplitude is not constant. The input carrier signal is amplitude-modulated by wide-band stationary Gaussian noise, and the signal, superposed with the additive white stationary Gaussian noise, enters the nonlimited phase-locked loop. Under the above assumptions the loop can be shown to constitute an n-dimensional vector Markov process, so that the process satisfies the n-dimensional Fokker-Plank equation. The probability density function depends on the effective loop signal-to-noise ratio and the effective modulation power.     

一种改进的直方图概率多假设多目标跟踪方法

直方图概率多假设跟踪（H-PMHT）方法及其变形泊松分布直方图概率多假设跟踪（P-HPMHT）方法的一个主要缺点是其量测模型仅考虑了背景杂波而没有考虑传感器噪声,从而导致在低信噪比条件下检测概率较低。针对这一问题,提出了一种带传感器噪声模型的H-PMHT方法,通过将传感器噪声引入量测模型,从而明显提高了对低信噪比目标的跟踪检测能力。该方法的计算量与目标数保持线性关系,仍然适用于目标数目较多的情况。仿真实验表明：该方法在误跟踪比率为1‰,信噪比为6 dB时,检测比率可提升近20%,信噪比为3 dB时,可提升近10%。     

大气湍流对APD直接检测接收机抗噪声性能影响

针对以往分析湍流信道对系统抗噪声性能影响的不足,给出了一种基于APD直接检测接收机的湍流噪声的处理方法.在APD直接检测接收机模型的基础上,推导了其信噪比和曼彻斯特编码系统的差错概率,并详细分析了湍流对接收机的抗噪声性能影响.结果表明,增大接收天线孔径可以减弱湍流的影响,但在天线孔径较小时,增加发射功率并不能有效地提高...     

Carrier loop architectures for tracking weak GPS signals

The performance of various carrier recovery loop architectures (phase lock loop (PLL), Doppler-aided PLL, frequency lock loop (FLL), and Doppler-aided FLL) in tracking weak GPS signals are analyzed and experimentally validated. The effects of phase or frequency detector design, oscillator quality, coherent averaging time, and external Doppler aiding information on delaying loss of lock are quantified. It is shown that for PLLs the metric of total phase jitter is a reliable metric for assessing low C/N performance of the tracking loop provided the loop bandwidth is not too small (~> 5 Hz). For loop bandwidths that are not too small, total phase jitter accurately predicts carrier-to-noise ratio (C/N) at which loss of lock occurs. This predicted C/N is very close to the C/N predicted by bit error rate (BER). However, unlike BER, total phase jitter can be computed in real-time and an estimator for it is developed and experimentally validated. Total phase jitter is not a replacement for BER, since at low bandwidths it is less accurate than BER in that the receiver loses lock at a higher C/N than predicted by the estimator. Similarly, for FLLs operating at small loop bandwidths, it is found that normalized total frequency jitter is not a reliable metric for assessing loss of lock in weak signal or low C/N conditions. At small loop bandwidths, while total frequency jitter may indicate that a loop is still tracking, the Doppler estimates provided by the FLL will be biased.     

Spectrum Distortion Due to Frequency Instabilities in a Two-Way Coherent Doppler Radar

The power spectral density of the intermediate frequency signal in a coherent Doppler navigation radar is derived. The effects of antenna parameters, periodic frequency instabilities, signal two-way transit time, and transmitter frequency modulation noise are considered Several examples based on the measured frequency modulation noise of a solid-state source transmitter are presented. The results indicate the degree of loss in signal-to-noise ratio, and spectrum broadening due to an increase in signal transit time and/or frequency modulation noise.     

Computer Optimization of FM/FM Telemetry Systems

In this correspondence, the threshold carrier power of FM/FM systems and the receiver IF bandwidth are optimized through a computer program. For a typical set of IRIG (inter-range instrumentation group) channels, it is demonstrated that the required carrier signal-to-noise ratio can be as low as 5 dB (much less than the conventional 12 dB value) in order to maintain the subcarrier signal-to-noise ratio at 12 dB, the threshold level. comparison of the optimized and the conventional taper analysis is tabulated.     

Interferences in Phase-Locked Loops

An analysis of the behavior of a second-order phase-locked loop is presented when an unwanted signal is added to the useful signal. Both signals are sinusoidal and unmodulated, and the analysis is made in the absence of additive noise. When the loop remains locked on the useful signal, a parasitic signal exists at the phase detector output. This signal produces a parasitic phase modulation of the VCO and a static phase error in the loop. The parasitic signal amplitude, the parasitic phase modulation index, and the static phase error are calculated. A necessary condition for the loop to remain in lock is derived. When the loop is initially unlocked, locking can occur either on the useful signal or on the unwanted signal, depending on the amplitude ratio and the frequency difference of the two signals. A formula allowing one to compute the pull-in time is obtained. When the loop locks on the useful signal, acquisition can be slower or faster than in the absence of an unwanted signal. The same phenomenon is observed when the loop locks on the unwanted signal.     

The Effect of Integrator Pole Position on the Performance of an Adaptive Array

The LMS adaptive array requires an integrator in each weight feedback control loop. In practice the integrator is often replaced by a low-pass filter, i.e., by a filter with a single pole at s = - ? (where s is complex frequency). The effect of this pole position on array performance is examined. It is shown that to obtain optimal performance from the array, ? must be less than k?2, where k is the loop gain and ?2 is the thermal noise power per element. When at exceeds k?2, the output signal-to-inter ference-plus-noise ratio from the array is degraded for intermediate values of interference power.     

ECCM capabilities of an ultrawideband bandlimited random noise imaging radar

Investigated here is high-resolution imaging of targets in noisy or unfriendly radar environments through a simulation analysis of the ultrawideband (UWB) continuous-wave (CW) bandlimited random noise waveform. The linear FM chirp signal was selected as a benchmark radar waveform for comparison purposes. Simulation of the recovery of various types of target reflectivity functions (TRFs) for these waveforms were performed and analyzed. In addition, electronic counter-countermeasure (ECCM) capabilities for both types of systems were investigated. The results are compared using the error between the interference (jamming)-free recovered TRF and the recovered TRF under noisy conditions as a function of the signal-to-interference/jamming ratio (SIR/SJR). Our analysis shows that noise waveforms possess better jamming immunity (of the order of 5-10 dB improvement over the linear FM chirp) due to the unique radar correlation processing in the receiver.     

Phase-Locked Loop Behavior near Threshold

The phase-locked loop behavior is analyzed following the quasilinearization Booton's method. When the loop is locked on an unmodulated input signal with a static phase error, the phase detector nonlinearity produces an interaction between the static phase error and the voltage-controlled-oscillator (VCO) noise phase fluctuations. Formulas allowing one to compute the static phase error increase and the VCO phase variance increase are derived. When the input signal is phase modulated, there is an interaction between the static phase error, the VCO noise phase fluctuations, and the input signal phase modulation. Formulas are obtained that allow one to compute the loop loss of performances (static phase error increase and VCO phase variance increase) and the coherent phase demodulator output signal-to-noise ratio decrease. Finally, a slight modification to Booton's procedure is proposed, leading to results in better agreement with experimental data.     

FSO链路中基于平均信噪比的自适应功率传输技术

文章提出了 1种基于平均信噪比(Signal-to-Noise Ratio,SNR)的自适应功率传输技术,旨在补偿大气湍流引起的闪烁效应,以提升自由空间光(Free-Space Optical,FSO)通信系统的性能。在无需大气湍流状态信息估计的情况下,根据接收信号的平均 SNR来调整发射信号的发射功率,实现基于自适应功率传输技术的大气湍流效应补偿。建立了不同湍流强度下具有不同噪声水平的信道模型,并将所提出的技术与固定阈值判决(Fixed Threshold Deci-sion,FTD)和自适应阈值判决(Adaptive Threshold Decision,ATD)技术进行了比较分析。仿真结果表明,在不同的湍流强度下,该技术的误码率(Bit Error Rate,BER)性能相较于 FTD有了显著的提高,并且接近于 ATD。因此,该技术可以有效提高 FSO链路的通信能力。     

风扇前传噪声及声衬特性研究

为了探究航空发动机的风扇噪声,通过试验研究了缩尺风扇前传噪声在不同转速工况下的频谱特性和周向模态特性,并对所设计的环形声衬降噪效果进行了验证。研究结果表明,基于试验测量数据分析得到的频谱特性和周向模态特性满足转静干涉噪声理论。随着转速的提高,风扇纯音噪声越显著,高转速工况下纯音噪声的能量约占风扇前传噪声的85%。在所研究的工况范围内,该声衬对目标频率及模态均有显著的降噪效果,且在85%转速工况时的纯音降噪效果最优,其主模态的传声损失约为59dB。该声衬对于设计频率附近的宽频噪声也有一定降噪效果,在85%转速时的宽频噪声平均传递损失约为3dB。     

Optimization and Performance of Multilevel Quantization in Automatic Detectors

The optimization and performance of the double-threshold or binary method of automatic radar detection has been the subject of numerous studies in the past, and the loss due to this approximation has been shown to be 1 to 2-1/2 dB. By quantization of the input video into more than two levels, this loss can be avoided; however, little information is available on appropriate design procedures and expected performance. In this paper empirical procedures are described to find good settings of the first and second threshold levels, and the corresponding detection performance is determined. These results, which are believed to be close to optimum, show that for the case of quantization into four levels (2 bits), the loss in decibels is reduced to approximately 1/3 of the corresponding value for the binary case. In some cases this is equivalent to a saving of as much as 1-1/2 dB in signal-to-noise ratio.     

Rotating Blades Radio Interference in a Helicopter-Borne CW Doppler Radar

Radio interference generated in a helicopter-borne continuous wave (CW) Doppler radar system due to the rotating blades is analyzed. This problem has been previously treated for the case of pulse Doppler radar systems with very narrow (near zero) beamwidth. In this case the strong interference component returning directly from the blades (with no ground reflection) need not be considered as it reaches the receiver when it is still blinded. In the case of a CW Doppler radar, however, this interference component must be included. Numerical calculations show that the total blade interference power level, dominated by the direct component, is higher than that of the direct ground clutter in the radar clutter region. It decreases approximately as (f - fo)-4 in the radar clear region. It stays, however, well above the thermal noise level which might cause false alarm and degrade the radar performance.