Frequency-Selective Limiters Using Nonlinear Bandstop Filters

This paper presents a novel design for frequency-selective limiters based on reflection-mode nonlinear bandstop filters. This technique enables the use of low-Q lossy resonators for high-Q bandstop filter applications. Frequency-selective limiters described here achieve fast switching, high levels of power limiting, and flexible channel bandwidth. The prototype for a third-order device indicates 2-dB insertion loss for low RF powers. At high RF powers, it demonstrates 18-dB limiting level, 3-dBm limiting threshold, and 300-MHz limiting bandwidth. The switching speed is less than 10 ns, thus the spike leakage is minimized. The single-channel limiters are cascaded to provide limiting in wideband channels. The multichannel device is able to compress the dynamic range of the signals over 30 dB.     

Frequency-Selective Limiting

In the usual microwave limiter, the presence of one more large signals above a certain threshold level produces a limiting action which can be explained as a change in insertion loss of the limiter so as to maintain a constant output power, regardless of the number of independent signals present. Experimental results of coincidence mode passive ferrite limiters in S band and C band are presented which show that they do not behave in this manner, but rather to a good approximation limit on a frequency-by-frequency or frequency-selective basis. A qualitative explanation of this phenomenon is presented, using the passive parametric limiter as a model.     

Nonrefrigerated Bulk-Semiconductor Microwave Limiters

A microwave limiter using in intrinsic silicon that is operable at room temperature is described. The microwave non-linearity observed for the device is discussed in terms of intrinsic impact ionization, a behavior that is hypothesized from low-frequency studies of the V-I characteristics for the silicon element used. Thus the operation is thought to consist of a transition from essentially a dielectric medium at low RF applied fields to a medium of substantial conductivity resulting from plasma generation by impact ionization at high RF applied electric fields in the tens of kilovolts rms per centimeter. An appropriate circuit structure for the limiter in the form of a slotted metal diaphragm, or resonant waveguide iris is also discussed. In its center is placed the silicon element. The device transmits low-power signals but, shunted by the conductive silicon medium at high incident fields, reflects high power. Details for the fabrication of the limiter as well as a microwave characterization technique to determine absolute values of electric-field strength within and conductivity of the silicon specimen are presented. Finally, the operational details of an experimental model at 9.35 GHz are presented which demonstrate limiting action of 0.4 dB at low power up to 15 dB at a burnout power of 8-kW peak, 8 watts average with l- /spl mu/ s pulse width. Recovery time is about 5 /spl mu/ s.     

固体微波限幅器

-用薄I层PIN二极管研制成了X波段集成和波导限幅器.实验表明,限幅门限小于12毫瓦,插入损耗小于0.2分贝,小信号时电压驻波比小于1.3.本文阐述了PIN二极管限幅器工作原理、设计和实验结果.     

线性限幅器的设计与研究

ADC芯片广泛用于现代无线电系统,已成为大量军事装备中不可或缺的部分。复杂电磁环境可导致接收机灵敏度下降甚至失效。接收机用的ADC 芯片耐受大信号能力有限,急需一种既能保证其正常工作,又不会影响其采样灵敏度的器件,为此开发了一种新的器件,线性限幅器。为满足高线性度的限幅要求,介绍了一种基于PIN二极管的小型化线性限幅器设计方案。用ADS 仿真设计,薄膜混合工艺、一体化陶瓷管壳装配实现,开发出了一系列频率覆盖10 MHz~5 GHz,尺寸仅5 mm×5 mm×2.5 mm 的高线性度限幅器。测试结果表明,P 波段300~500 MHz产品损耗小于0.6 dB,输出P-1 大于11 dBm,输入0 dBm 信号时OIP3 大于35 dBm,输入25 dBm 信号时漏功率低于12 dBm,最大可承受功率5 W。ADC 芯片的可靠性大大提高,具有广阔应用前景。     

Practical Microwave Power Limiters

The near future holds considerable promise for the utilization of solid-state limiters as receiver protection elements. This correspondence describes the performance of ferrimagnetic limiters in the actual role of diode protection. The peak powers involved in the tests were up to 25 kw.     

YIG Filters as Envelope Limiters

Envelope limiters are used in such applications as FM demodulation and power leveling. Recently, the envelope-limiting properties of yttrium-iron-garnet (YIG) filters were reported for the special cases of unmodulated and pulsed input signals. Measured data is presented hereon the response of a YIG limiter to AM carriers having modulation index of the order of 50 percent. Sinusoidal, square-wave, and low-pass noise modulating signals were used in the measurements. It was found that a YIG filter will give good envelope Iimiting for modulating frequencies in the submegacycle range. At these low frequencies the carrier and the side frequencies are not limited selectively. At higher modulating frequencies where the limiting is frequency selective, the YIG filter will not remove the variations. In fact, in the particular filter tested, the modulation index was increased, rather than decreased, at modulating frequencies greater than about 750 kc/s. A graph is given showing the measured factor of reduction (or increase) of modulation index, as a function of modulating frequency. The response of the limiter as a function of carrier frequency, modulating frequency, and input power is shown by oscilloscope displays produced by sweeping the carrier frequency or input power. In addition, selected photographs of output envelope waveforms are given.     

Waveguide Subsidiary Resonance Ferrite Limiters

This paper gives an analysis of E-plane waveguide ferrite limiters under subsidiary resonance conditions. Since Suhl's high-power damping term is power sensitive, it is necessary to evaluate it in every part of the ferrite structure before forming the dispersion relation. This is done in this paper by dividing the waveguide assembly in small elements transverse and parallel to the direction of propagation and calculating it in each region. The dispersion relation in each section along the direction of propagation is then formed in conjunction with the appropriate Suhl damping constant by establishing the transverse resonance condition. The total output power is obtained by forming the input/output relation for each section one at a time along the structure. The theory has been found useful in describing both the onset of limiting and the dynamic range of a ferrite limiter mounted on one of the narrow walls of the waveguide.     

多层频率选择表面的分析

本文利用矩量法和广义散射矩阵理论分析了多层频率选择表面的散射特性。通过引入广义波导概念,可以统一分析单元形状为任意的多层频率选择表面。作为示例,计算了几种不同结构频率选择表面的散射特性,结果与文献中给出的数据相符。     

Single-Layer High-Order Miniaturized-Element Frequency-Selective Surfaces

A new miniaturized-element frequency-selective surface is presented in this paper. This frequency-selective surface is made up of a 2-D periodic array of metallic loop and a wire grid of the same period printed on either side of a very thin substrate. Unique features of the new design include localized frequency-selective properties, high-order frequency response achieved by a single substrate, lack of passband harmonics in the frequency response, and very low frequency response sensitivity to the incidence angle. High-order frequency response is accomplished through the application of a thin substrate that allows considerable couplings between the elements on the two sides of the substrate. The layers' couplings in conjunction with each layer characteristics are designed to produce a high-Q bandpass frequency response, in addition to a transmission zero. It is shown that by inserting variable capacitors in the gap between the metallic loops, the center frequency of the passband can be tuned over nearly an octave. In addition, using a cluster of loops as the unit cell and modifying the parameters of the loops within the cluster, a dual-band characteristic from a single-layer miniaturized-element frequency-selective surface can be achieved. A prototype sample of the miniaturized-element frequency-selective surface, whose unit cell can be as small as lambda₀ /12, is fabricated to verify the design performance through a standard free-space measurement setup. The transmission characteristic of the structure is measured and compared with numerical simulation results.     

Characteristics of Ferrite Microwave Limiters

Microwave ferrites that exhibit a nonlinear RF absorption as a function of RF power level can be utillzed in the construction of a passive microwave device which will allow small RF signals to be transmitted with very little attenuation but which will attenuate large RF signals considerably. Such a device tends to "limit" the amplitude of the microwave energy passing through the device and is therefore called a ferrite microwave limiter. One application of the ferrite limiter is in the protection of crystal detectors in pulsed radar sets. However, when a rectangular pulse of X-band RF energy is transmitted through the limiter, the output waveform is no longer rectangular but consists of a leading edge spike of 0.1-µsec duration followed by a plateau of highly attenuated RF energy. At the present time the leading edge spike is the major obstacle in the successful use of the ferrite microwave limiter as a TR cell in the protection of crystal detectors. Experimental techniques used to improve the performance of the limiter are presented, and the performance characteristics of an X-band ferrite microwave limiter are shown.     

Wide-Band Oscillating Limiters at X-Band and L-Band Frequencies

Injection-locked tunnel diode oscillators have been designed and tested as oscillating limiters. Units have been operated and tested at X-band and L-band frequencies. Most of the units tested had output power variations that were less than 0.5 dB for input power variations in excess of 30 dB. Bandwidths of 30 percent with 10 dB of gain are predicted from theory and have been realized in practice. The locked oscillator Iimiters have conventional noise figures on the order of 5 to 7 dB at 11GHz. Measurements on typical units when used in an experimental 160 megabit pulse code modulation system showed good agreement with theoretical predictions.     

Application of Sequential Estimation Algorithm Based on Branch Metric to Frequency-Selective Channel Equalization

In this work, a sequential estimation algorithm based on branch metric is used as channel equalizer to combat intersymbol interference in frequency-selective wireless communication channels. The bit error rate (BER) and computational complexity of the algorithm are compared with those of the maximum likelihood sequence estimation (MLSE), the recursive least squares (RLS) algorithm, the Fano sequential algorithm, the stack sequential algorithm, list-type MAP equalizer, soft-output sequential algorithm (SOSA) and maximum-likelihood soft-decision sequential decoding algorithm (MLSDA). The BER results have shown that whilst the sequential estimation algorithm has a close performance to the MLSE using the Viterbi algorithm, its performance is better than the other algorithms. Beside, the sequential estimation algorithm is the best in terms of computational complexity among the algorithms mentioned above, so it performs the channel equalization faster. Especially in M-ary modulated systems, the equalization speed of the algorithm increases exponentially when compared to those of the other algorithms.     

Switched-Diversity FSK in Frequency-Selective Rayleigh Fading

This paper presents the measured performance of a 64 kbit/s switched-diversity FSK receiver subjected to simulated frequency-selective Rayleigh fading. The single receiver input is switched between two, three, or four uncorrelated Rayleigh-fading signals whenever the instantaneous receiver output falls below a threshold. The optimum level of this threshold relative to the mean signal level is relatively insensitive to frequency selectivity and vehicle speed. A nearly optimum threshold may be determined using an AGC amplifier and fixed comparator. Switched diversity is a powerful tool for combating frequency selectivity and Rayleigh fading. Two-branch switched diversity can achieve 10^-2BER, with 6 dB lower SNR than that needed without diversity. In the absence of frequency selectivity, two-branch switched diversity can perform within 3 dB SNR of maximal-ratio diversity and within 1 dB of selection diversity. Switched diversity can achieve BER's lower than the irreducible single-channel BER produced by frequency selectivity. Four-branch switched diversity typically requires 4 dB lower SNR to achieve a given BER than does two-branch switched diversity.     

Multiple-Access Strategies for Frequency-Selective MIMO Channels

In this paper, we consider frequency-selective coherent multiple-input multiple-output (MIMO) multiple-access fading channels. Assuming that each of the users employs orthogonal frequency-division multiplexing (OFDM), we introduce a multiple-access scheme that gradually varies the amount of user collision in signal space by assigning different subsets of the available OFDM tones to different users. The corresponding multiple-access schemes range from frequency-division multiple access (FDMA) (each OFDM tone is assigned to at most one user) to CDMA (each OFDM tone is assigned to all the users). We quantify the effect of signal space collision between the users by computing the ergodic capacity region for the entire family of multiple-access schemes. It is shown that the ergodic capacity region obtained by a fully collision-based scheme (CDMA) is an outer bound to that corresponding to any other multiple-access strategy. In practice, however, minimizing the amount of user collision in frequency is desirable as this minimizes the receiver complexity incurred by having to separate the interfering (colliding) signals. Our analysis shows that the impact of collision on spectral efficiency depends critically on the channel's spatial fading statistics and the number of antennas     

Interaction of Signals in Ferromagnetic Microwave Limiters

A weak secondary signal is partially absorbed in a ferromagnetic microwave limiter that is saturated by a strong primary signal; the absorption is greatest when the two signals are close in frequency. The width of this absorption is determined here, and is found to be proportional to the spin wave linewidth and to the square root of the excess power in the primary signal. The theory of this effect is presented and is found to agree well with experiment.     

频选快衰落信道的Turbo均衡算法

针对频选快衰落信道,本文提出卡尔曼滤波信道跟踪、软输出判决反馈均衡及软输入软输出信道解码迭代处理的Turbo均衡算法以充分利用已获得的信息,实现信道估计、信道均衡与信道解码的迭代更新,并克服传统判决反馈均衡器误差传播的缺陷.仿真表明,本算法能有效地跟踪快衰落信道,经两次迭代就可获得较为满意的码间干扰消除效果.     

Frequency-Selective Predistortion Linearization of RF Power Amplifiers

This paper presents a frequency-selective RF vector predistortion linearization system for RF multicarrier power amplifiers (PAs) affected by strong differential memory effects. Differential memory effects can be revealed in two-tone experiment by the divergence for increasing tone-spacing of the vector Volterra coefficients associated with the lower and upper intermodulations tones. Using large-signal vector measurement with a large-signal network analyzer, a class-AB LDMOS RF PA is demonstrated to exhibit a strong differential memory effect for modulation bandwidth above 0.3 MHz. New frequency-selective RF and baseband predistortion linearization algorithms are proposed to separately address the linearization requirements of the interband and inband intermodulation products of both the lower and upper sidebands. Theoretical verification of the algorithms are demonstrated with Matlab simulations using a Volterra/Wiener PA model with memory effects. The baseband linearization algorithm is next implemented in a field-programmable gate array and experimentally investigated for the linearization of the class-AB LDMOS PA for two carrier wideband code-division multiple-access signals. The ability of the algorithm to selectively linearize the two interband and four inband intermodulation products is demonstrated. Adjacent channel leakage ratio of up to 45 dBc for inband and interband are demonstrated experimentally at twice the typical fractional bandwidth.     

Modelling and Simulation of Direction-Selective and Frequency-Selective Mobile Radio Channels

This paper presents a new approach to the design of simulation models for spatio-temporal mobile radio channels, which are important for the design, optimization, and test of antenna array based mobile communication systems. The proposed spatio-temporally correlated simulation model is completely deterministic and enables the emulation of the desired fading behaviour with comparably little effort. Moreover, it can be configured flexibly to simulate not only different kinds of theoretical channels with given spatial, temporal, and frequency correlation properties, but also real-world fading channels for mobile communication systems with adaptive antennas.