螺旋线行波管薄膜衰减器高频衰减特性的研究

利用HFSS仿真软件对螺旋线行波管中衰减器的衰减量分布情况进行了模拟分析。由于直接模拟螺旋线行波管中不同涂碳层厚度下衰减器的衰减分布情况难度大,计算量大,涂碳厚度不易模拟,因此,本文利用等效替换的方法进行了模拟计算,通过对单根夹持杆进行波导测试,获得S21,找到不同S21下的等效体电率或损耗角正切,并将他们对应的这些材料替换到行波管内,计算相应的衰减常数,得出不同位置下的衰减常数。     

A Simple Equivalent Circuit Analysis of the Dielectric Loss in a Helical Slow-Wave Structure of a Traveling-Wave Tube

The electromagnetic field analysis of a helical slow-wave structure (SWS) is carried out based on a tape-helix model incorporating the effects of space-harmonic propagating modes and the surface current on the helix over the actual metallic area of the tape. Using this analysis, closed-form expressions are derived for the shunt capacitance per unit length and the shunt conductance per unit length of the transmission-line equivalent circuit of the structure. The analysis is interpreted for the circuit attenuation constant contributed by the loss of the dielectric helix-support rods. The analysis is accurate, amenable to easy computation, and validated against published results. The analysis is subsequently used for investigating the dielectric loss in an SWS due to the backward-wave ($-$1) space-harmonic mode of propagation.      

A Simple and Accurate Analysis of Conductivity Loss in Millimeter-Wave Helical Slow-Wave Structures

Electromagnetic field analysis of a helix slow-wave structure was carried out and a closed form expression was derived for the inductance per unit length of the transmission-line equivalent circuit of the structure, taking into account the actual helix tape dimensions and surface current on the helix over the actual metallic area of the tape. The expression of the inductance per unit length, thus obtained, was used for estimating the increment in the inductance per unit length caused due to penetration of the magnetic flux into the conducting surfaces following Wheeler’s incremental inductance rule, which was subsequently interpreted for the attenuation constant of the propagating structure. The analysis was computationally simple and accurate, and accrues the accuracy of 3D electromagnetic analysis by allowing the use of dispersion characteristics obtainable from any standard electromagnetic modeling. The approach was benchmarked against measurement for two practical structures, and excellent agreement was observed. The analysis was subsequently applied to demonstrate the effects of conductivity on the attenuation constant of a typical broadband millimeter-wave helical slow-wave structure with respect to helix materials and copper plating on the helix, surface finish of the helix, dielectric loading effect and effect of high temperature operation – a comparative study of various such aspects are covered.     

Gain stability of traveling wave tubes

The long-term gain stability of traveling wave tubes (TWT's) with helix slow-wave structures is examined. A major variable in the gain of TWT's is the stability of the attenuator material that is placed in the tube to damp oscillations and improve input-to-output isolation. Thin carbon layers are often used for this purpose in TWT's and are deposited onto the helix support rods by several different techniques that produce a variability in the material structure and properties. The carbon layers are also susceptible to physical damage due to the environment in the tube during conditioning and long-term operation. Modification of the electrical conductivity of the layer by energetic particle bombardment and chemical erosion decreases the net RF loss in the tube and causes the gain to increase with time. The presence of impurity gases and rapid conditioning procedures produce gain increases due to the lattice damage of the attenuator material of up to 10 dB in a TWT in the first several hundred hours of operation. Properly designed attenuator loss-patterns and minimization of the gas evolution in the TWT causes these effects to saturate and the gain to stabilize quickly. Techniques to ensure long-term stability of these layers are discussed     

High-frequency analysis of broad-band 0° hybrid-junction: Distributed parameter approach

The broad-band 0° hybrid junction constructed with a twisted-wire transmission-line transformer is analyzed by using a distributed parameter approach, with the help of the transmission-line theory. Such an analysis provides some useful information concerning the behavior of the device at higher frequencies when the line length becomes comparable to a wavelength.     

Large signal behavior of high power traveling-wave amplifiers

The results of an experimental investigation on the large signal behavior of a kilowatt power level helix type traveling-wave amplifier tube are presented. Operation with and without attenuators was investigated using a movable electromagnetic probe to measure power level along the tube. Quite different effects of drive power and beam voltage on the saturation level were found for operation with and without attenuators. The maximum power level is lower for attenuator operation. Also, power levels do not continue to increase with increasing beam voltage and drive power. In contrast, attenuator-less operation produces the highest efficiency, and the power levels continue to rise with increasing beam voltage and drive power. Conversion efficiencies as high as 25 per cent are obtained with an attenuator and as high as 40 per cent without an attenuator. Efficiency calculations based on small-signal theory can be made to agree reasonably well with the experimental attenuator-less operation efficiencies by assuming an appropriate ratio of the ac component of beam current to the dc component of beam current, i/I0.     

An equivalent transmission-line model containing dispersion forhigh-speed digital lines-with an FDTD implementation

The increase in processor speeds in the last few years has created a growing need for the accurate characterization of waveform propagation on lossy printed-circuit-board (PCB) transmission lines. Due to the dispersive nature of pulse propagation on lossy transmission lines, approximations of the classic transmission-line model can fail in this application (i.e., lossless or DC losses approximations). This paper shows how an equivalent transmission-line model can be used to analyze dispersive transmission lines for high-speed digital applications. The equivalent-circuit elements of this transmission-line model incorporate the frequency dependence of the per unit length impedance and admittance caused by the finite conductivity of the conductors as well as the dielectric losses. We show that these equivalent circuit elements can be readily implemented into finite-difference time-domain (FDTD) transmission-line codes, and we present such a FDTD implementation. S-parameters and pulsed waveforms for a circular wire, coplanar waveguides (CPW) and microstrip lines are shown. Finally, we present approximate expressions for analytically obtaining the resistance and inductance per length of a microstrip line     

Prediction of gain peak wavelength for Er-doped fiber amplifiersand amplifier chains

We present a simple rule for predicting the gain peak wavelength (GPW) of an Er-doped fiber amplifier. For a given fiber type, the GPW is determined solely by the operating gain per unit length. Using this rule coupled with a simple Er-doped fiber amplifier model and measured modeling parameters, the GPW is predicted for a particular Er-doped fiber. The result is verified by direct measurement in a fiberloop. We demonstrate that the gain peak does not vary with pump power, pump wavelength or signal power as long as the gain per unit fiber length is fixed. Extension of this theory to include the wavelength dependences of other components in an amplifier chain is also discussed and demonstrated     

Theoretical and experimental results of guided wavelength of a slot on a low-permittivity substrate

In the letter theoretical results for the guided wavelength and characteristic impedance of different slots are given for a substrate of relative permittivity ?r = 2.17. A formula is given for the first parameter. A comparison of theory and experiment, according to a model of a transmission-line applied to the slot, is presented.     

MEMS可调式光衰减器性能测试及其分析

采用微细加工的方法研制出了一种可调式微机械光衰减器 ,对该衰减器的性能进行了测试 ,并对影响其光学性能的因素作了分析。测试表明 ,该衰减器的插入损耗<3dB ,衰减范围 0～ 4 0dB ,回波损耗 <- 45dB ,衰减精度可达 0 1dB。     

Design of Tip Loss Profile on Support Rod for a Helix TWT

This paper describes the design of tip loss profile on support rod for a high gain, high efficiency helix TWT. The tip loss profile at sever ends for the input and the output section of the helix slow wave structure has been designed for return loss at severs ends better than -20 dB. The experimental results have been carried out in the input and the output section of the helix assemblies for the high gain helix TWT to find the return loss in the region of carbon coated tip loss on the support rods. The experimental result has been compared with the simulated performance for the return loss at the sever ends. The design of tip loss profile on the support rod for helix TWT has been carried out in real situation using Ansoft HFSS. A good agreement has been found in the simulated and experimental results.     

Theory of long term gain growth in traveling wave tubes

The properties of the attenuator materials used in traveling wave tubes (TWT) are a major factor in determining the gain of the tube. Carbon is a commonly used attenuator material which is typically deposited on the dielectric rods used to support the helix slow wave structure in TWTs. The deposited carbon layers are susceptible to ion bombardment induced damage during operation and outgassing of the tube, which increases the electrical resistivity of the layer and reduces the rf loss in the tube. The total gain in the tube grows during this process until the bombardment ceases or the layer damage saturates. A simple theory is developed that relates this gain change to the gas evolved in the tube that results in the particle bombardment of the layers during operation. The model accurately describes the observed time dependent behavior of the gain, and the time constant for outgassing the tube is found for several tubes ranging from S to Ka band. The time constant increases significantly for smaller, high frequency tubes with a low conductance and poor pumping speeds. The model shows that stabilization of the gain occurs after several time constants have passed, which may take hundreds to thousands of hours     

Design of gain-clamped doped-fiber amplifiers for optimal dynamicperformance

This paper provides a detailed analysis of gain-clamped doped-fiber amplifiers and design guidelines in a wavelength division multiplexed (WDM) networking environment. A simple dynamic model of the doped-fiber amplifier allows us to derive explicit expressions for the small-signal response, which help identify and optimize the most critical parameters for best dynamic performance. The most important parameter is the pump power, which should be chosen 1-2 dB's above its required open-loop value, with all channels present, for the required signal gain. In an all-optical networking scenario with input power per channel as high as -3 dBm the required pump power may well exceed 20 dBm. Thus optimization of other parameters such as laser wavelength and loop loss are important. For best dynamic performance either the loop loss should be extremely small, implying a very large laser flux, or the laser gain variation in response to a perturbation should be large. Accordingly, the laser wavelength should be placed either close to the unity-gain region of the clamped gain profile, or at its peak. Finally, the small signal model for a chain of clamped amplifiers is provided, and it is shown that long chains are vulnerable to low-frequency input signal perturbations     

A SiGe MMIC variable gain cascode amplifier

A silicon-germanium variable gain cascode amplifier has been developed to combine the functionality of an amplifier and an attenuator into one monolithic microwave integrated circuit (MMIC). The cascode amplifier, which was designed for a 7-11 GHz frequency range, achieved a gain of 12.5 dB, an input return loss of 7.5 dB, and an output return loss of 12.5 dB. The cascode amplifier exhibited 16 dB of gain control.     

Propagation characteristics of single-mode optical fibers with arbitrary complex index profiles

A rapidly converging numerical technique for the evaluation mode characteristics of circularly symmetric optical fibers with an arbitrary complex refractive index profile is presented. This method is based on transmission-line principles. From Maxwell's equations, we derive a transmission-line equivalent circuit for the optical fiber refractive index profile and we demonstrate how it can be used to determine the mode effective index (normalized propagation constant) of cylindrical dielectric waveguides. To illustrate the effectiveness of the procedure, we have applied it to circularly symmetric fibers with complex step, parabolic, and segmented optical refractive index profiles. We have used this method to evaluate and manipulate the gain in a typical 980-nm pumped erbium-doped fiber as well as for calculating attenuation of optical fibers when radial loss factors are present.     

Simplified Analysis of Coupled Transmission-Line Networks

A relatively simple method is presented for analyzing coupled transmission-line networks by using network graphs and graph transformations. The network graph symbolism is easy to draw and to manipulate. All the graphs consist only of inductor, capacitor, and transformer symbols, and straight lines, which represent unit elements. The method of analysis is illustrated by several two-wire-line and multiwire-line examples. Also presented are several new useful transmission-line transformations and a graph equivalent for the general coupled transmission-line network. The graph-transformation method has four principal advantages: 1) explicit open-wire-line equivalent circuits of coupled line networks can be obtained relatively easily and without knowledge of network synthesis techniques; 2) the form of equivalent circuits can often be obtained without using any algebra; 3) at each step of the analysis, a positive-real network in graph form is available; consequently, in many analysis problems several equivalent circuits for the same network are derived; and 4) multiport networks are as easily dealt with as two-port networks.     

Strip Type Components for 2000 Megacycle Receiver Head-End

Recent experimental work has evolved some components using air-spaced strip type transmission line that have been used successfully in connection with a variable attenuator, cavity and crystal mixer. Bandwidth, VSWR, and NF measurements are comparable with a commercial receiver head-end presently in use. The asymmetric air-spaced strip-above-ground transmission line used results in a simple configuration for coupling the line to the cavity. It also facilitates the application of a variable attenuator using a ferrite slab of high attenuation per unit length and having good VSWR properties.     

Equivalent circuit analysis of noise in bulk semiconductor devices

Equivalent transmission-line analogs may be developed to advantage for the analysis of noise in bulk semiconductor devices. We discuss first a transmission-line analog for the law of conduction and diffusion of a single species of charge carriers that experience small disturbances from equilibrium. Through the use of Nyquist's theorem it is possible to obtain the power spectra of the noise sources of the equivalent transmission-line circuit at thermal equilibrium. Next, an equivalent circuit can be obtained if there is more than one charge carrier. This circuit is generalized to the case of drifted carriers and applied to the metal-semiconductor-metal (MSM) diode and to the small-signal analysis of the Gunn effect. We show how the equivalent circuit for the Gunn effect gives, by inspection, a lower bound on the noise measure achievable with a Gunn diode. We find that the Gunn diode has noise measures exceeding the lower bound. A traveling-wave Gunn device achieves the lower limit.     

Perturbation theory for multistrip acoustoelectric surface-wave amplifier

The `multistrip perturbation theory? of the coupler has been extended to the multistrip amplifier. A study of the acoustoelectric gain shows that, if the number of strips per wavelength is very large, the gain becomes that predicted by Adler, but the number of strips per wavelength required to reach this condition increases drastically with the conductivity of the semiconductor; it also shows that the width of the strips on the semiconductor should be as large a fraction as possible of their period, and that, if the conductivity of the semiconductor is low and the number of strips per wavelength is small, gain can be obtained from electrons drifting at a velocity less than the sound velocity in the opposite direction to the surface wave, and, in some cases, even in the same direction.     

Gain peak wavelength measurements using a polarization scrambledfiber loop configuration

We present a quick and accurate method to measure the gain peak wavelength (GPW) of concatenated optical amplifiers in a long haul optical fiber communication system. This method utilizes a simple fiber-amplifier loop with polarization scrambling. We verify that the GPW of an amplifier is completely determined by the operating gain per unit erbium doped fiber length and changes linearly with compressed gain near 1.558 μm. The data presented, indicates how the GPW can be controlled for a broad range of operating gains by adjusting the erbium fiber length. The temperature and pump power dependence of GPW were found to be negligible for typical undersea system amplifier conditions. The gain bandwidth for concatenated amplifier systems appears to be a function of compression not operating gain, implying that wavelength division multiplexing systems requiring a flattened gain response should take this into consideration