带阻滤波器英文翻译
安徽建筑大学
毕业设计(翻译)
专业通信工程
班级10通信①班
学生姓名高路
学号10205040109
指导教师吴东升
2014年6月6日
安徽建筑大学毕业设计（翻译）
Microwave filters
Microwave Engineering Pozar
A filter is a two-port network used to control the frequency response at a certain point in an RF or microwave system by providing transmission at frequencies within the passband of the filter and attenuation in the stopband of the filter. Typical frequency responses include low-pass, high-pass, bandpass, and band-reject characteristics. Applications can be found in virtually any type of RF or microwave communication, radar, or test and measurement system.
The development of filter theory and practice began in the years preceding World War II by pioneers such as Mason, Sykes, Darling, Fano,
Lawson, and Richards. The image parameter method of filter design was development in the late 1930s and was useful for low-frequency filters in the radio and telephony. In the early 1950s a group at Stanford Research Institute, consisting of G.Matthaei, L.Young, E.Jones, S.Cohn, and others, became very active in microwave filter and coupler development. A voluminous handbook on filters and couplers resulted from this work and remains a valuable reference [1]. Today, most microwave filter design is done with sophisticated computer-aided design (CAD) packages based on the insertion loss method. Because of continuing advances in network synthesis with distributed elements, the use of
low-temperature superconductors and other new materials, and the incorporation of active devices in filter circuits, microwave filter design remains an active research area.
We begin our discussion of filter theory and design with the frequency characteristics of periodic structures, which consist of a transmission line or waveguide periodically loaded with reactive elements. These structures are of interest in themselves because of their application to slow-wave components and traveling-wave amplifier design, and also because they exhibit basic passband-stopband responses that lead to the image parameter method of filter design.
Filter design using the image parameter method consist of a cascade of simpler two-port filter sections to provide the desired cutoff
reactive翻译frequencies and attenuation characteristics but do not allow the specification of a particular frequency response
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安徽建筑大学毕业设计（翻译）
over the complete operating range. Thus, although the procedure is relatively simple, the design of filte
rs by the image parameter method often must be iterated many times to achieve the desired results.
A more modern procedure, called the insertion loss method, uses network synthesis techniques to beginning with low-pass filter prototypes that are normalized in terms of impedance and frequency. Transformations are then applied to convert the prototype designs to the desired frequency range and impedance level.
Filter Design By The Insertion Loss Method
A perfect filter would have zero insertion loss in the passband, infinite attenuation in the stopband, and a linear phase response (to avoid signal distortion) in the passband. Of course, such filter do not exist in practice, so compromises must be made: herein lies the art of filter design.
The insertion loss method, allows a high degree of control over the passband and stopband amplitude and phase characteristics, with a systematic way to synthesize a desired response. The necessary design trade-offs can be evaluated to best meet the application requirements. If,
for example, a minimum insertion loss in most important, a binomial response could be used; a Chebyshev response would satisfy a requirement for the sharpest cutoff. If it is possible to sacrifice the
attenuation rate, a better phase response can be obtained allows filter performance to be obtained by using a linear phase filter design. In addition, in all cases, the insertion loss method allows filter performance to be improved in a straightforward manner, at the expense of a higher order filter. For the filter prototypes to be discussed below, the order of the filter is equal to the number of reactive elements.
Characterization By Power Loss Ratio
In the insertion loss method a filter response is defined by its insertion loss, or power loss ration, PLR :
PLR?Pinc1 (1) ?2Pload1??(?)
2Observe that this quantity is the reciprocal of S12 if both load and source are
matched. The insertion loss (IL) in dB is
IL=10logPLR （2）
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安徽建筑大学毕业设计（翻译）
Microwave Filters We know that ?（? is an even function of ?, therefore it can be expressed as a polynomial in ?2. Thus we can write 2
M(?2) （3）?（??22M(?)?N(?)2
Where M and N are real polynomials in?2. Substituting this from in (1) gives the following:
PLRM(?2) (4) ?1?2N(?)
For a filter to be physically realizable its power loss ratio must be of the form in (4). Notice that specifying the power loss ratio simultaneously constrains the magnitude of the reflection coefficient, ?（?. We now discuss some practical filter responses. Maximally flat: This characteristic is also called the binomial or Butterworth response, and is optimum in the sense that it provides the flattest possible passband response for a given filter complexity, or order. For a low-pass filter, it is specified by PLR?1?k2(?2N)
(5) ?c
Where N is the order of the filter and ?c is the cutoff frequency. The passband extends from ??0 to c; at the band edge the power loss ratio is 1?k2. If we choose this as the -3dB point, as is common, we h
ave k=1, which we will assume from now on. For c, PLR?k2(c)2N, which shows that the insertion loss
increases at the rate of 20NdB/decade. Like the binomial response for multisection quarter-wave matching transformers, the first (2N-1) derivatives of (5) are zero at ??0.
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