超宽带带通滤波器设计：缝隙线耦合转换器分析

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"缝隙线耦合转换器用于设计超宽带带通滤波器" 在无线通信领域，设计高效、性能优异的滤波器是至关重要的，而缝隙线耦合转换器在此过程中扮演着核心角色。缝隙线（Slotline）是一种电磁波传输方式，它通过介质板上的开槽来传播电磁能量，常用于微波电路设计。缝隙线转换器则能够将微带线（Microstrip Line）与缝隙线之间进行有效耦合，从而实现不同类型的滤波响应。本文《Synthesis Design of Ultra-Wideband Bandpass Filters With Composite Series and Shunt Stubs》发表在2009年的IEEE Transactions on Microwave Theory and Techniques期刊上，作者Rui Li、Sheng Sun和Lei Zhu提出了一种针对超宽带（Ultra-Wideband, UWB）带通滤波器的直接综合设计方法。他们利用串联和并联传输线 stubs（即开路和短路负载）的复合结构，设计出一种单级带通原型滤波器。该滤波器由两组串联的系列开路端和并联短路端的 stubs 组成，并通过一个多段非均匀传输线连接。所有stubs设置有相同的电气长度，即在较低截止频率时的π/2。中间连接线由具有阶梯阻抗的传输线组成，包含n个等间距的段，每段的电气长度为λ/4。这种设计目标是构建一个具有Chebyshev等纹波响应的超宽带带通滤波器，并在通带内产生+3dB的传输极点。通过导出传输函数，作者建立了合成方法，并系统地描述了设计此类滤波器的过程。这种方法允许设计师精确控制滤波器的带宽、中心频率、选择性和通带内的响应特性。缝隙线与微带线间的耦合使得滤波器能够在超宽带范围内实现良好的频率选择性，同时减小尺寸和复杂性。这种基于缝隙线耦合转换器的滤波器设计技术对于实现高性能的UWB通信系统至关重要，例如在无线个人区域网络（WPAN）、雷达系统以及无线传感器网络中，它可以有效地抑制不需要的信号干扰，提高信噪比，确保数据传输的稳定性和可靠性。通过这种方式，可以优化滤波器的性能，使其满足日益增长的超宽带通信应用需求。

684 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 3, MARCH 2009

Synthesis Design of Ultra-Wideband Bandpass Filters

With Composite Series and Shunt Stubs

Rui Li, Student Member, IEEE, Sheng Sun, Member, IEEE, and Lei Zhu, Senior Member, IEEE

Abstract—This paper presents a direct synthesis procedure for

efﬁcient design of a class of ultra-wideband bandpass ﬁlters with

composite series and shunt transmission-line stubs. The proposed

single-stage bandpass prototype is formed by cascading two sets

of series open-ended and shunt short-ended stubs through a mul-

tisection nonuniform transmission line. All the stubs are set with

an identical electrical length, i.e.,

, at lower cutoff frequency

and the middle connecting line is composed of stepped-impedance

transmission line with

sections, and each section has an electrical

length

. The proposed ﬁlter topology aims to construct an ultra-

wide bandpass ﬁlter with Chebyshev equal-ripple responses and

( +3)

in-band transmission poles. Based on the derivation of the

transfer function, a synthesis approach is established and system-

atically described to design this type of ﬁlters according to the spec-

iﬁcations such as lower/upper cutoff frequencies. Next, a multi-

stage bandpass ﬁlter prototype is proposed and the synthesis design

procedure is also presented. The implementation is achieved by

using hybrid microstrip line and slotline sections. Compared with

traditional stub ﬁlters, the proposed ﬁlters achieve higher order

transmission zeros and thus provide higher selectivity. As design

examples, two single-stage and one three-stage bandpass ﬁlters are

designed and fabricated to conﬁrm the theoretical predictions.

Index Terms—Bandpass ﬁlter, equal-ripple response, multistage,

series/shunt stubs, single stage, synthesis design, ultra-wideband.

I. INTRODUCTION

INCE THE Federal Communications Commission (FCC)

in the U.S. authorized the unlicensed use of ultra-wide-

band devices in the frequency band of 3.1–10.6 GHz in 2002

[1], great interest has been aroused from both academic and

industrial areas toward ultra-wideband technology. As a key

component in the ultra-wideband wireless communication sys-

tems, microwave bandpass ﬁlters with high performance, com-

pact size, and low cost are highly demanded. Numerous ultra-

wideband bandpass ﬁlters with various desired features have

been reported [2]–[15]. Nevertheless, these ﬁlters are usually

designed relying on the cut-and-try approach where the overall

ﬁlter layout with electrically large size is simulated again and

Manuscript received October 15, 2008; revised December 10, 2008. First pub-

lished February 13, 2009; current version published March 11, 2009.

R. Li and L. Zhu are with the School of Electrical and Electronic Engineering,

Nanyang Technological University, Singapore 639798 (e-mail: liru0003@ntu.

edu.sg; ezhul@ntu.edu.sg).

S. Sun was with the School of Electrical and Electronic Engineering, Nanyang

Technological University, Singapore 639798. He is now with the Institute of

Microwave Techniques, University of Ulm, 89081 Ulm, Germany (e-mail: sun-

sheng@ieee.org).

Color versions of one or more of the ﬁgures in this paper are available online

at http://ieeexplore.ieee.org.

Digital Object Identiﬁer 10.1109/TMTT.2009.2013312

again using the commercial software towards the speciﬁed ultra-

wide passband. As a result, there is a demand for developing a

systematic synthesis approach such that these bandpass ﬁlters

with a fractional bandwidth of 110% can be efﬁciently designed.

To address this issue, the conventional high-pass prototype

with short-circuited shunt stubs [16] has been adopted in [2]–[6]

to explore various bandpass ﬁlters with ultra-wide bandwidth.

In [2], this type of wideband ﬁlter was initially presented and

then systematically synthesized in [3]. However, the resultant

ﬁlter block in [3], composed of high-pass, low-pass, and iso-

lation sections, occupied a large circuitry area. Hence, exten-

sive work has been carried out toward miniaturizing the overall

ﬁlter size and improving the electrical performance at the same

time. In [4], an electromagnetic (EM) bandgap structure was

formed on the connecting line between two adjacent stubs to

suppress the upper spurious harmonics. An extra cross-coupling

route was purposely introduced between the input and output

ports in [5] and [6] to enhance the ﬁlter selectivity and to im-

prove the group-delay performance. However, in constructing

these ﬁlters on microstrip-line topology, via-holes are always

required to realize the short-ended terminals for all shunt stubs.

As a dual of the high-pass prototype with shunt short-circuited

stubs, a high-pass ﬁlter with series open-circuited stubs was the-

oretically studied in [17]. Later, this ﬁlter network was imple-

mented using a 3/4 wavelength parallel-coupled line resonator

to synthesize and design an ultra-wideband bandpass ﬁlter [15].

For the readers’ reference, it needs to be pointed out that the ini-

tial generating functions with bandpass ﬁltering behavior were

presented in [18] for an ideal transmission-line ﬁlter network.

However, no work has been reported thus far to physically im-

plement such a microwave ﬁlter based on any transmission-line

structure and to experimentally demonstrate its real ﬁltering per-

formances.

In this paper, an alternative class of ultra-wideband bandpass

ﬁlters with single and multiple stages implemented using com-

posite series and shunt stubs is proposed. In Section II, a syn-

thesis approach is developed to efﬁciently design the single-

stage bandpass ﬁlters in the closed-form format following the

studies in [16]–[18]. Fig. 1(a) and (b) depicts the general trans-

mission-line models with an odd- and even-integer

, respec-

tively. In the synthesis procedure, the transfer functions are at

ﬁrst analytically derived and regulated to exhibit the Cheby-

shev equal-ripple frequency responses in the ultra-wide pass-

bands. Under equalization of the insertion-loss functions, all

the normalized element values in the prototype ﬁlters can be

accordingly determined. Next, this procedure is applied to syn-

thesis design a multistage bandpass ﬁlter. Since there has been

no reported work in practical implementation for these ﬁlter

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