14-02-2013, 12:28 PM
A Novel Cross-Shape DGS Applied to Design Ultra-Wide Stopband Low-Pass Filters
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Abstract—
This letter presents a novel low-pass filter with an
ultra-wide stopband. The proposed filter is comprised of a new
cross-shape defected ground structure (CSDGS). By using this
structure, the filter not only supports conventional DGS performances
with a sharp rejection, but also exhibits an ultra-wide
stopband. For the deigned low-pass filter, an insertion loss of less
than 2 dB fromdc to 3.5 GHz and the rejection is better than 20 dB
from 4.3 to 15.8 GHz. Predicted performances show widened and
deepened stopband beyond the low passband. Furthermore, it is
confirmed by measurement.
Index Terms—Cross-shape, defected ground structure (DGS),
low-pass, stopband, ultra-wide.
INTRODUCTION
LOW-PASS filters have been widely used to suppress harmonics
and spurious signals. The conventional microstrip
stepped-impedance resonator (SIR) filters, can only provide
Butterworth and Chebyshev characteristics with a gradual
cutoff frequency response [1]. In order to achieve a sharp
cutoff frequency response, more sections are needed [2], but
increasing sections will also increase the loss in the passband
and circuit size.
Microstrip transmission lines with an electromagnetic
bandgap (EBG) structure exhibit stopband and slow-wave characteristics,
which can be used as stopband or low-pass filters.
Recently, many EBG structures [3], [4] have been developed
for microwave circuits, which mostly have periods as photonic
crystals (PCs). However, EBG structures have the disadvantage
of many design parameters and difficulties in finding its
equivalent circuits. Defected ground structure (DGS) which has
etched defected in the ground plane, can also provide bandgap
characteristics, making a slow-wave structure [5]. Moreover,
the equivalent circuit and parameters can be extracted with
a one-step Butterworth low-pass prototype [6]. However, the
stopband bandwidth of photonic bandgap (PBG) and DGS are
enhanced by using periodic structures, corresponding to larger
Manuscript received November 21, 2005; revised February 14, 2006. This
Center, Cheng Shiu University, Kaohsiung 833, Taiwan, R.O.C.
Digital Object Identifier 10.1109/LMWC.2006.873594
sizes and transmission losses in passband. Moreover, the high
attenuation rates are also obtained with cells in series.
As reported in [7], wideband performance of filters was generated
by using three cells in series. However, their cell configuration
was composed of a traditional microstrip low-pass
filter and their proposed EBG cell, resulted in larger size and
loss. Huang and Lee [8] have designed the triple EBG structures
for a bandstop filter, but there was a spurious response
and passband ripple in their performances. In addition, more
cell-sections were needed for a sharp rejection. In [2], the elliptic-
function low-pass filters using SIR hairpin resonators provide
a wide-band stopband and a sharp cutoff frequency response,
but they’re designed as a compositing configuration,
corresponding to larger size and are difficult to use.
AnSIR microstrip filter with tapped-line excitation at the input
and output is developed to further extend the upper rejection band
[9]. By properly allocating the two transmission zeros, the first
and/or second spurious resonances in this filter are effectively
cancelled. In this letter,we propose a novel cross-shape defected
ground structure (CSDGS) low-pass filter. The proposed filter
shows a sharper cutoff frequency response by using only one unit
cell of CSDGS configuration. Furthermore, additional attenuation
poles are added to suppress the high-order harmonics. An
ultra-wide stopband bandwidth can be also obtained by SIR effect,
due to different aperture widths in CSDGS configuration.
The measured results agree well with simulated results.
II. CROSS-SHAPE DGS (CSDGS) DESIGN
For microwave circuits, the suppression of harmonics can be
realized by the construction of DGS, PBG or low-pass filters.
Since a single cell or resonator suppresses one harmonic, the
second and third harmonics can be suppressed simultaneously
by using cascaded configurations of several basic cells. However,
cascaded configurations give rise to higher insertion loss
and are harmful for miniaturized circuits. For examples, in [2],
the additional attenuation poles were implemented by using two
cascaded resonators to obtain a higher cutoff frequency and attenuation
at the second harmonic. In [8], it was an EBG microstrip
Bragg reflector whose design is restricted by the Bragg
reflection condition, resulting in larger size.