05-09-2012, 03:51 PM
Compact UWB Bandpass Filter on Hybrid Microstrip/Slotline Structure with Improved Out-of- Band Performances
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Abstract
A compact ultra-wideband (UWB) bandpass filter
with good in-band and out-of-band performances is proposed
and implemented on hybrid microstrip/slotline structure. A
multiple-mode slotline resonator with stepped-impedance
configuration is first properly constructed, aiming to allocate its
first three resonances occurring at desired positions within the
UWB passband. Two 50 Ω microstrip feed lines are then placed
perpendicular to this slotline resonator on the opposite side of the
substrate. As a result, an initial back-to-back microstrip-slotline
bandpass filter is constituted with the targeted UWB passband.
Later the slotline resonator is twisted and an additional coupling
between two open-ends of feed lines is introduced, resulting in
miniaturizing its overall circuitry size and enhancing the roll-off
skirts out of the passband. In final, defected ground structures
(DGSs) are formed on the two feed lines to improve the upper
stopband performance. Two UWB bandpass filters are designed,
fabricated and measured. Measured results demonstrate an
UWB passband from 3.25 to 10.21 GHz with in-band return loss
>10.43 dB and group delay variation < 0.34 ns.
INTRODUCTION
In modern wireless communication systems, ultra-wideband
(UWB) technology has been gaining much more attention
since the Federal Communication Committee (FCC) in USA
authorized the unlicensed use of short-distance
communication in the spectrum of 3.1~10.6 GHz in 2002 [1].
As an essential component, UWB bandpass filters with good
in-band and out-of-band performances are in high demand. In
the past years, numerous UWB bandpass filters have been
reportedly developed by using different geometrical structures
and design methodologies [2]-[10]. Among them, one class of
UWB bandpass filters based on multiple-mode resonator
(MMR) has been considered more attractive for its advanced
features, such as simple design procedure, low in-band loss,
compact size and low fabrication cost.
RESULTS AND DISCUSSION
As shown in Fig. 2©, the two initially-implemented UWB
bandpass filters suffer from very narrow upper-stopband and
very poor rejection at high frequencies due to unexpected
high-order harmonic resonances of the slotline resonator. To
address this issue, these two initial filters need to be internally
or externally integrated with microstrip-line DGSs or lowpass
filters in the two feed lines as discussed in [9, 10]. In this work,
the two identical DGSs are installed along the two feed lines,
aiming at exciting a preferred stopband beyond the desired
UWB passband and eventually suppressing those unwanted
harmonic resonances at high frequencies (10.6 to 20.0 GHz).
After optimization design is carried out via Agilent ADS
simulator, the two Bowl-shaped UWB bandpass filters with
and without DGSs are fabricated on the Roger’s RT/Duriod
6010 with relative permittivity: εr = 10.8 and thickness: h =
0.635 mm, respectively. Fig. 3(a) illustrates the top- and
bottom-view photographs of these two filters. The defected
ground structure (DGS) is composed of three transverse slots
that are formed on the ground plane underneath the feed-line
strip conductor. As explained above, these two DGSs are used
to produce a highly attenuated stopband beyond the desired
UWB passband.
CONCLUSION
In this paper, a novel UWB bandpass filter with improved
out-of-band performances and compact size is presented using
the multiple-mode slotline resonator and microstrip-to-slotline
transitions. This slotline resonator with stepped-impedance
configuration is designed with the target of allocating its first
three resonances within the UWB passband. As this resonator
is twisted and two feed lines are moved in proximity to each
other, the overall circuitry size is largely reduced and the
rejection skirts outside the passband are significantly
sharpened due to the emergence of transmission zeros. In final,
two Bowl-shaped UWB bandpass filters with/without DGSs
implemented along the two feed lines are designed, fabricated
and measured.