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A Compact UWB filter for Wireless communication

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Ultra-Wide Band (UWB) is a promising technology for many wireless applications due to its large bandwidth, good ratio of transmission data and low power cost. the main goal of this work is to design an UWB filter suitable for that purpose. In order to achieve that goal, one UWB filter configuration have been investigated, designed and characterized after analyzing the typical filter parameters, such as the return loss, insertion loss and attenuation characteristics over the full frequency band. Setting the dimensions of the proposed filter in a miniaturized size is a requirement as per user specification. The size of this filter has also been studied because of its important aspect on the frequency behavior.

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									International Journal of Scientific Research Engineering &Technology (IJSRET)
Volume 1 Issue2 pp 004-010 May 2012                              www. ijsret.org         ISSN 2278 - 0882




                 A Compact UWB filter for Wireless communication

                                                 Manidipa Nath
                                           AICTR, Department of ECE,
                                                New Delhi 700031
                                           manidipa.deoghar@gmail.com




ABSTRACT                                                   where fc is the central frequency and BW is the
                                                           bandwidth. UWB can operate between 3.1 and 10.6
 Ultra-Wide Band (UWB) is a promising technology           GHz at limited transmission powers for indoor
for many wireless applications due to its large            communications, as defined in FCC.
bandwidth, good ratio of transmission data and low         In 2002, the Federal Communications Commission
power cost. the main goal of this work is to design        (FCC) of the United States released the frequency
an UWB filter suitable for that purpose. In order to       band 3.1-10.6 GHz for ultra-wideband (UWB)
achieve that goal, one UWB filter configuration have       commercial communications. So recently, more
been investigated, designed and characterized after        attention has been paid to applications of ultra-
analyzing the typical filter parameters, such as the       wideband (UWB)            technology on wireless
return loss, insertion loss and attenuation                communication system. UWB technology is
characteristics over the full frequency band. Setting      promising and attractive for local area networks,
the dimensions of the proposed filter in a                 position location and tracking, and radar systems,
miniaturized size is a requirement as per user             because UWB has thecharacteristics of low cost, high
specification. The size of this filter has also been       data transmission rate and very low power
studied because of its important aspect on the             consumption. Many UWB devices and circuits have
frequency behavior.                                        been proposed and investigated widely. It is
                                                           important to reducetheir size and weight in order to
Keywords- UWB filter, resonator, miniaturization,          integrate them with other components as a compact
matching, stub, shortin                                    system. Compact and broadband bandpass filter
                                                           (BPF) is a key passive component and highly
  I.   INTRODUCTION                                        demanded in a UWB system. A planar BPF, based on
                                                           a microstrip structure, can provide the advantages of
As defined by the Federal Communications                   easy design, low cost, compact size, and is widely
Commission (FCC), UWB technology[1] is to                  used in a variety of RF/microwave and millimeter-
transmit and receive information over a large              wave systems to transmit energy in passband and to
bandwidth                                                  attenuate energy in one or morestopbands. So,
These are the two conditions of UWB technology:            compact UWB microstrip BPF can be used in a UWB
                                                           communication system. UWB filters must have a
BW > 500MHz                                                fractional bandwidth of more than 70%, and it is very
or                                                         difficult to achieve such a wide passband with a
BW /fc >0:2                                                traditional parallel-coupled transmission line
                                                           structure. Therefore, there is a requirement for UWB
                                                           BPF with a strong coupling structure that can be
                                                           easily fabricated. In this design, a dual-line coupling

                                                 IJSRET @ 2012
International Journal of Scientific Research Engineering &Technology (IJSRET)
Volume 1 Issue2 pp 004-010 May 2012                                www. ijsret.org         ISSN 2278 - 0882

structure has been used to implement a strong                third resonant mode at the edges of the UWB
coupling between the input/output port and the               passband. The parallel-coupled lines are modified to
resonator, which is more compact than the inter-             obtain the ultra-wide passband. This could be done
digital coupling structure. A compact UWB                    by adjusting the coupling length, Lc [5], for example.
microstrip BPF and Notched BPF with low insertion            The second type is a hybrid coplanar waveguide
loss have been presented and analyzed. In addition,          (CPW) and microstrip structure. This type of
the UWB filters have extremely compact size of 25            structure consists of a CPW MMR on one side and a
mm X 10 mm for BPF and 25 mm X 28 mm for                     microstrip input and output on the other side [6]. The
Notched BPF when the length of the feed lines is             CPW MMR is responsible for generating the first and
ignored.                                                     third resonant mode for the UWB passband, which is
 After the release of UWB, bandpass filters with a           similar to a microstrip MMR in [5]. Its geometry can
passband of the same frequency range (3.1 GHz -              be varied. Fig. 2 shows the CPW MMR in [6]. The
10.6 GHz, a fractional bandwidth of 110%) were               third type of filter which is also able to have a
challenges for conventional filter designs. Before mid       fractional bandwidth of 110% is the broadside-
2003, the bandwidth of the passband for a bandpass           coupled microstrip-CPW structure [7] shown in Fig.
filters was extended from 40% to 70% [2]. These              3. There is a broadside-coupled microstrip line
filters are named broad bandpass filters. They were          on one side of the substrate [see Fig. 3 (a)] and an
not covering the whole UWB frequency range yet. In           open-end CPW on the other side of the substrate [see
[3], a bandpass filter covering the whole UWB                Fig. 3 (b)]. The length of the coupled line equals to
frequency range with a fractional bandwidth of 110%          λg/2 in order to obtain a 110% bandwidth. The last
was realized by fabrication signal lines on a lossy          type of filters that has a bandwidth as high as around
composite substrate. A successful transmission of the        100% is the combination of a highpass filter and a
UWB pulse signal was demonstrated using the                  lowpass filter [8]. In [8], a stepped-impedance
proposed bandpass filter. This is one of the early           lowpass filter is embedded into a highpass filter with
reported filters that possess an ultra-wide passband.        quarter-wavelength short-circuited stubs, achieving a
However, it has a high insertion loss in the passband        passband from 3 GHz to 10 GHz. New fabrication
due to the lossy substrate. Not much research work           technique, such as Low Temperature Co-fire Ceramic
was reported in 2003 and 2004. In 2004, a ring               (LTCC), is applied in UWB bandpass filter designs
resonator with a stub was proposed which shows a             [9]. In [9], a LTCC bandpass filter shows a
bandwidth of 86.6% [4]. A bandpass filter covering           bandwidth of 48.75%. This filter has a small physical
the whole UWB frequency band was a challenge for             size due to the multi-layer configuration. However,
microwave filter designers and researchers in that           the bandwidth of the passband is relatively small
period of time.                                              compared to other UWB bandpass filters and the
                                                             insertion loss is as high as around 2.2 dB.
In 2005, there are 11 conference papers in total
published in International Microwave Symposium,              In 2006, microstrip MMR based UWB bandpass
International Conference on Ultra-Wideband, Asia-            filters are further optimized with improvement in the
Pacific Microwave Conference, or European                    rejection of the upper stopband. It can be done by
Microwave Conference. In the same year, there are            introducing interdigital microstrip coupled lines at
four journal publications. There are mainly four types       the two sides of the MMR in [10]. A highpass filter
of structures that are able to realize an ultra-wide         consisting of a transmission line with two embedded
passband. One is a microstrip structure shown in Fig.        U-shaped slots is cascaded with a lowpass filter
1 [5]. It consists of a microstrip multi-mode resonator      which is a dumbbell-shaped defected ground
(MMR) and a parallel-coupled line at each end of the         structure array in the ground plane, to obtain a
network. The MMR has two identical high-                     passband from 3 GHz to 10.9 GHz [11]. With novel
impedance sections with a length of quarter guided           highpass and lowpass structures, the bandpass filter
wavelength at two sides and a low-impedance section          obtains a wider bandwidth than the filter taking a
with a length of half guided wavelength in the               similar approach in 2005 [8]. With regards to the
middle. The MMR in the filter generates first and            UWB bandpass filter designs by cascading a highpass

                                                   IJSRET @ 2012
International Journal of Scientific Research Engineering &Technology (IJSRET)
Volume 1 Issue2 pp 004-010 May 2012                                   www. ijsret.org        ISSN 2278 - 0882

and a lowpass filter, a systematic consistent and             the application in hand-held devices. A system
analytical method is proposed [12]. There are a good          integrating both filters and antennas in UWB
number of new structures proposed that exhibit an             frequency range is very attractive to wireless
ultra-wide passband [13] – [16]. In [13], 3λg/4               communications using signals in this frequency band.
parallel-coupled line resonators shown in Fig. 4 are          UWB was originally developed for military
used to realize a passband from 3 GHz to 10 GHz.              communications and radar. In the field of UWB
With the introduction of lumped components to a               technology different methods and structures [20-
microstrip line, a miniaturized UWB BPF with a                24]has pushed development of new UWB filters .
length of 0.18λg is realized at a fractional bandwidth        Lumped-element filter design is generally unpopular
of 127% at a center frequency of 6.5 GHz [14]. The            due to the difficulty of its use at microwave
small physical size is attributed to the lumped               frequencies along with the limitations of lumped-
components used. A broadside coupled line in                  element values [25]
suspended substrate stripline [15] can also be used to
realize an UWB bandpass filter. A filter with short-            II.     THEORY
circuited stubs could giverise to a UWB bandpass
filters with a bandwidth of 110% [16].                        The size is another important factor in this work,
                                                              because the final application requires a small filter
In 2007, there are 26 papers reporting new UWB                with a diameter around 3.1 cm. This restriction is the
bandpass filters which is much more than the                  hardest specification due to the relationship between
previous two years (15 papers in 2005, 18 papers in           the size and the frequency.For lower frequencies as
2006). UWB bandpass filters with a notch stopband             required in the specifications, the size should be
from 5 GHz to 6 GHz for filtering the wireless local-         bigger.Thus, the design of a small filter becomes a
area network (WLAN) is a new topic branched out in            challenging issue. UWB was originally developed for
this area [17] - [19]. Additional components are              military communications and radar. In the field of
introduced providing the notch stopband at the                UWB technology different methods and structures [2-
desired frequency. In [17], an embedded open-circuit          6]has pushed development of new UWB filters .
stub is proposed providing a sharp notch stopband. It         Lumped-element filter design is generally unpopular
is integrated into a UWB bandpass filter providing            due to the difficulty of its use at microwave
the stopband from 5 GHz to 6 GHz. A stub is                   frequencies along with the limitations of lumped-
introduced in the broadside-coupled microstrip-CPW            element values [25]. Hence, conventional microstrip
structure [18] to generate a notch stopband at WLAN           filters are often used. The new proposed filter design
frequency range. Other than adding stubs to the               is based on quarter wavelength short-circuited stub.
structure, in [19], a notch stopband is generated in the      In order to reduce filter size, bending connecting line
UWB passband by an asymmetric parallel-coupled                and let five short-circuited stubs via the same hole is
line at two sides of a microstrip MMR.                        designed .Consequently,half hexagon UWB filter was
                                                              simulated and optimized for its best achievable
There are three main existing approaches to realize a         performance.
UWB bandpass filter. One is a microstrip or CPW
MMR with the assistance of coupling mechanisms,               The paper focuses the on systematic design and
such as microstrip coupled line or coupling at the            realization of a ultra wideband filter in printed circuit
transit between a microstrip line and a CPW.                  configuration.The UWB filter has as one input and
Broadside-coupled microstrip line with a CPW at the           one output port.The filter is of hexagonal shaped
back is another important configuration. The third            microstrip configuration.The length and width of
one is a direct or indirect combination of a lowpass          fingers of the connecting lines are taken as design
and a highpass filter. In terms of miniaturization, the       parameter for optimization. The MOM simulation
employment of LTCC or lump components is an                   tools used to investigate the performance of the this
effective means to significantly reduce the size of the       filter and the combined                 responses for
structure. For future development and research in this        ultrawideband.It       is designed as per FCC
area, miniaturization of UWB filters is important for         recommended band from 3.1-10.6 GHz. It was

                                                    IJSRET @ 2012
International Journal of Scientific Research Engineering &Technology (IJSRET)
Volume 1 Issue2 pp 004-010 May 2012                                   www. ijsret.org        ISSN 2278 - 0882

observed that the design dimensions are critical in           stubs and the characteristic impedances of the
deciding the filter responses. The line dimension and         connecting lines are choosen at 3.1 GHz.
coupling gaps are optimized to meet the specification
and final pcb design is generated. The UWB filter is           III.     DESIGN
designed to provide an Insertion Loss ≤ 1 dB and
average roll off of 30 dB/decade. Simulated results           To meet the design criterion particularly the
predicts performance of the filter as per FCC                 bandwidth and size a half hexagonal microstrip
Standard. The filter hardware based on the optimized          structure with shorting pin configuration has been
design has been fabricated and tested.The                     choosen. The filter design has been implemented on a
measurement results are quite encouraging.                    high frequency circuit board.Here the substrate used
                                                              is 25 mil(dielectric constant 10, loss tangent 0.009).A
The structure under consideration consist of a TEM            detail analysis has been done to find the response of
mode or quasi-TEM mode transmission line                      the structure under consideration(Fig 2) using
resonator elements arranged in a half hexagonal               moment method and FDTD solver. It has been seen
pattern. Each resonator element has an electrical             that the bandwidth criterion (3.1-10.6 GHz.) is
length of 90 degree at the midband frequency and is           fulfilled with this structure and the size is perfect to
short circuited at one end. The resulting filter is           put in a predefined package for testing and
compact and the tolerance required in their                   measurement.The shorting pin is used to suppress the
manufacture are relatively relaxed. The second pass           unwanted mode that may lead to additional loss for
band of this filter is centered at about 3 times the          this filter configuration.Final design has been
midband frequency of the desired first pass band              optimized several times for getting best filter
while there is no spurious response in between.               response over the frequency band of operation”.
Another advantage is that the filter can be fabricated
in structural forms which are self supporting so that
dielectric material is not required to be used.

An exact analysis of the structure is very tedious.
Hence a synthesis procedure is followed which
involves a number of simplifying approximations that
permit straightforward, easy to-use design
calculations. However these approximate design
equations are found to be sufficiently accurate for
most practical applications.

The new proposed filter design is based on quarter
wavelength short-circuited stubs. Here five short-
circuited stubs was designed for an optimum
distributed band pass filter performance whose
connecting lines are non-redundant [26]. Thus, the               Figure 1.Simulation result of UWB filter(before
filter can exhibit a frequency selectivity equivalent to                          optimization)
that of a conventional 9-pole Chebyshev filter. But an
optimum distributed high-pass filter requires a
greater area. In order to reduce the filter size , the
connecting line had been reduced.Thus, let 9-pole
decrease to 5-pole, even though frequency selectivity
of 5-pole is not better than 9-pole. However, circuit
dimension can be reduced very much. The
characteristic impedances of these short-circuited



                                                    IJSRET @ 2012
International Journal of Scientific Research Engineering &Technology (IJSRET)
Volume 1 Issue2 pp 004-010 May 2012                                 www. ijsret.org         ISSN 2278 - 0882




                                                                Figure 4. UWB filter configuration with package.

       Fig2. Simulated S-Parameters of UWB Filter              V.     MEASUREMENT

                                                              The fabricated filter was measured for transmission
                                                              and reflection performance with the help of Network
                                                              Analyzer (E8363B).The measured attenuation and
                                                              VSWR plot of the filter is shown in figure(3-4).The
                                                              fractional bandwidth is 0.76 instead of the design
                                                              value 0.868 & simulation value 0.78, which indicates
                                                              a shrinkage of bandwidth of 12.6 % as a result of
                                                              various approximation involved in the design
                                                              equations and due to fabrication inaccuracies.The
                                                              insertion loss is found to be 3.5dB (average) over the
                                                              band. The fabrication process is required to be better
                                                              to improve this loss figure. The other performance is
                                                              seen to be satisfactory.
   Figure 3.Simulation Result of UWB Filter(after
                   optimization)

 IV.      CHARACTERIZATION

The filter layout has been fabricated using CER-10
with best precession available.The Final circuit after
integration and packaging undergone for testing.Inital
measurement results shows good filter characteristic
over the whole UWB band.The measured return loss
over the band is 1.0 dB(average).This loss can be
further reduced using low loss substrate and SMA
connectors.


                                                               Figure 5 Attenuation Measurement of UWB Filter




                                                    IJSRET @ 2012
International Journal of Scientific Research Engineering &Technology (IJSRET)
Volume 1 Issue2 pp 004-010 May 2012                              www. ijsret.org      ISSN 2278 - 0882

                                                         passband are equal to 3.9 GHz and 11.2 GHz against
                                                         their counterpart frequencies of 3.935 GHz and 10.81
                                                         GHz in the simulation. Furthermore, the UWB filter
                                                         has achieved the return loss of less than -10 dB from
                                                         4 to 10.5 GHz.

                                                         VII.    CONCLUSION
                                                         Compact UWB microstrip BPF have been designed
                                                         using the structure having half hexagon resonator
                                                         with the five stubs. The compact UWB filters have
                                                         beenfabricated, and tested. The fabricated UWB BPF
                                                         has a 7.3 GHz passband, -10 dB return lossbandwidth
                                                         of 6.5 GHz. These results have indicated a very
                                               Fi        goodagreement        between      simulation    and
        gure 6. VSWR Plot of UWB Filter                  measurements.

                                                         VIII.     REFERCES

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                                               IJSRET @ 2012
International Journal of Scientific Research Engineering &Technology (IJSRET)
Volume 1 Issue2 pp 004-010 May 2012                               www. ijsret.org        ISSN 2278 - 0882

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