Fourth-order tri-band bandpass filter usingsquare ring loaded resonators J.-Z.. Liang A tri-band bandpass filter BPF using a novel square ring loaded res-onator SRLR is presented.. Moreov
Trang 1Fourth-order tri-band bandpass filter using
square ring loaded resonators
J.-Z Chen, N Wang, Y He and C.-H Liang
A tri-band bandpass filter (BPF) using a novel square ring loaded
res-onator (SRLR) is presented The SRLR can generate a tri-band response
by tuning its geometric parameters Moreover, it can build the
high-order tri-band BPFs using the proposed resonators because of the
suffi-cient coupling between adjacent resonators A fourth-order Chebyshev
tri-band BPF centred at 2.4, 3.5 and 5.2 GHz is designed and
fabri-cated Measurement results agree well with the full-wave EM simulated
results
Introduction: In recent years, the tri-band BPF has become one of the
most important RF devices to meet the increasing communication
requirements Many different methods have been introduced and
reported to design tri-band BPFs A widely used method is to utilise a
single resonator with controllable resonant frequencies, such as the
stepped impedance resonator (SIR) [1-3] and the stub loaded resonator
(SLR) [4-6], applying their multiband behaviours The three desired
fre-quencies can be conveniently controlled by the tri-section SIR[2, 3] For
the SLR, it is also attractive in dual-band[4, 5]and tri-band[6]BPF
design owing to its flexibility Although these designs are effective,
all of them are just second-order BPFs and they are hard to extend to
high-order BPFs In this Letter, a novel square ring loaded resonator
(SRLR) is proposed for high-order tri-band BPF applications The
novel SRLR is flexible for high-order tri-band BPF designs To verify
its performance, a fourth-order Chebyshev tri-band BPF that can be
operated at 3.5 GHz WiMAX band and 2.4/5.2 GHz WLAN bands is
designed, fabricated, and tested
L2
Z0
L3
Z in,odd
L1
L2
Z0
L3
Z in,even
L1
d
c
L2
L1 Z0
L3 W1
L2
L1 Z
0
L3
square ring folded line
Fig 1 Layout of square ring loaded resonators
a Original SRLR
b Odd-mode equivalent circuit
c Even-mode equivalent circuit
d Odd symmetrical SRLR
Properties of square ring loaded resonator: As shown inFig 1a, the
original SRLR consists of two open folded microstrip lines and a
square ring Since the original SRLR is even symmetrical to the
dashed line, the resonator frequency can be extracted by the even and
odd-mode method The odd-mode and even-mode equivalent circuits
are shown in Figs 1b and c, respectively The odd-mode equivalent
circuit contains two resonant circuits The two resonant frequencies
can be obtained as follows:
4(L1+ L2) √ , f1e odd2= 3c
4(L1+ L3) √1e (1) where c is the speed of the light in free space, and 1edenotes the
effec-tive dielectric constant of the substrate For even-mode excitation, the
required resonant frequencies can be written as:
These three frequencies can be used to design a tri-band BPF However,
it is found the original SRLRs are not suitable for building high-order
tri-band BPFs, because the couplings between the resonators are not
so efficient Therefore, the odd symmetrical SRLR is put forward in
added in the SRLR for fine-tuning resonant frequencies It is found
that the proposed new odd symmetrical SRLR can offer the required
couplings while it can keep the similar frequency characteristics with
the original SRLR.Fig 2shows the EM simulated frequency responses
of the odd symmetrical SRLR FromFig 2a, it can be seen that fodd1can
be kept constant while feven1, fodd2 change when tuning L2 Correspondingly, as shown inFig 2b, fodd1, fodd2change in the same direction while feven1changes in the opposite direction when tuning W2 According to the above analysis, when we design a tri-band BPF, basic structure parameters (such as L1, L2, L3) of the SRLR can be first decided by sovling (1) to (2), and then the resonant frequencies can be determined by adjusting L2 and W2 slightly Therefore, the required three frequencies can be simultaneously obtained Fig 3 shows the simulated electric current density at f1(2.4GHz), f2(3.5GHz), and f3(5.2GHz) It can be seen that the electrical current density level
is higher at the vertical parts of the SRLR, which is mainly atributed to
a stronger magnetic coupling between the odd symmetrical SRLRs
–30 –20 –10
–40
0
fodd2
feven1
fodd1
L2=4mm
L2=5mm
L2=6mm
L 2 =7mm
L2=8mm
fodd2
f even1
fodd1
W2=0.4
W2=1.4
W 2 =3.4
W2=5.4
W 2 =7.4
Fig 2 Simulated insertion loss of odd symmetrical SRLR for varying L2and
W1
a L2
b W1
2.4GHz 3.5GHz 5.2GHz
0 db –2 db –6 db –10 db –14 db –18 db –22 db –26 db –30 db –34 db –38 db
Fig 3 Simulated electric current density at 2.4/3.5/5.2GHz
W1
W3
G1
W2
G2
G3
W6
L2
L7
L3
L4
L5
L5
L10
L9
L8
Fig 4 Layout of tri-band BPF Circuit dimensions (in mm): W1¼ 2.7, W2¼ 0.3, W3¼ 1.0, W4¼ 2.8, W5¼ 3.3,
W6¼ 6.2, L1¼ 30.9, L2¼ 7.8, L3¼ 6.2, L4¼ 7.3, L5¼ 6.6, L6¼ 26.1, L7¼ 9.7,
L8¼ 6.9, L9¼ 6.8, L10¼ 5.9, G1¼ 0.2, G2¼ 0.4, G3¼ 0.8
Tri-band bandpass filter design and result: A fouth-order Chebyshev tri-band BPF applying the proposed novel SRLR is designed on a sub-strate (1r¼ 2.65 and h ¼ 1.0 mm) Given the three operating frequen-cies are centred at 2.4, 3.5 and 5.2 GHz, the 3dB fractional bandwidths are 8.4, 8.0 and 4.8%, respectively.Fig 4describes the layout of the proposed filter After an efficient optimisation process using IE3D soft-ware, the dimensions for this tri-band BPF are found Its simulated and measured results are shown inFig 5 Good agreements are obtained The simulated results are centred at 2.4/3.5/5.2 GHz, their 3dB frac-tional bandwidths are 8.5, 8.0 and 5% In the measurements which are performed on an Agilent 8917ES network analyser, the three passbands are centred at 2.4/3.5/5.2 GHz, with the 3dB fractional bandwidths of 8.6, 7.8 and 4.9%, respectively The measured insertion losses at the
Trang 2passband centre frequencies are 1.57, 1.60 and 1.77 dB, respectively.
The return losses of the three bands are below 215 dB
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 –50
–40
–30
–20
–10
0
S21
S11
frequency, GHz
simulation measurement
Fig 5 Simulated and measured results of tri-band BPF
Conclusion: A novel square ring loaded resonator is proposed and has
been used to design a tri-band BPF A fourth-order BPF with tri-band
performance centring at 2.4/3.5/5.2 GHz is designed, fabricated and
measured to verify performance From the measured results, the
designed filter has exhibited good tri-band bahaviour This filter can
be applied to various tri-band designs such as wireless local area
networks
#The Institution of Engineering and Technology 2011
17 March 2011
doi: 10.1049/el.2010.3724
One or more of the Figures in this Letter are available in colour online
J.-Z Chen, N Wang, Y He and C.-H Liang (Science and Technology
on Antenna and Microwave Laboratory, Xidian University, Xi’an, People’s Republic of China)
E-mail: xjtucjz@gmail.com References
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