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Published in IET Microwaves, Antennas & Propagation
Received on 2nd February 2013
Revised on 25th May 2013
Accepted on 18th June 2013
doi: 10.1049/iet-map.2013.0061

ISSN 1751-8725

Compact ultra-wideband bandpass filters with narrow

notched bands based on a ring resonator
Wenjie Feng1,2, Wenquan Che1, Quan Xue2
1
Department of Communication Engineering, Nanjing University of Science & Technology, 210094 Nanjing,
People’s Republic of China
2
State Key Laboratory of Millimeter Waves (Hong Kong), City University of Hong Kong, Hong Kong,
People’s Republic of China
E-mail: yeeren_che@163.com

Abstract: In this study, compact ultra-wideband (UWB) bandpass filters with single/dual-notched bands are proposed based on a
ring resonator and transversal signal-interference concept. Broad passband and narrow notched bands with controlled even/odd-
mode resonator frequencies can be adjusted conveniently by changing the characteristic impedance of the ring resonator.
Transmission zeros are introduced to improve selectivity and harmonic suppression of the proposed UWB bandpass filters
with notched bands. To verify the presented strategies, three prototypes (εr = 2.65, h = 0.5 mm) with 3 dB fractional
bandwidth >120% are designed and fabricated. Simple structure and good in/out-of-band performances can be achieved in the
proposed UWB bandpass filters.

1 Introduction single ring resonator in compact UWB filters with single/

multi-notched bands.
Increasing attention has been paid recently to the In this paper, compact UWB bandpass filters with single/
development of ultra-wideband (UWB) systems since the dual-notched bands are proposed based on a ring resonator
Federal Communications Commission’s decision to permit and open/shorted stubs. A broad passband with 3 dB
unlicensed operation band from 3.1 to 10.6 GHz in 2002 fractional bandwidth >120% can be realised with a
[1]. UWB bandpass filters with high performance and low three-quarter wavelength ring resonator and a shorted stub.
cost are a key component in UWB communication systems. Two transmission zeros are used to improve selectivity and
Various UWB filters employing multi-mode resonators, harmonic suppression based on the transversal
complementary split-ring resonator and multi-layer signal-interference concept. Furthermore, another two UWB
aperture-coupled patches have been designed and analysed bandpass filters with single/dual-notched bands for WiMax
[2–8]. In [9–16], cascaded low-pass/high-pass filters, band, WLAN band and satellite-communication systems
electromagnetic loaded bandgap and T/Y-shaped resonators 8.0 GHz band are proposed using the three-quarter
were used to extend the upper stopband bandwidth and wavelength ring resonator and an open stub. The bandwidth
improve selectivity of the passband. In addition, UWB and centre frequencies of the notched bands can be easily
bandpass and differential filters have been realised based on adjusted by changing the electrical length and characteristic
the transversal signal-interference concept in [17–24], by impedance of the open stub. All the structures are designed
introducing intentionally a passband constructive and fabricated on a dielectric substrate with εr = 2.65, h =
interference and out-of-band signal energy cancellations to 0.5 mm and tan δ = 0.002. Good agreement is found
produce power transmission zeros, high-selectivity filtering between the theoretical and measured results.
responses and harmonic suppression.
In addition, since the frequency band of the UWB indoor
limit (3.1–10.6 GHz) covers existing wireless 2 Design of proposed UWB bandpass filter
communication systems, such as the 3.5 GHz band with single centrally loaded shorted stub
WiMax, 5.2–5.8 GHz wireless local-area network
(WLAN) and some 8.0 GHz band satellite-communication Figs. 1a and b show the top view and the circuit of the UWB
systems, it is desirable to introduce single or multiple bandpass filter with a three-quarter wavelength ring resonator,
notch bands to avoid interferences from existing wireless two different transmission paths with electrical length 2θ1 and
communication systems in design of UWB bandpass θ1, characteristic impedance Z1 are tapped between ports 1 and
filters. A number of UWB bandpass filters with notched 2. A centrally loaded shorted stub with electrical length θ2 and
bands have been investigated and discussed in [25–32]. characteristic impedance Z2 is attached to the centre of path
However, there has been little work on the application of 1. Owing to the symmetry of the ring resonator, the even/

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Fig. 1 Proposed UWB bandpass filter with the three-quarter wavelength ring resonator and a shorted stub
a Top view
b Equivalent circuit
c Even-mode equivalent circuit
d odd-mode equivalent circuit

odd-mode equivalent circuits of the ring resonator are shown in To understand the relationship between the bandwidth of
Figs. 1c and d [33] for further theoretical analysis, respectively. the passband for the UWB filter based on the ring resonator
in a better manner, the loaded quality factor QL of the
2.1 Even/odd-mode analysis passband against θ2, Z1 and Z2 is shown in Figs. 3a and b.
It may be noted that the QL of the passband decreases as Z1
When the even/odd-mode signals are excited from ports 1 to increases, whereas it increases as θ2, Z2 increase. In
2, a virtual open/short appears along the centre of the addition, the loaded quality factor QL and the 3 dB
three-quarter wavelength ring resonator, and the even/ bandwidth Δf3 dB for the passband can be related by [34]
odd-mode input admittance Yine/Yino of Figs. 1c and d is

 QL = f0 /Df3dB (3)
tan u1/2 tan u1 − Z1 cot u2 /2Z2
Yine =j +j
 (1)
Z1 Z1 + Z12 tan u1 cot u2 /2Z2 The loaded quality factor QL can be expressed as QL = f3 dB
(θ2, Z1, Z2). Once Z0 and θ1 are determined, we can adjust θ2,
cot u1 cot u1/2 Z1 and Z2 to satisfy the demand of QL. Obviously, the
Yino = −j −j (2) required 3 dB bandwidth Δf3 dB and the transmission
Z1 Z1
characteristic for the passband can be simultaneously
The resonance frequencies for the even/odd-mode can be obtained and further optimised based on the above discussion.
calculated when Yine/Yino = 0 or ∞, and the even/odd-mode
resonator frequencies for the ring resonator under weak 2.2 Proposed UWB bandpass filter with single ring
coupling and against θ1 are shown in Figs. 2a and b. The resonator
bandwidth for the ring resonator is mainly determined by
the even-mode resonator frequencies feven1 and feven2, and Based on the above theoretical analysis, the final parameters
the even-mode resonator frequency feven3 is a transmission for the filter circuit of Fig. 1b are given as follows: Z0 =
zero near the upper passband for improving selectivity. 50 Ω, Z1 = 60 Ω, Z2 = 70 Ω, f0 = 7.7 GHz, θ1 = 90° and θ2 =
However, there are two unknown parameters (θ1, θ2) in (1), 23°. In addition, the structure parameters for the UWB
hence, it cannot solve the expressions for the three even bandpass filter (15 × 12 mm, 0.56λg0 × 0.45λg0) in Fig. 1a
modes directly. Moreover, another two odd-mode resonator are listed below: L1 = 10.35, L2 = 2.1, W0 = 1.37, W1 = 1,
frequencies fodd1 (θ1 = 120°, 4f0/3) and fodd2 (θ1 = 180°, 2f0) W2 = 0.8 and d = 0.7 mm. The photograph, measured results
can be realised. We note that the odd-mode resonator and simulated results are shown in Fig. 3c, where good
frequency fodd2 is mainly caused by the 180° phase agreement is found between the measured and simulated
difference of the two transmission paths at 2f0 [20–22]. In results. The measured insertion loss is <0.95 dB whereas
addition, Figs. 2c–f illustrate |S21| against θ2 and different the return loss is over 14 dB from 3.7 to 11.6 GHz (3 dB
characteristic impedances Z1 and Z2. As we can see, the fractional bandwidth is ∼123.4%, 2.4–11.9 GHz). Three
bandwidth of the filter increases as Z1 increases; decreases transmission zeros (located at 13, 14.6 and 15.7 GHz) are
as f0 as θ2, Z2 increase. In this way, the bandwidth for the realised to improve selectivity and harmonic suppression.
passband of the UWB filter with the ring resonator can be Furthermore, over 15 dB upper stopband is achieved from
controlled conveniently by changing the characteristic 12.85 to 18.5 GHz (2.4f0). The measured group delay is
impedances Z1, Z2 and θ2 when Z0 and θ1 are fixed. <0.20 ns from 2.7 to 11.5 GHz.

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Fig. 2 Resonance frequencies for the even/odd-mode

a Equivalent circuit of the ring resonator under weak coupling
b Analysis of resonator frequencies against θ1, Z1 = 60 Ω, Z2 = 70 Ω, θ1 = 4.5θ2
c |S21| under weak coupling, Z1 = 60 Ω, Z2 = 70 Ω, θ2 = 20°
d |S21| against θ2 at f0, Z1 = 60 Ω, Z2 = 70 Ω
e |S21| against Z1, Z2 = 70 Ω, θ2 = 20°
f |S21| against Z2, Z1 = 60 Ω, θ2 = 20°
f0 = 7.7 GHz

In addition, Table 1 illustrates the comparisons of measured open stub with characteristic impedance Z3 and electrical
results for several UWB bandpass filter structures. Compared length θ3 is connected in the centre of path 1. The even/
with other UWB filters [3–24], the effective circuit size of the odd-mode equivalent circuits for the ring resonator with the
proposed UWB bandpass filter is only 0.19λg0 × 0.19λg0, and open/shorted stubs are shown in Figs. 4c and d, and the
the 3 dB bandwidth is 123.4%. Moreover, to extend the upper even-mode input admittance Yine of Fig. 4c can be
stopband, lowpass/bandstop networks can be cascaded to illustrated as
improve the upper performance of the UWB filter with the
single ring resonator [9–13] further. tan u1/2
Yine = j
Z1



3 Design of proposed UWB bandpass filter tan u1 − Z1 cot u2 /2Z2 + Z1 tan u3 /2Z3
with single/dual-notched bands +j


Z1 + Z12 tan u1 cot u2 /2Z2 − Z12 tan u1 tan u3 /2Z3
3.1 Proposed UWB bandpass filter with (4)
single-notched band
The odd-mode input admittance Yino of Fig. 4d is the same as
To cancel the interference from WLAN signals (5.2–5.8 (2). Similarly, when the Yine/Zine = 0, the even-mode resonator
GHz), a UWB bandpass filter with a notched band is frequencies of Fig. 4c can be calculated. Figs. 5a and b show
further proposed as shown in Figs. 4a and b, where a shunt the equivalent circuit and the even/odd-mode resonator

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Fig. 3 Bandwidth of the passband for the UWB filter based on the ring resonator
a Simulated QL for the passband against θ2, Z1 and Z2 = 70 Ω
b Simulated QL for the passband against θ2, Z2 and Z1 = 60 Ω
c Photograph, measured and simulated results of the UWB bandpass filter

Table 1 Comparisons of measured results for several UWB bandpass filter structures
Filter structure Transmission poles (|S11|) 3 dB bandwidth, % Effective circuit size (λg0) Upper stopband, dB Group delay, ns

[3] 5 60 0.75 × 0.17 >12 (2.5f0) <0.6

[4] 4 72.6 0.65 × 0.25 >15 (2.3f0) <0.4
[5] 3 90 0.50 × 0.20 – <0.6
[6] 5 104 0.75 × 0.18 – <0.35
[7] 3 75 0.75 × 0.50 >20 (2.1f0) <0.29
[8] 6 110 0.75 × 0.50 >15 (1.9f0) <0.7
[14] 5 83 0.50 × 0.50 >25 (2.3f0) <0.5
[15] 3 65 0.75 × 0.50 >20 (2.5f0) <0.25
[17] 3 27.6 0.75 × 0.50 >15 (1.5f0) <0.69
[18] 4 75 0.50 × 0.25 >20 (2.5f0) <0.7
[19] 4 123 0.25 × 0.25 >20 (2.3f0) <1.5
[20] 3 124.6 0.25 × 0.18 >15 (2.5f0) <0.25
[21] 3 110 0.50 × 0.25 >13 (2.4f0) <0.60
[22] 3 117.6 0.75 × 0.25 >15 (2.1f0) –
[23] 5 61.7 0.50 × 0.50 >15 (2.7f0) <0.70
[24] 3 36.3 0.39 × 0.19 >15 (2.8f0) <0.70
this work 4 123.4 0.19 × 0.19 >15 (2.4f0) <0.20

frequencies for the ring resonator with shorted/open stubs addition, the even/odd-mode resonator, the ring resonator of
under weak coupling. A transmission zero ( ftz1) located at Fig. 1a does not change compared with Fig. 2b. The
5.6 GHz, and the third harmonic (3ftz1) of the transmission simulated frequency responses of the notched band against
zero can be used to improve the upper stopband further. θ3 and Z3 are shown in Figs. 5c–e, and the centre frequency
Actually, the electrical length θ3 of the open stub is nearly of the notched band decreases as θ3 increases, whereas it
a quarter-wavelength of the centre frequency for the increases as Z3 increases, indicating that the centre
single-notched band when Z0, Z1, Z2, θ1 and θ2 are fixed, frequency and the bandwidth of the notched band for
and it can be seen as an independent bandstop filter for the WLAN band can be easily adjusted by changing θ3 and Z3
single-notched band [34], as shown in Figs. 5c–e. In when Z0, Z1, Z2, θ1 and θ2 are fixed.

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Fig. 4 Proposed UWB bandpass filter with single-notched band

a Top view
b Equivalent circuit
c Even-mode equivalent circuit
d Odd-mode equivalent circuit
L1 = 9.95, L2 = 2.84, L3 = 3.11, L4 = 2.5, L5 = 2.4 mm and W0 = 1.37, W1 = 1.25, W2 = 1.1, W3 = 0.6 mm, d = 0.7 mm, 20 × 12 mm and 0.74λg0 × 0.45λg0

3.2 Proposed UWB bandpass filter with dual-notched bands can be acquired when Z4/Z3 = tanθ3
dual-notched bands tanθ4. To meet the two notched bands centre frequencies
located at 3.5 and 8.0 GHz, Z3 should be less than Z4 [35],
To demonstrate the above strategies further, a UWB bandpass and the centre frequency of the first notched band ( ftz1) can
filter with dual-notched bands to cancel the interferences for be seen as the fundamental resonant frequency of the
3.5 GHz band WiMax and some 8.0 GHz band satellite- stepped impedance resonator; the centre frequency of the
communication systems is proposed and shown in Figs. 6a second notched band ( ftz2) is the spurious resonance
and 6b, where a stepped impedance resonator [35] with frequency of the stepped impedance resonator, in which ftz2/
characteristic impedances Z3, Z4 and electrical lengths θ3, ftz1 < 3 (Z3 < Z4) [35]. In addition, the dual-notched bands
θ4 is introduced to realise the dual-notched bands, and the against the characteristic impedances Z3, Z4 and electrical
even-mode input admittance Yine of Fig. 6c is given by lengths θ3, θ4 are shown in Figs. 7c–f, the centre frequency

 of the first notched band decreases as θ3, Z3 increase and
tan u1/2 tan u1 − Z1 cot u2 /2Z2 the centre frequency of the second notched band also
Yine = j +j
 decrease as θ4, Z4 increase. In addition, the centre
Z1 Z1 + Z12 tan u1 cot u2 /2Z2


 frequency of each notched band can be adjusted
+ Z1 Z3 tan u4 + Z1 Z4 tan u3 / 2Z3 Z4 − 2Z32 tan u3 tan u4 independently when the other notched band is fixed.


 Moreover, the bandwidth of the notched bands can also be
− Z12 Z3 tan u4 + Z12 Z4 tan u3 / 2Z3 Z4 − 2Z32 tan u3 tan u4
adjusted by changing the characteristic impedances Z3, Z4
(5) and electrical lengths θ3, θ4 when Z0, Z1, Z2, θ1 and θ2 are
fixed.
where the odd-mode input admittance Yino of Fig. 6d is also
the same as (2). Similarly, when the Yine/Zine = 0, the
even-mode resonator frequencies of Fig. 6c can be 3.3 Measured and simulated results of proposed
calculated. The equivalent circuit of the UWB bandpass UWB bandpass filters with single/dual-notched
filter with dual-notched bands and even/odd-mode resonator bands
frequencies under weak coupling is shown as Figs. 7a and Based on the above theoretical analysis, the final parameters
b, two transmission zeros are located at 3.5 GHz ( ftz1) and for the UWB filter circuit with a notched band of Fig. 4b
8.0 GHz ( ftz2), respectively. As discussed in Section 3.1, are: Z0 = 50 Ω, Z1 = 55 Ω, Z2 = 60 Ω, Z3 = 75 Ω, f0 = 7.7
when Z0, Z1, Z2, θ1 and θ2 are fixed, the input admittance GHz, θ1 = 90°, θ2 = 30° and θ3 = 112°; and the final
YSIR is parameters for the UWB filter circuit with dual-notched
bands of Fig. 6b are: Z0 = 50 Ω, Z1 = 55 Ω, Z2 = 60 Ω, Z3 =
Z3 tan u4 + Z4 tan u3 43 Ω, Z4 = 90 Ω, f0 = 7.7 GHz, θ1 = 90°, θ2 = 30°, θ3 = 125°
YSIR = jY3
 (6)
Z4 − Z3 tan u3 tan u4 and θ4 = 100°. The photographs, measured results and
simulated results of the UWB filters with single/
When YSIR → ∞, the two centre frequencies ( ftz1, ftz2) of the dual-notched bands are shown in Fig. 8. For the UWB filter

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Fig. 5 Equivalent circuit and the even/odd-mode resonator frequencies for the ring resonator with shorted/open stubs under weak coupling
a Equivalent circuit of the ring resonator with single-notched band under weak coupling
b |S21| under weak coupling, C0 = 0.02 pF, Z1 = 55 Ω, Z2 = 60 Ω, Z3 = 75 Ω, θ2 = 30° and θ3 = 112°
c Notched band against θ3, Z1 = 55 Ω, Z2 = 60 Ω, Z3 = 75 Ω and θ2 = 30°
d Notched band against Z3, Z1 = 55 Ω, Z2 = 60 Ω, θ2 = 30° and θ3 = 112°
e Notched band against θ3 and Z3, Z1 = 55 Ω, Z2 = 60 Ω, θ2 = 30°
C0 = 0.02 pF, Z0 = 50 Ω and f0 = 7.7 GHz

Fig. 6 Proposed UWB bandpass filter with dual-notched bands

a Top view
b Equivalent circuit
c Even-mode equivalent circuit
d Odd-mode equivalent circuit
L1 = 9.95, L2 = 2.84, L3 = 3.16, L4 = 3.6, L5 = 2.4, L6 = 3.5, L7 = 1.5, L8 = 2.5 mm, W0 = 1.37, W1 = 1.25, W2 = 1.1, W3 = 1.4, W4 = 0.5 mm, d = 0.7 mm, 20 × 12 mm
and 0.74λg0 × 0.45λg0

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Fig. 7 Equivalent circuit of the UWB bandpass filter with dual-notched bands and even/odd-mode resonator frequencies under weak
coupling
a Equivalent circuit of the ring resonator with single-notched band under weak coupling
b |S21| under weak coupling, C0 = 0.02 pF, Z1 = 55 Ω, Z2 = 30 Ω, Z3 = 43 Ω, Z4 = 90 Ω,θ2 = 30°, θ3 = 123° and θ4 = 100°
c First notched band against θ3 and Z3, Z1 = 55 Ω, Z2 = 60 Ω, Z4 = 90 Ω, θ2 = 30° and θ4 = 100°
d First notched band against θ4 and Z4, Z1 = 55 Ω, Z2 = 60 Ω, Z3 = 43 Ω, θ2 = 30° and θ3 = 123°
e Second notched band against θ4 and Z4, Z1 = 55 Ω, Z2 = 60 Ω, Z3 = 43 Ω, θ2 = 30° and θ3 = 123°
f Second notched band against θ3 and Z3, Z1 = 55 Ω, Z2 = 60 Ω, Z4 = 90 Ω, θ2 = 30° and θ4 = 100°
C0 = 0.02 pF, Z0 = 50 Ω, f0 = 7.7 GHz

with single-notched band (Fig. 8a), the measured 3 dB 11.6 GHz. The slight frequency discrepancies between the
fractional bandwidth is ∼126% (2.3–12 GHz), a notched measured and simulated results are mainly caused by the
band is located at 5.7 GHz with 10 dB bandwidth of 7.0% limited fabrication precision and experimental errors. To
(5.5–5.9 GHz). The group delay is 2.2 ns at 5.7 GHz. further highlight the advantages of our work, Table 2
Furthermore, over 15 dB upper stopband is achieved from illustrates the comparisons of measured results among our
12.6 to 19.2 GHz (2.5f0). The measured group delay is design and several other UWB bandpass filter structures
<0.22 ns from 2.1 to 12 GHz. with single/dual-notched bands [25–32]. The proposed
For the UWB filter with dual-notched bands (Fig. 8b), the UWB filters with single/dual-notched bands have more
measured 3 dB fractional bandwidth is ∼127% (2.2–12 GHz); compact effective circuit sizes (0.35λg0 × 0.19λg0, 0.35λg0 ×
dual-notched bands are located at 3.5 and 8.1 GHz with 10 0.30λg0) and simpler structures. In addition, in our work,
dB bandwidth of 10% (3.4–3.75 GHz) and 4.3% (7.95–8.3 more transmission zeros have been realised to improve the
GHz). The group delay is 2.1 ns at 3.5 GHz and 1.6 ns at upper stopband based on transversal signal-interaction
8.1 GHz. The measured group delay is <0.25 ns from 2.2 to concept.

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measured responses of the filters are observed, indicating
the validity of the design strategies.

5 Acknowledgments
This work is supported by the National Natural Science
Foundation of China (60971013), the 2012 Distinguished
Young Scientist awarded by the National Natural Science
Foundation Committee of China (61225001) and the 2010
and 2012 Innovative Projects for Graduates of Jiangsu
Province (CX10B_125Z, CXZZ12_0196).

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