Ultra-wideband bandpass filter based on transversal signal-interaction concepts
Based on the transversal filter concepts, a passband can be achieved by the following relations [6]:
u1 ( f0 ) = u2 ( f0 ) + 2np, (n = 0, 1, 2 . . .)
W.J. Feng, W.Q. Che and T.F. Eibert An ultra-wideband (UWB) bandpass filter based on transversal signalinteraction concepts is proposed. By cascading a planar Marchand balun and two transmission lines with different electrical lengths, a new UWB bandpass filter (fractional bandwidth 86 –111.5%) with two transmission zeros close to the passband is realised. Good agreement is observed between the simulated and measured results.
Introduction: As one of the most important microwave components, filters with high performance and compact size are extremely desirable in UWB technology. Recently, some filter structures using transversal signal-interaction concepts were proposed [1– 4]. In these filter structures, the input signal is split and propagates through two different feedforward signal paths. High-selectivity filtering responses and harmonic suppression can be achieved by forcing signal energy cancellations to produce transmission zeros. In our previous work [4], we proposed a novel UWB bandpass filter by cascading two planar Marchand baluns (fractional bandwidth 74 – 114%), but the selectivity for the filter is not very good and the filter size is still a little large. In this Letter, we propose a new UWB bandpass filter by cascading only one Marchand balun with two different transmission lines. Owing to the intrinsic 1808 phase difference of the Marchand balun, a new UWB filter (fractional bandwidth 86 – 111.5%) with two transmission zeros close to the passband is analysed and realised.
port 2 Zin
L1
Zout S2
W0
g2
g3
W1
port 1
q
1
plane of symmetry
q O/C
where f0 is the centre frequency of the filter. The UWB filter consisting of a planar Marchland balun and two different transmission lines with electrical lengths 3u (path 1) and u (path 2) is shown in Fig. 1b. At the centre frequency f0 of the filter, u1 ( f0) ¼ u2 ( f0) ¼ 3u ¼ u + 1808 (u ¼ 908); a passband can thus be achieved referring to (2). In addition, for the second harmonic (2f0) of the UWB bandpass filter, u1 (2f0) ¼ 6u ¼ 5408, u2 (2f0) ¼ 2u + 1808 ¼ 3608. When the signals propagate along paths 1 and 2 from port 1 to port 2, they are in the same magnitude but with out-of-phase response because of the 1808 phase difference, rejection bands can thus be achieved. In addition, for the two transmission paths, the ABCD matrix of path 1 is Ms1 × M3u , while the ABCD matrix of path 2 is Ms2 × Mu , and Ms1 , Ms2 , M3u and Mu can be acquired from [7]. After the conversions among Y-, ABCD-, and Sparameters, the frequency responses of the UWB bandpass filter can be computed. The simulated frequency responses of the UWB filter are shown in Fig. 1c. Here three different values of Zout ¼ Z1 (Zin ¼ Zo ¼ 50 V) are chosen, corresponding to Zoe ¼ 111.2 V, 90.6 V, 79.5 V (Zoo ¼ 33 V) [3, 4], respectively. The fractional bandwidth for the UWB filter ranges from 86 to 111.5% (|S11| . 10 dB), and two transmission zeros are located separately at both sides of the passband, leading to a quasi-elliptic function that improves the passband and out-of-band performances for the filter, and the transmission zero located at 2f0 is created by the two shorted stubs.
2
Zoe, Zoo
port 1
0º
4
Zoe, Zoo
(2)
S1 g0
L4
0
L2
L2
port 3 180º
via hole to ground W
g1
L3
port 2 L6
L7
t
3
L5
Zout
W2
a a
path 1, q1= 3q q
q+2q
0º
Zoe, Zoo
Z0
port 1
Z1
S2
q
port 2
q O/C
Zoe, Zoo
g3
Z0 d
g2
via hole to ground
S1
Z1
180º
q
q
path 2, q2=q +180º
b 0
b
magnitude, db
–10
S11
Fig. 2 Top view of proposed UWB bandpass filter (Fig. 2a); and bottom view of proposed UWB bandpass filter (Fig. 2b)
–20
W0 ¼ 1.37 mm, W1 ¼ 1.62 mm, W2 ¼ 0.65 mm, L1 ¼ 14.7 mm, L2 ¼ 1.85 mm, L3 ¼ 4.55 mm, L4 ¼ 1.2 mm, L5 ¼ 11.15 mm, L6 ¼ 4.5 mm, L7 ¼ 1.35 mm, g0 ¼ 0.15 mm, g1 ¼ 0.8 mm, g2 ¼ 0.3 mm, g3 ¼ 0.3 mm, S1 ¼ 3.8 mm, S2 ¼ 5.05 mm, d ¼ 0.7 mm, t ¼ 1.14 mm
–30 –40
S21
Z in = Z o = 50 W, Z out = Z 1 = 60 W Z in = Z o = 50 W, Z out = Z 1 = 90 W Z in = Z o = 50 W, Z out = Z 1 = 120 W
–50 –60
0
0.5
1.0
1.5
2.0
2.5
f/f 0
c
Fig. 1 Conventional planar Marchand balun (Fig. 1a); UWB filter cascading planar Marchand balun and two different transmission lines (Fig. 1b); and simulated frequency responses of UWB filter (Fig. 1c)
Filter design: Fig. 1a shows the circuit of the planar Marchand balun [5]. When an ideal signal is transmitted from port 1 to ports 2 and 3, power division with equal amplitude but 1808 phase difference can be realised for S21 and S31 (S21 ¼ 2S31), and the relationship between Zoe and Zoo for different Zin and Zout can be simplified as [4, 5]: (Zoe − Zoo)/(Zoe + Zoo) = 1/ 1 + 2Zout/Zin
(1)
To satisfy the requirement of the UWB band (3.1 – 10.6 GHz), the parameters for the Marchand balun filter are chosen as: Zo ¼ 50 V, Z1 ¼ 68 V, Zoe ¼ 104 V, Zoo ¼ 33 V. The patterned ground-plane technique in [8] is used to meet the needed even/odd-mode values. The proposed planar Marchand balun UWB filter (1r ¼ 2.65 and h ¼ 0.5 mm) is shown in Figs. 2a and b. The simulated results for the UWB bandpass filter are shown in Figs. 3a and b; two transmission zeros are observed to locate at 2.3 and 10.9 GHz. The insertion loss is less than 0.45 dB while the return loss is over 12.5 dB (3.9 – 9.8 GHz). Moreover, owing to 1808 phase difference for the two transmission paths at 2f0 , over 13.5 dB roll-off skirt rejection is achieved (11 – 15.8 GHz), and the group delay is less than 0.4 ns in the whole passband (3.1 – 10.6 GHz). Three transmission poles in the passband are realised for the Marchland balun UWB bandpass filter. Experimental results: Fig. 3c shows a photograph of the UWB filter with size of 30 × 15 mm (er ¼ 2.65, h ¼ 0.5 mm, and tand ¼ 0.002).
ELECTRONICS LETTERS 24th November 2011 Vol. 47 No. 24
The measured results of the filter are shown in Fig. 3. As shown in Fig. 3a, two measured transmission zeros are located at 2.4 and 10.9 GHz. The measured insertion loss is less than 1.2 dB and return loss is greater than 12 dB within the passband (3.1 – 10.63 GHz). Over 13 dB roll-off skirt rejection is achieved (11– 16 GHz). The group delay is less than 0.5 ns in the whole passband, as shown in Fig. 3b. Compared with the former UWB bandpass filter in [4], the selectivity for the proposed filter has been much improved and almost 50% circuit size reduction has been achieved. 0
magnitude, dB
–20
W.J. Feng and W.Q. Che (Department of Communication Engineering, Nanjing University of Science & Technology, Nanjing 210094, People’s Republic of China)
S21
–30
Acknowledgments: The authors acknowledge the financial support of the National Natural Science Foundation of China (60971013) and the support of the Technische Universita¨t Mu¨nchen (TUM), Germany, under the IGSSE Project 5.02. # The Institution of Engineering and Technology 2011 25 August 2011 doi: 10.1049/el.2011.2658 One or more of the Figures in this Letter are available in colour online.
S11
–10
Conclusion: A novel UWB filter using a single Marchand balun based on transversal filter concepts is proposed. The filter shows the advantages of simple topology, high selectivity and adjustable bandwidth. The validity of the design strategies has been verified.
–40
E-mail:
[email protected] simulated results measured results
–50
T.F. Eibert (Lehrstuhl fu¨r Hochfrequenztechnik, Technische Universita¨t Mu¨nchen, Germany)
–60 0
2
4
6
8
10
12
14
16
References
frequency, GHz
a 1.0 simulated result measured result
group delay, ns
0.8 0.6 0.4 0.2 0 2
4
6 8 10 frequency, GHz
12
14
b
c
Fig. 3 Measured and simulated results of proposed UWB filter (Fig. 3a); group delay of proposed UWB filter (Fig. 3b); and photograph of proposed UWB filter (Fig. 3c)
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