PIERS Proceedings, Taipei, March 25–28, 2013
660
A 60 GHz Marchand Balun with Floating Ground Centre-tap in
CMOS Technology
Leijun Xu1 , Henrik Sjöland2 , Markus Törmänen2 ,
Tianhong Pan1 , and Xue Bai1
1
Jiangsu University, Zhenjiang 212013, China
2
Lund University, Lund 22100, Sweden
Abstract— A novel Marchand Balun with floating ground centre-tap is proposed for siliconbased millimeter-wave applications. Implemented in 65 nm CMOS technology, the Marchand
Balun is designed with broadside coupling lines by using two top metals. To reduce the size, the
balun was folded and both ends of the two secondary lines are connected together to floating
ground, two by-bass capacitances are connected between the real ground and floating ground.
The balun is simulated by ADS Momentum, different width of the lines are compared to find
the optimal value. The proposed balun has small size and floating ground centre-tap for feeding
DC bias conveniently. Compared with transformer balun, the layout of this Marchand balun is
more symmetric due to the open-end of its primary side, which also improves the bandwidth and
imbalances of the balun.
1. INTRODUCTION
With the increase of the circuit frequency, more balanced structures are used in millimeter-wave
designs for reducing noise and high order harmonics, such as Mixers and Amplifiers. As a passive
component, on-chip baluns play an important role in the design of balanced circuits. To reduce
size and provide a simple DC bias for active devices, transformer baluns are commonly used in
millimeter-wave applications [1, 2] for their easy connected centre-tap to bias and compact structure.
However, one of the problems for millimeter-wave transformer is that its primary turn connection
to ground will lead asymmetrical layout, which will degrade the performance of the balun at such
high frequency. Compared with transformer balun, the Marchand balun doesn’t have this problem
due to its open-ended primary side, as shown in Fig. 1, it is also widely used in millimeter-wave
circuits [3–5]. But Marchand baluns with centre-tap for DC bias are seldom reported due to the
inherent structure.
In this paper, a novel Marchand balun with floating ground centre-tap is proposed for siliconbased millimeter-wave applications. The balun uses a stacked structure and bended lines to reduce
the size and provide a centre-tap for DC bias. It overcomes the drawback of the planar Marchand
balun where it is difficult to make a grounded centre-tap, meanwhile, it has symmetric layout for
both primary and secondly sides. In the following sections, the design of this balun is discussed in
detail.
2. BALUN DESIGN
The conventional Marchand balun (Fig. 1) consists of two symmetrical quarter-wave coupled lines,
where the primary line is open-ended and the two secondary lines are connected to ground separately. Implemented in a 65 nm CMOS process, this balun uses the top two metal layers as
broadside coupled lines. Unlike edge coupled lines, broadside coupling has large coupled area and
Input
Output1
Output2
Figure 1: The structure of Marchand balun.
Figure 2: The proposed Marchand balun with
centre-tap.
Progress In Electromagnetics Research Symposium Proceedings, Taipei, March 25–28, 2013
661
confines the electric field between the two lines. The coupled lines of the balun are bent so that
the two separated grounds can be connected together as a centre-tap with the additional benefit of
reduced size. To allow DC bias, two symmetric capacitances are connected between the centre-tap
and ground, the structure is shown in Fig. 2.
The centre frequency of the balun is 60 GHz, and the value of each capacitance is 5 pF. The
balun was implemented in a ST 65 nm standard CMOS process with 7 metal layers, where the
coupled lines use the two top metal layers of copper, M7 and M6. The thickness of each metal layer
is 0.9 µm and the isolation between them is 0.6 µm. The layout of the balun is shown in Fig. 3,
occupying an area of 150 µm × 100 µm excluding centre-tap capacitances.
3. LAYOUT OPTIMIZATION
To get good insertion loss and balance of the balun, the width and length of the coupled lines were
optimized using the EM simulation tool ADS Momentum. The insertion loss, amplitude and phase
imbalance are three main figures of merit for the balun, and to find suitable line dimensions, they
were simulated for different length and width of the primary line at the frequency of 60 GHz, see
Fig. 4 to Fig. 6.
When the line length increases, the amplitude imbalance decreases and phase imbalance increases
monotonically. For a given line width (w), on the other hand, the insertion loss has a minimum
value for a certain line length. Meanwhile, the minimum insertion loss and amplitude imbalance
decrease, whereas the phase imbalance increases with increased line width. Considering the above
results, the figures of merit are traded off and we chose a line length of 320 µm and a width of 6 µm
2.4
w=5µm
w=6µm
w=7µm
w=8µm
2.2
Insertion loss (dB)
2.0
1.8
1.6
1.4
1.2
1.0
280
290
300
310
320
330
340
350
360
370
Line length ( µm)
Figure 3: The layout of the balun.
Figure 4: Insertion loss with different line length and
width.
4.0
0.8
w=5µm
w=6µm
w=7µm
w=8µm
3.0
Phase imbalance (°)
Amplitude imbalance (dB)
0.7
0.6
0.5
0.4
w=5µm
w=6µm
w=7µm
w=8µm
3.5
2.5
2.0
1.5
1.0
0.5
0.3
0.0
0.2
280
290
300
310
320
330
340
350
360
370
Line length ( µm)
Figure 5: Amplitude imbalance with different line
length and width.
280
290
300
310
320
330
340
350
360
370
Line length ( µm)
Figure 6: Phase imbalance with different line length
and width.
2.0
Amplitude imbalance (dB)
Insertion loss (dB)
1.8
1.6
1.4
1.2
0.58
4.0
0.56
3.5
0.54
3.0
0.52
2.5
0.50
2.0
0.48
1.5
0.46
1.0
0.44
0.5
0.42
0.0
-0.5
0.40
1.0
50
52
54
56
58
60
62
64
66
68
70
Freq (GHz)
Figure 7: Insertion loss of the proposed balun.
Phase imbalance (°)
PIERS Proceedings, Taipei, March 25–28, 2013
662
50
52
54
56
58
60
62
64
66
68
70
Freq (GHz)
Figure 8: Amplitude and phase imbalance of the proposed balun.
for the balun. The results for the optimized balun are shown in Fig. 7 and Fig. 8.
The insertion loss of the designed balun is 1.1 dB at 60 GHz and less than 1.2 dB in the band
from 57 GHz to 64 GHz. In this frequency band, the amplitude imbalance is less than 0.5 dB and
the phase imbalance is less than 2.5◦ .
4. CONCLUSIONS
A millimeter wave Marchand balun with floating ground centre-tap for DC bias was proposed,
Implemented in a 65 nm CMOS process, the balun was optimized for low insertion loss and imbalances at 60 GHz. The balun has wide bandwidth and compact size, and it can be applied in
millimeter wave circuits such as amplifiers and mixers which need DC bias, resulting in area efficient
implementations.
ACKNOWLEDGMENT
The authors would like to thank the department of Electrical and Information Technology, Lund
University, Lund, Sweden, for supporting this research. The authors would also like to thank ST
Microelectronics for their support. This work is also supported by Jiangsu Provincial Natural
Science Foundation of China (Grant No. BK2011466), National Natural Science Foundation of
China (Grant No. 61273142), Natural Science Fund for Colleges and Universities in Jiangsu Province
(10KJB510002), and the Priority Academic Program Development of Jiangsu Higher Education
Institutions (PAPD).
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