Ayan Karmakar

Harsimran Singh

Kamaljeet Singh

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IOSR Journal of Electronics and Communication Engineering (IOSR-JECE) 
ISSN: 2278-2834, ISBN: 2278-8735. Volume 4, Issue 2 (Nov. - Dec. 2012), PP 07-11 
www.iosrjournals.org 
                                                       www.iosrjournals.org                                                      7 | Page 
 
Broadband Branch Line Balun using Meander line on Silicon 
Substrate 
 
Harsimran Singh
1
, Ayan Karmakar
2
 and Kamaljeet Singh
2
 
1Student of Dept. of Electronics and Communication, Rayat and Bahra Institute of Engineering and 
Biotechnology, Mohali, Punjab, India) 
2MEMS Development Division, Semi-Conductor Laboratory (SCL), Mohali, Punjab, India)   
 
Abstract : This article presents the design of L-band branch line balun using microstrip configuration on high 
resistive silicon substrate (ρ>8KΩ-cm). In comparison to earlier designs better performance is achieved by 
meandering all the four branches and appending a quarter wavelength short circuited stub at the upper right 
corner of the structure. The circuit is designed using circuit simulator of Agilent’s advanced design system 
(ADS) and further verified using Ansoft’s FEM and ADS’s MoM based electromagnetic solvers. Comparative 
study is carried out among various simulation techniques and analyzed. Fabrication methodology along with 
design steps are detailed in this article. Proposed design saves around 60% print area as compared to 
conventional topology. Further comparison with other reported topologies have also been discussed. 
Keywords - Branch line balun, high resistive silicon, microstrip line, short circuited stub, via hole. 
 
I. INTRODUCTION  
Baluns are generally used to convert unbalanced input to balanced output. Balun configurations are 
being used in various circuits such as balanced mixers, antenna, push pull amplifier, frequency multiplier and 
antenna feed networks [1-5]. Rapid development in digital wireless communication system demands wider 
bandwidth without compromising the size.  
Conventional branch line balun [6] is composed of two quarter wavelength lines and two half 
wavelength lines. It has quite simple structure as compared to other types of baluns and can provide low 
insertion loss with promising return loss characteristics. Further viability of Si as a substrate has been 
established [7] by the author. 
Present article proposed the branch line balun having meandered branches. It consists of two quarter 
wavelength lines, two half-wavelength lines and a quarter wavelength short circuited stub with via-hole. It is 
compact and around 60 % print area is less as compared to conventional topology [8]. Present design is carried 
out on Si substrate and optimized to cater for the desired specifications. The most critical part is the 
implementation of via topology for which fabrication aspects has been detailed. 
 
II. BALUN DESIGN 
The Proposed balun comprises of a pair of quarter wavelength  transmission line(branch 1&3) and a pair 
of half  wavelength transmission line(branch 2&4) and a quarter wavelength short circuited stub attached at the 
upper right corner, as shown in the Fig.1. Various branch line impedances are governed by the following equation 
[9]: 
Z1=Z2 ∕ ((√2×Z2)-Z0                                                                   …..(1) 
Where Z0 (50Ω) is the characteristic impedance of the input/output line and Z1 & Z2 are the impedances of the 
branch-1 and branch 2-4 respectively. One of them is chosen based on ease of fabrication and the other one can 
be determined from the Equation (1). In this study the branch line impedance are selected as Z1= 86.10 Ω and Z2= 
60Ω. 
The circuit is designed on the high-Resistive Si-substrate (εr=11.8 & tanδ = 0.001) of 675 μm thick considering 
metal (Al/Au) thickness as 1-2 μm. The effective permittivity (εre) of the substrate is calculated as: 
                                                                                             ..... (2) 
Where, εr is the dielectric constant of the substrate and „w‟ and „h‟ are the width of the microstrip and height of 
the substrate, respectively. 
Broadband Branch Line Balun using Meander Line on Silicon Substrate 
www.iosrjournals.org                                                             8 | Page 
         
Figure 1: Layout of the designed Balun 
Based upon above calculation balun has been designed at the centre frequency of 1.575GHz. Optimizing the 
design for the specified target, the physical parameters were found out to be as, the width of λ/4 line and λ/2 is 
112.71 μm and 350.350 μm respectively with an overall area of 14.6 mm × 16.3 mm. The critical dimension of 
the structure is 18.87 μm. The width of the short circuited stub is 880.201 μm and with a via-hole cross-section 
of 880.2 μm ×100 μm. The conventional balun is of size 17.2 mm × 34.3 mm. Fig.2 shows the 3D structure of 
the design built in the FEM environment. Compared to conventional structure, a compact structure with 60 % 
less area is achieved by incorporating short circuited stub & meandering. 
 
 
Figure 2: Isometric view of the designed balun 
 
III. RESULTS 
  The simulation study and optimization is being carried out using circuit simulator, MoM and FEM based 
electromagnetic solvers [10] [11]. The main design challenge was to optimize the performance catering to 
undesired and cross-coupling phenomena between various branches.  
Fig.3 shows the return loss (S11) behavior of the designed structure. It shows better than 20 dB return loss over a 
wide band of frequencies (1.4 GHz to 1.9 GHz). There is a significant degradation of matching of around 6 dB in 
case of FEM in comparison to two other environments. It is basically due to Si substrate which is modeled for 
losses accurately in FEM based solvers. The losses are primarily due to undesired surface waves and leaky waves 
contributing in higher dielectric losses. FEM basically solves the problem using volumetric discretization method 
by evaluating the approximate solution of partial differential equations and integral equations for the given 
boundary conditions. 
Broadband Branch Line Balun using Meander Line on Silicon Substrate 
www.iosrjournals.org                                                             9 | Page 
 
                          Figure 3: Input Return loss (S11). 
 
Fig 4 shows the insertion loss (S21, S31) at two balanced output ports. It‟s clear from the graph that a flat (almost 
constant) insertion loss nature is at the two output ports of the balun structure for wide band (1.4 GHz to 1.9 
GHz). Here also FEM results show around 1 dB more loss because of the above said effects. And, in practice we 
used to always follow this result as a reference in many cases as it takes into consideration various practical 
situation during the simulation. 
 
                   Figure 4: Insertion loss (S21, S31) at two balanced output ports 
 
Fig. 5 shows the phase difference between the two output ports which is -180.669° in case of schematic 
simulation, -176.194° and -176.089° in case of MoM based simulation and FEM based simulation respectively. 
The obtained 180±5° phase bandwidth of the designed balun is around 299 MHz (1.364 to 1.663 GHz) in case of  
schematic simulation, 355 MHz (1.488 to 1.843 GHz) in case of MoM simulation and 369 MHz (1.499 to 1.868 
GHz) in case of FEM simulation. In case of FEM analysis, the effect of via-hole is very dominant for phase 
analysis. This is because of via-hole offers a parasitic inductance which in turn is a phase lagging component.  
The shape and size of the via-hole basically dictates the parasitic inductance for a given height of substrate, which 
can be accurately computed using only FEM 
 
Figure 5: Output Phase Difference 
Broadband Branch Line Balun using Meander Line on Silicon Substrate 
www.iosrjournals.org                                                             10 | Page 
A surface current density (Jsur) distribution plot is shown in Fig. 6. It depicts that there is a current crowding 
(densely populated current vectors) at the bending of the lines or at the corner of the line/stub. In case of 
meander line, there a clear indication of mutual coupling vectors (oppositely directed current vectors). 
 
   Figure 6: Surface current density distribution of the balun 
 
Detailed simulation study on four different substrates other than Hi-Res Si is also performed to validate the 
versatility of the design.  Table I shows the comparative study where all the parameter values are depicted. It has 
been observed that there is shift in centre frequency with variation of substrate characteristics keeping all the 
other parameters as fixed. For the lower dielectric constant substrate the tuning frequency shifts to a lower 
value.  
TABLE I.  COMPARITIVE STUDY OF BALUN‟S PREFORMANCE ON DIFFERENT SUBSTRATES 
Feature 
Alumina 
(25 mil) 
5 μm Oxide on 
CMOS grade Si. 
5 μm Polymide 
[PI 2525] on 
CMOS grade Si. 
5 μm Nitride on 
CMOS grade Si. 
Centre 
Frequency(GHz) 
1.9 1.8 1.8 1.8 
Max Return loss(dB)  -25.627 -27.961 -28.611 -32.279 
Max Insertion 
loss(dB) 
-3.003 -3.026 -3.023 -3.012 
Output phase 
difference (deg°) 
-181.375 -179.169 -179.353 -180.337 
  
Also the simulated parameter values obtained for the designed balun is compared with other designs     
incorporated on silicon substrate in Table II 
TABLE II.  COMPARISION OF DESIGNED BALUN WITH OTHER BALUN DESIGNS 
Type of Balun design 
S11(d
B) 
S21(d
B) 
S31(d
B) 
Phase 
difference 
(deg°) 
Conventional balun (on silicon 
substrate)[6] 
-
38.05
7 
-3.471 -3.404 -179.595 
Meandered balun (on silicon 
substrate)[6] 
-
33.67
9 
-3.549 -3.394 -180.336 
Proposed Balun -23.08 -2.954 -3.113 -176.194 
 
The comparison shows that the better results are obtained for the designed balun as compared to other balun 
design which are simulated on silicon substrate with a wide band nature and very compact size (60 % lower area 
than other baluns). 
 
IV. FABRICATION PROCESS 
Proposed fabrication steps are described as below in Fig.7 detailing required processes. At Step-5 the 
via-hole can be opened using the DRIE (Deep Reactive Ion Etching) process and for the backside metallization 
evaporation technique can be utilized. In this way, we can use the simple CMOS technology to realize structure 
with desired design tolerances in respect of structural dimensions 
 
Broadband Branch Line Balun using Meander Line on Silicon Substrate 
                                                       www.iosrjournals.org                                                      11 | Page 
          
Figure 7: Steps for the proposed fabrication process 
 
V. CONCLUSION 
This article presents the design of a wideband balun structure of compact size, which can be realized 
using the MEMS technology and the proposed fabrication process is compatible with standard CMOS foundry. 
The designed three port circuit configuration has an added advantage of totally planar without any off-chip 
resistor as compared to conventional topologies. Better performance parameters are achieved by meandering all 
the four branches and attaching a short circuited stub at the top right corner of the balun. As the structure is 
targeted on Si-substrate, so integration of electronics (ASIC: Application Specific Integrated Circuits) will be 
easier to implement. As per author‟s knowledge, no other relevant circuit is designed on high permittivity 
substrates for wideband application. Three different analysis techniques are employed and comparative study is 
being carried out. This further establishes silicon as a viable MIC substrate for the development of system and 
sub-system.  This design can be used for L1 band (1.575 GHz) GPS system and L-band long range surveillance 
radars for feeding network of mono-pulse antennas.     
                                    
REFERENCES 
[1] Microwave Engineering-D.M Pozar, Wiley & Sons, Student Edition 2010. 
[2] Ghali, H. and T. A. Moselhy, “Miniaturized fractal rat-race, branch-line, and coupled-line hybrids," IEEE Trans. Microwave Theory 
Tech., Vol. 52, 2513-2520, 2000. 
[3] Ang, K. S., Y. C. Leong, and C. H. Lee, “Multisection Impedance-transforming coupled-line baluns," IEEE Trans. Microwave 
Theory Tech., Vol. 51,536-541, 2003. 
[4] Zhang, Z. and S. Xu, “A novel balun structure with a composite right left-handed transmission line," Microwave Opt. Technol. Lett., 
Vol. 45, 422-424, 2005.   
[5] RF and Microwave Coupled Line Circuits-Mongia, Bahl, Bhatia and Hong, Artech House, 2010. 
[6] Tseng C.F and Lu S.C “A Broadband Balun using meander line” Progress in Electromagnetic Research Symposium, Suzhou, 
China. ISSN 1559-9450, 501-504 September 12-16, 2011.  
[7] Kamaljeet Singh, Ayan Karmakar & K. Nagachenchaiah,” Dual Band(1:4) Wilkinson Power Divider on Silicon”, International 
Journal of  Electronics ,Vol.1, No.1,pp.4-6, May 2012.  
[8] Kamaljeet Singh, Ayan Karmakar & K. Nagachenchaiah,”Design and Development of X-band Planar Balun on Si substrate”, 
submitted for Journal Publication, “unpublished”.  
[9] Li, J. L., S. W. Qu, and Q. Xue, “Miniaturized branch-line balun with bandwidth enhancement," IEE Electron. Lett. Vol. 43, pp 
931-932, 2007. 
[10] ADS 2009, Agilent Technologies. 
[11] HFSS ver. 11, Ansoft Corporation.  
 


Broadband Branch Line Balun using Meander line on Silicon Substrate

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Broadband Branch Line Balun using Meander line on Silicon Substrate

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