Broadband Planar Marchand Balun
3.2 Realization of the Marchand Balun
The Marchand balun was fabricated using ceramic Al2O3 (εr=9.8) with CPW structure. The substrate thickness is 15 mils. We use 2 circuit models for the realization. Figure 3-8 is the couple-line model; it consists of 2 couple-line sections
and one 4
λ transmission line, the transmission line transfers the impedance and
enhances the bandwidth. Each electrical length of the transformer and couple-line is
4
λ at center frequency. Figure 3-9 shows the response of couple-line balun. We
design two layouts to implement this couple-line balun. The first layout, the ground plane appears only at one side of each couple-line, and we fold the two couple-lines to right and left side, as shown in Figure 3-10. The input port is at the bottom of the circuit, and the two output ports are on the upper right and upper left of the circuit.
The couple-lines are with Lange type of layout with four fingers for the first and six fingers for the second couple-line. Because the odd mode impedance of the second couple-line is too low to use 4-lined couple-line, we use six-finger couple-line instead.
Figure 3-11 is the simulated results where an EM simulator Sonnet is used, Figure 3-12 is the measured response, and Figure 3-13 shows the phase imbalance and amplitude imbalance of the measured responses. The S11 is less than -10dB in the range of 1.852 GHz to 6.429 GHz, and the fractional bandwidth is about 110%.
Figure 3-8: Couple-line model.
Figure 3-9: The response of couple-line model.
Figure 3-10: The first layout of couple-line balun.
Figure 3-11: The simulation result of the 1st layout of couple-line balun.
Figure 3-12: The measured data of the 1st layout of couple-line balun.
Figure 3-13: The measured phase imbalance (top) and the amplitude imbalance (bottom) of the 1st layout of couple-line balun.
The second layout of the couple-line balun is shown in Figure 3-14. In this layout, the couplers are still with four and six fingers respectively. However, this layout has ground plans at both sides of each coupler. Because this layout does not fold, it is
4
λ longer than the folded layout. However the bandwidth of the non-folded
structure is much better than the folded one. Figure 3-15 shows the simulated responses by EM simulator Sonnet, Figure 3-16 depicts the measured response and Figure 3-17 shows the phase imbalance and amplitude imbalance of the measured responses. The upper frequency of the input return loss better than -10dB is 7.375 GHz, but the frequency of phase balance better than 10o is just 6.73 GHz, so the fractional bandwidth (Δf) is 130.7% not 140%.
Figure 3-14: The second layout of couple-line balun.
Figure 3-15: The simulation result of the 2nd layout of couple-line balun.
Figure 3-16: The measured magnitude response of the 2nd layout of couple-line balun.
Figure 3-17: The measurement of the phase unbalance (top) and the amplitude unbalance (bottom) of the 2nd layout of couple-line balun.
Figure 3-18 is the Marchand balun model. It consists of an unbalanced transmission line section, an open-circuited series stub, and two series connected short-circuited shunt stubs, each transmission line is
4
λ at center frequency. Figure
3-19 is the response of the Marchand balun. We use different line width and gap to implement the characteristics z1, z2, zs1, zs2 in Figure 3-18. Figure 3-20 is the first layout of Marchand balun, in this layout, the minimal width of line is 2.1 mil. Figure 3-21 is the simulated result of the first layout of Marchand balun by EM simulator, HFSS, Figure 3-22 depicts the measured magnitude response, and Figure 3-23 shows the phase imbalance and amplitude imbalance of the measured responses. The S11 is less than -10dB in the range of 1.519 GHz to 6.887 GHz, and the fractional bandwidth of the type1 circuit is 127.8%.
We can see Figure 3-24 is the second layout of Marchand balun, the minimal width of line is 3.3 mil. Figure 3-25 shows the simulated result of the second layout by HFSS. Figure 3-26 depicts the measured magnitude responses, and Figure 3-27 shows the measured phase imbalance and amplitude imbalance of the balun. The response of S11 is very good, and it is less than -20 dB in the most of the frequency range. The amplitude imbalance is within 0.65dB, and phase imbalance is less than 2 ﾟ over the frequency range of 1.121 GHz to 5.477 GHz. The fractional bandwidth of this circuit is 132%. In Figure 3-18, the two series connected short-circuited shunt
stubs, zs1,zs2 equivalent to one short-circuited shunt stub with zs = 128Ω. So, it is the 3 orders Marchand balun but in Figure 3-26 the return loss seems like 4 orders.
Beside this point, CPW realization Marchand balun is a good way to implement a broadband planar balun. Figure 3-28 and Figure 3-29 are the realized CPW baluns.
Figure 3-18: The Marchand balun model.
Figure 3-19: The response of Marchand balun.
Figure 3-20: The first layout of Marchand balun.
Figure 3-21: The simulation result of the 1st layout Marchand balun.
Figure 3-22: The measured magnitude response of the 1st layout of Marchand balun.
Figure 3-23: The measured phase imbalance (top) and the amplitude imbalance (bottom) of the 1st layout of Marchand balun
Figure 3-24: The second layout of Marchand balun.
Figure 3-25: The simulation result of the 2nd layout of Marchand balun.
Figure 3-26: The measured magnitude response of the 2nd layout of Marchand balun.
Figure 3-27: The measured phase imbalance (top) and the amplitude imbalance (bottom) of the 2nd layout of Marchand balun.
Figure 3-28: The realized baluns *cm.
Figure 3-29: The realized baluns *cm.
Chapter 4 Conclusion
In Chpter2, the measured results of the modified equal split Wilkinson power divider is improved compared with the 2-way Wilkinson power divider. The return loss of the 3-way one section power divider is about -15 dB , it is good for a microwave circuit.
The insertion losses of the three outputs are balanced about -5.5 dB. The isolations between the neighborhood of the output ports are better than the isolation between the output ports, port2 and port4.
The 24-way power divider is a very symmetric geometry but each phase of the 24-way power divider is not identified. We think the cable of the network analyzer and substrate are the important factors. The sharp curve of the cable of network analyzer and the substrate’s flexibility influence the phase results.
In Chapter 3, we show two kinds of layouts for couple-line balun. One is folded structure, the two output ports go out at the top of it. And other is non-folded, the output ports are located at the middle of the balun. The results show the response of the non-folded structure is better than the folded. The possible reason is the couple-line in the non-folded balun is more close to the defined Co-planar waveguide
(CPW) couple-line structure. In the non-folded balun, the couple-line sees the ground plane at both sides, but in the folded one, it dose not.
In the later part of Chapter 3, we use different line width to implement Marchand balun. In our circuit model, there are 3 orders at most. However, the last circuit seems like 4 orders. Beside the point, the bandwidth and balance of amplitude and phase are good.
Compared with couple-line and Marchand baluns, we evaluate the performances of bandwidth, amplitude balance, phase balance and circuit size, the Marchand balun is a good choice for a broadband balun.
Reference
[1] David Pozar, Microwave Engineering, 3rd edition, John Wily & Sons, N.Y. 2005.
[2] Dimitrios Antsos, Rick Crist and Lin Sukamto, “A Novel Wilkinson Power Divider with Predictable Performance at K and Ka-band”, 1994 IEEE MTT-S Digest.
[3] Nobuo Nagai, Eiji Maekawa and Koujiro Ono, “New n-Way Hybrid Power Dividers”, IEEE Transactions on Microwave Theory and Techniques, Vol.
MTT-25, No. 12, Dec 1977.
[4] 張秀琴,”W-band Switch and Ka-band Power Divider” 交通大學電信研究所, 2005
[5] 紀 鈞 翔 , “ LTCC Broadband Mixer and LTCC Filters for Wireless LAN Applications”, 交通大學電信研究所, 2004
[6] Rajesh Mongia, Inder Bahl and Prakash Bhartia, “RF and Microwave Coupled-Line Circuits”, 1999 Artech House.
[7] Cloete, J.H., “Exact Design of the Marchand Balun”, Microwaves J., Vol. 23, May 1980, pp. 99-110.
[8] Goldsmith, C. L., A. Kikel, and N. L. Wilkens, “ Synthesis of Marchand Baluns Using Multilayer Microstrip Strucures”, Int. J. of Microwave and Millimeter-Wave Computer-Aided Engineering, Vol. 2, July 1992, pp.179-188.