Highly Integrated Power Distribution Networks on Multilayer LTCC for Ka-band Multiple-Beam Phased Array Antennas J. Kassner, R. Kulke, P. Uhlig, M. Rittweger, P. Waldow, R. Münnich*, H. Thust* IMST GmbH, Carl-Friedrich-Gauss-Str. 2, 47475 Kamp-Lintfort, Germany, Tel. +49-2842-981 244, Fax +49-2842-981 499, www.ltcc.de,
[email protected] * University of Ilmenau, Fakultät für Elektrotechnik u. Informationstechnik, Fachgebiet Mikroperipherik, Postfach 100 565, 98694 Ilmenau, Germany Abstract Nowadays broadband multimedia providers look for high flexibility of satellite systems, e.g. bandwidth on demand and reconfigurable footprint of the antenna supplying the favoured areas on the ground. The only promising approach to this demanding task is the use of phased array antennas with multiple beams. One key component of such a system is the power distribution network. Among several possible solutions LTCC (low temperature co-fired ceramics) was selected for its compactness and RF performance. In this paper the design and measurement results of a module with two stacked distribution networks (each with 1 input and 4 output ports) are presented. Key words: LTCC, multilayer, power distribution network, Wilkinson, divider
Introduction Divider module A
The phased array antenna concept of the project HIFE [1], part of the ESA ARTES-3 program, motivates the development of the power distribution network in multi-layer LTCC presented in this paper. Applications like satellite communication with broadband multimedia services are requiring solutions of that kind. The innovative antenna concept provides multiple beams with different directions of radiation. The main advantage is the flexible use of bandwidth exclusively in the desired ground zones. A multiple beam concept implies that every antenna element has to receive the information of all participating beams. In order to maximize miniaturization, a compact distribution network was needed [2]. Within the EASTON project [3], funded by the German Space Agency, a power distribution module on multi-layer LTCC has been developed, which fits the requirements of the HIFE project.
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Fig. 1: Block diagram of multibeam antenna
The signals of two beams (1,2 and 3,4) are distributed simultaneously by one divider module each (A and B). The control modules provide the additional phase shift and amplitude variation needed for the appropriate radiation of every single antenna element. Afterwards the four beam signals are combined and sent to the antenna amplifier module (SSPA) across the divider module B. Thus the divider module shown in Fig. 1 includes two stacked distribution networks and four additional RF transmission lines. All parts have to be well shielded from each other. Multilayer LTCC was chosen as carrier substrate due to its RF performance and mechanical stability. Fig. 2 illustrates the internal construction of the LTCC. The two
System requirements In Fig. 1 the block diagram of the phased array antenna is shown [1]. Operating frequency is the Ka-band satellite downlink at 19 GHz with a bandwidth of 2 GHz. The system includes four steerable beams (No. 14) distributed to four control modules.
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power distribution networks are realized in strip line technology (SL1 and SL2). The RF transmission lines are situated on top of the substrate in micro strip technology. A2: h = 130 µ m AX: h = 200 µm
A prototype of a single Wilkinson divider has been fabricated on LTCC. In Fig. 4 the electrical behaviour of the divider structure with transitions to 50 Ω coplanar ports for onwafer measurement are shown. The comparison of simulation and measurement gives an excellent agreement.
h(MS) = 330µm
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AX: h = 200 µm A2: h = 130 µ m A2: h = 130 µ m
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Fig. 2: Cross section of the divider module (material: Dupont 951, εr = 7,8)
The ground planes ensure high isolation between the different parts of the circuit. As an additional system constraint all outer ports have to be on the same level (SL1) at predefined positions to fit to the surrounding modules. Optimised wave guide transitions provide the required layer interconnects.
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Power distribution network Each network includes one input and four output ports. A binary tree is formed of three Wilkinson dividers [2]. The resistors thereof are buried components to provide short connections without additional parasitics. Wider strip lines give lower ohmic losses. Thus the inner line impedance of the strip lines was decided to be 30 Ω. Therefore the resistor value of the Wilkinson divider is 60 Ω. The optimisation of the divider design was done with the IMST in-house developed 3D FDTD TM field simulator EMPIRE . Fig. 3 shows the Wilkinson divider in the software environment with visualised field distribution of the perpendicular electric field for two different field excitations. Via chains around the structure suppress unwanted cross-talk.
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Fig. 4: Measurement and simulation of a single Wilkinson divider
In the next step three dividers were combined to a 1 in 4-distribution network. Optimum chamfered bends and interconnecting lines lead to an overall satisfying electrical behaviour. Port 5 Port 4
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Fig. 5: 1 in 4-distribution network, electric field
Fig. 3: Wilkinson divider, electric field
The structure together with the field distribution of the perpendicular electric field is shown in Fig. 5. The field excitation at Port 2
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gives a good imagination of the increasing isolation between adjacent output ports. The power is reduced by 3 dB at every divider stage. This relationship is illustrated nicely by the declining amplitude of the electric field. Additional wave guide transitions have been designed to achieve the desired interconnects to the outer ports. In a final step the two power distribution networks are stacked on top of each other inside the multi layer ceramic. The four connecting RF transmission lines are added to the top layer. Fig. 6 is an exploded view of the LTCC module.
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Fig. 7: Measurement and simulation of divider module
The simulated and measured S-Parameters in Fig. 7 show very good results at the desired frequency of 19 GHz. The transmission loss is 1 to 2 dB higher than predicted. The not well enough known ohmic losses of the conductor paste is a possible reason for this effect. The simulated ohmic losses are not frequency selective and have been calculated with the skin effect at 19 GHz.
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Fig. 8: Amplitude balance of divider module
The amplitude balance of the networks is in the range of +/-0.6 dB and differs about 1 dB between the two distribution networks, see Fig. 8. The wave guide transitions for the layer change to layer 1 add some loss to the distribution network on layer 2.
Fig. 6: Different layers of divider module
Layer 1 contains all outer ports in a cavity, which can also be used for measurement purposes with on-wafer probes. Ground planes at different layers are interconnected by via holes to avoid separate ground areas. The divider module has been fabricated in LTCC. DuPont 951 was chosen as material system for the substrate, the conductors, the vias and the resistors.
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References
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[1] J. Butz, M. Spinnler, J. Christ, U. Mahr, “Highly integrated RF-modules for Ka-band multiple-beam active phases array antennas”. IEEE MTT-S CDROM, pp. 61-64, Seattle, June 2002 [2] R. Kulke, W. Simon, G. Möllenbeck, J. Kassner, P. Uhlig, S. Holzwarth, P. Waldow: "Power Distribution Networks in Multilayer LTCC for Microwave Applications", Proceedings of the International Symposium on Microelectronics (IMAPS), pp. 321-324, Baltimore, October 2001 [3] „EASTON: Entwicklung eines Verteilernetzwerkes mit integrierter Antenne auf mehrlagigem LTCC Substrat“, a German DLR Project
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Fig. 9: Phase balance of divider module
The measured phase balance shown in Fig. 9 gives reasonable good results for each single power distribution network. A phase difference of 30 deg between the networks is subject to improvement in further development stages. The phase difference of the transitions between layers was already compensated for in the design phase by adding delay line segments between the input and the first divider stage in layer 1. Fine tuning of this compensation will reduce this phase difference even more. Nevertheless the prospects are good that in a further design step these minor deviations can be corrected. Conclusion A complex divider module embedded in a satellite communication system with two power distribution networks and additional RF transmission lines has been developed. LTCC was selected to be the appropriate ceramic multilayer technology for this RF module due to its low ohmic losses compared to HTCC. Thick film resistors have been utilized in the Wilkinson dividers as buried components in inner layers. The performance of the divider module has been verified on basis of Sparameters. The measurements showed good agreement with the specified and simulated values. The module demonstrates the high abilities of LTCC at Ka-band frequencies and its advantages of compactness and robust design. Acknowledgement This work has been funded by the German Space Agency under contract number 50 YB 0007 [3].
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