HIGHLY COMPACTED X-BAND T/R MODULE USING LTCC MULTILAYER CERAMIC Qi Zhang, Yuefei Dai China National Electronic Technology Group No. 38 Research Institute No. 88, Pihe Rd., Shushan, Hefei, Anhui, China 230031 mscherry@163.com, 0086-551-5132998 (Fax) Keywords: T/R Module, LTCC technology, GaAs MMIC, X-band. Abstract Transmit/Receive (T/R) Modules are the key components for active phased array radar applications. A complete T/R Module for airborne or spaceborne active phased array radar operating in X-band was developed and tested. Besides highly sophisticated Gallium Arsenide monolithic microwave integrated circuits (GaAs MMICs), the key components for T/R Modules, Low Temperature Co-fired Ceramic (LTCC) substrate technology has been applied to reduce module size and weight. The paper has discussed in detail the module architecture, the assemble technology, and the optimum design. Finally, an extract of measurement results coming from the T/R Module first prototypes has been given. The design resulted in a very small and low-weight module, that enables a phase/gain quantisation of 5.625º and 0.5dB respectively. The module's output power is more than 8 Watts and the noise figure is as low as 4dB in X band. 1 Introduction T/R Modules set up system performance in a phased array. Their main three functions are to boost output power of the transmitted signal up to its final radiated power, establish system noise figure for receive, and provide beam steering control. Work on the development of T/R Modules for military and commercial active array applications has been going on in the United States since the 1980s. The major goals were reduced cost, increased power, and improved efficiency. In this paper, the use of multilayer LTCC allows dense packaging. And by the application of the GaAs MMIC technique, prototype modules have been built. The complete low power chipset, consisting of Low Noise Amplifier, Gain Block, T/R Switches, Phase Shifter and Digital Attenuator was especially designed by the Miltichip Module (MCM) technology with special regard to the performance requirements of the T/R Modules for airborne or spaceborne environment. T/R Modules are sized to fit within the lattice of a phase array, which is a function of frequency. A good rule of thumb is that within the plane of the array, the modules must stack together to meet a half-wavelength spacing. Depending on the system design the size of the module presented in this paper was defined to 75 22 10mm 3. And it was mounted in aluminium housing in order to guarantee low weight, low costs and good thermal conductivity. 2 LTCC technology In the fabrication of T/R Module, the most important objectives are: costs, weights and size reduction and the module reproducibility. Therefore the MCM technology is an ideal integrated technique for such kind of units. Present interest in T/R & MCM technology has focused on different packaging technologies such as laminate, ceramic, and thin film technologies. One such ceramic technology, LTCC offers significant benefits over other packaging technologies for used: high level of integration, buried components, low losses, robustness, cavities for mounting MMICs, TCE close to GaAs (7.0 Vs. 6.5), good thermal conductivity due to thermal management and others. LTCC technology has been set-up in Alenia Spazio on 1994. Its tape systems are composed of a glass/ceramic dielectric tape provided in rolls with shrinkage-matched metallization pastes, and it processes all the different layers in parallel. Once all the layers are processed, they are stacked, laminated and then co-fired at temperatures between 800ºC-900ºC to form a high density, fully integrated substrate. In this design, Ferro's A6-M tape with mixed metal conductors has been utilized as LTCC materials systems. It has a dielectric constant of 6 and very low dielectric loss (<0.002@10GHz). Typical physical properties are listed in Table 1. In such microwave frequency applications conductor loss is a significant contributor to total loss. Utilizing high conductivity metals and minimizing non-conducting additives in conductor pastes can further reduce total loss. Materials A6-M Thickness (mils) 5,10 Shrinkage (x y, %) 15.5 Shrinkage (z, %) 27 Flexural Strength (psi) 28000 Hermetic Yes Table 1: Typical physical properties of A6-M.
3 T/R Module design The T/R Module consists of a receive path, a transmit path, controller and driver, power supply and interface circuits. Figure 1 shows a block diagram of the T/R Module illustrating the architecture of the transmit and receive sections of the system. Isolator Limiter LNA Gain Block 6-bit Attenuato Circulator Power Amplifier Driver Amplifier Switch Phase Shifter Figure 2: Floor plan of the module. Figure 1: Block diagram of the T/R Module. A low-loss T/R Switch and a 3-port Circulator direct the path of RF current from the source and the antenna in the transmit and receive sections, respectively. And an 6 bit Phase Shifter is also shared by the two sections permitting beam forming on the array, that will be designed for minimum loss with 90 degrees of phase shift at 5.625 degree steps. A single pole double throw switch connects the Phase Shifter with the receiving arm. The receiver front-end consists of a Limiter, a Low-noise Amplifier, a small-signal gain amplifier and a 6-bit Attenuator. They were all realised on a single GaAs MMIC. The LNA will have high gain with very low noise figure operating at low DC power. Limiter is needed for protection of LNA from high power RF signal. The first stage was biased for minimum noise performances, whilst the other stages provide the gain as needed by the receiver. The transmit side includes a Drive Amplifier and a Power Amplifier. In the transmit mode, the module amplifies a low level, X-band signal up to a power level of greater than 8 watts at the antenna. The control of these elements (e.g. T/R Switches, Phase Shifter, Digital Attenuator) is done via complementary signals generated out of the serial command data by a module's ASIC. It was especially designed to perform all functions. The main characteristic of this architecture is the special power supply mode between transmit path and receive path, which means the transmit gain path will be turned off during receive, and the receive amplifier path is biased off during transmit. That is done by a external modulation circuitry. This module is built by the chip-on-substrate technology to get the repeatability for the packaging of MMIC chips. Figure 2 shows the fabricated T/R Module substrate which size is 65mm x 20mm. By using the multilayer, the supply lines were designed to have very low resistance in order to keep the voltage drop especially on the high power line very low. In addition, some special attention was also paid to improve layout and performance: - Shortened RF transitions; - Arranged vias and ground to inhibit RF reflections; - Re-arranged and shrunk cavities to inhibit resonance; - Used coplanar structures for better isolation; - Maintained optimum design guidelines throughout package. The PA has a RF output power of 39dBm causing a thermal energy of 8W, which has to be dissipated from the transistors through the substrate and the aluminium housing. The junction temperature for proper operation and reliability must be kept below 125ºC. A special heat management for the PA was introduced: The amplifier chips have been dieattached with solder on a Copper Molybdenum carrier (Shown in the Figure 3). This carrier provides a good heat sink, and the thermal expansion coefficient of this carrier material corresponds well with the thermal expansion coefficient of GaAs. Figure 3: Copper Molybdenum of Power Amplifier 4 Results The first prototypes were tested and a typical 39.0dBm power output was obtained. This leads to an excellent overall efficiency of more than 18% for the module in full radar operation with a TX duty cycle of 10% at 25ºC. The main performance characteristics of the presented module can be seen in Table 2. Figure 4 shows the performance of the Phase Shifter. Tests were conducted for 5.625º and, in the worst case, a Max 4º RMS error was obtained.
Rx gain Tx output power Rx gain setting range Rx gain step size Overall noise figure >25dB 39dBm or 8Watts >30dB 0.5dB (6bit) <4.0dB Phase control range 360º Phase quantisation Overall module efficiency @10% Tx duty cycle 6bit or 5.625º step >18% Table 2: Main electrical characteristics of the T/R module. electrical and environmental conditions on an aircraft or in space. A further aspect that is getting more and more important is the affordability of these modules in volumes of several hundreds or even thousands per SAR system. In this paper, we discussed the use of LTCC technology to design a totally integrated T/R Module. Measured results including gain and phase characteristics of the first prototype lot show good performance, so that it is applicable to the active phased array antenna system. And it has an advantage of the repeatability for the packaging of MMIC chips in the case of the mass production. Acknowledgements The authors would like to thank Pei Li, Dezhi Zhang, Chunming Lv and Jing Song of China National Electronic Technology Group No. 38 Research Institute for their encouragement, and thanks to the many others that contributed to the design, fabrication, and test of the first prototype lot of the T/R Module. References Figure 4: Phase step available in the Phase Shifter. The photograph of the final circuit is shown in Figure 5. This T/R Module has a very compact size of 75 22 10mm 3, and a extremely low weight of less than 25 grams by the use of multilayer LTCC. [1] C. Hemmi, T. Dover, F. German, and A. Vespa: Multifunction wide-band array design, IEEE trans. Antennas Propagat., vol. 47, pp. 425-431, Mar. 1999. [2] P. Eskelinen, Introduction to RF Equipment and System Design, Artch House, 2004. [3] R. Kulke et al.: Integration Techniques for MMICs and Chip Devices in LTCC Multichip Modules for Radio Frequencies, IMAPS Symposium, Boston, 2000 [4] M. E. Davis, Space Based Radar Core Technology Challenges for Affordability, 2001 Core Technologies for Space Systems Conference Dig., Colorado Sparings, Colorado, Nov.2001 [5] W. N. Edelstein, C. Andrivos, F. Wang, D. Rutledge, Current Status of the High-Efficiency T/R Module for SAR, Earth Science Technology Conference, June, 2003. [6] C. Wang, C. T. Rodenbeck, M. R. Coutant, and K. Chang, A novel broadband T/R module for phased array applications in wireless communications, in IEEE MTT-S Int. Microwave Symp. Dig., Seattle, WA, Jun 2002. Figure 5: Photo of the T/R Module. 5 Conclusion T/R Modules suitable for SAR instruments have to fulfil very demanding requirements as given by the challenging
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