Design of a Variable X-band RF Power Splitter

IPAC2015, Richmond, VA, USA JACoW Publishing doi:10.18429/JACoW-IPAC2015-WEPHA025 DESIGN OF A VARIABLE X-BAND RF POWER SPLITTER Hao Zha, Igor Syratchev, Dmitry Gudkov, Alexej Grudiev, CERN, Geneva, Switzerland y1 s14 s11 ρs (y2 ) = (s21 ) + (s24 ) 41 1−ρs44 y3 s34 s31 Abstract The design of the variable high RF power X-Band splitter is presented. The RF power division ratio is adjusted by mechanically changing the position of a special RF short circuit piston. The piston is mounted on a step-motor providing the precise linear movement. Throughout the design, special measures were taken to reduce the maximum electric field on the copper surface, as well as to maximise the frequency bandwidth of the device. (2) INTRODUCTION High RF power X-band test stands for testing the normal conducting CLIC accelerating structures are now in operation at CERN. In general, these tests require a precise adjustment of the driving RF power level [1]. The RF splitter with arbitrary division ratio will be used to meet this need without changing the RF power provided by RF source itself. The RF splitter is a three port device, where one output port is connected to the DUT (Device Under Test) itself and the other is connected to the matched RF load, which absorbs the excessive RF power. The division of RF power between the two is adjusted by mechanically changing the position of a special RF short circuit piston (see Fig. 5 in [2]). The piston is an RF contact free device and is mounted to a step-motor providing the precise movement. Thus a continuous adjustment of the RF power is achieved. PRINCIPLE OF OPERATION The schematic of the splitter RF circuit is shown in Fig. 1. For the known parameters 𝑠𝑖𝑗 in the scattering matrix (1), the output power in each port 𝑦𝑛 can be calculated using equation (2). Parameter 𝜌 in equations (1 and 2) is the reflection from the RF short circuit piston. The output power in the ports 𝑦1 , 𝑦2 and 𝑦3 depends on the RF phase of the reflection 𝜌 . By controlling the position of the RF short circuit piston, this phase can be changed, thus the output power at ports 2 and 3 can be adjusted. In order to minimize the overall reflection, the RF splitter should be matched, thus 𝑦1 should be equal to zero. Consequently 𝑠11 and 𝑠14 should be equal to zero as well, but not 𝑠41 , otherwise 𝑦2 and 𝑦3 will be de-coupled. This condition (𝑠14 ≠ 𝑠41 ) breaks the symmetry of the RF circuit and makes matching of the device impossible. s11 y1 y2 s21 (y ) = (s 31 3 y4 s41 s12 s22 s32 s42 7: Accelerator Technology T06 - Room Temperature RF s13 s23 s33 s43 s14 1 s24 0 s34 ) ( 0 ) ρy4 s44 Figure 1: The 4 port network proposal for RF splitter. The way to approach matching is to have a 5-port network where two ports are connected to the piston. It can be proven that a solution for the symmetric network exists in this case. A schematic of such a 5-port network is shown in Fig. 2. It uses 3 symmetric RF splitters to split the input port to 4 output branches. Each of the symmetric RF splitters is designed in a way that every port sees the combined impedance of the two others. The distance between the first and second symmetric RF splitter is a quarter-wavelength. According to transmission line theory, the total impedance seen by the input port 1 1 is + , where 𝑍𝑖 is the impedance of each port 𝑍2 +𝑍4 𝑍3 +𝑍5 by itself. As shown in Fig. 2, port 2 and 3 are output ports, thus 𝑍2 = 𝑍3 = 1 . The RF short circuit pistons bring reactance, so that the values of 𝑍5 and 𝑍4 are purely imaginary. The input port will be matched if 𝑍5 ∗ 𝑍4 = 1. That can be achieved by having 180 phase difference in reflection between the two short circuit pistons. The power division ratio is now determined only by the reflection phase of the pistons and can be varied from 0 to 1. This network design is very compact and has a broad frequency bandwidth. However, two synchronised short circuit pistons are needed, thus a certain complication of the mechanical design is unavoidable. (1) WEPHA025 3167 Content from this work may be used under the terms of the CC BY 3.0 licence (© 2015). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI. 6th International Particle Accelerator Conference ISBN: 978-3-95450-168-7 Content from this work may be used under the terms of the CC BY 3.0 licence (© 2015). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI. 6th International Particle Accelerator Conference ISBN: 978-3-95450-168-7 IPAC2015, Richmond, VA, USA JACoW Publishing doi:10.18429/JACoW-IPAC2015-WEPHA025 RF power at each port versus the piston position are shown in Fig. 4(b). After carefully tuning of all components, the system achieved a good matching state where the maximum reflection is less than -35 dB. The bandwidth of the device varies between 180 MHz and 270 MHz depending on the division ratio between the two ports. For 100 MW peak RF power, the maximum surface field is 48 MV/m. This field level is considered safe for X-band RF pulses shorter than 2 microseconds. Figure 2: The 5 port network proposal for RF splitter. The system shown in Fig. 2 can be further simplified, if one port can accommodate two different waveguide modes, like two TE11 modes in a circular waveguide with orthogonal polarisations. In this case, only one short circuit piston can be used to reflect both modes. The RF circular polarizer (see Fig. 3) is a device, which can provide such mode merging [3]. Here, RF signals that are coming from left (port 4) and right (port 5) will excite modes with orthogonal polarization in the connected circular waveguide. The circular waveguide is terminated by the short circuit piston, which reflects both polarisations equally. Ports 4 and 5 are completely decoupled, so that the phase of reflected signals will be identical for both. This RF circular polarizer is used to substitute the two channels with short circuit pistons in the 5-port network shown in Fig. 2. Figure 4: the RF splitter design using WR90 waveguide: (a) geometry; (b) power at each port versus position of the piston. Figure 3: RF polarizer. RF DESIGN AND OPTIMIZATION Figure 4 shows the design of an RF variable splitter, which implements the concept of RF circular polariser. It is comprised of several waveguide-based components: three symmetrical splitters, two H-bends and the circular polarizer. All components are designed using WR90 waveguide transverse dimension (22.86 mm x 10.16mm) and the diameter of the circular waveguide is 18 mm. To provide the most compact design, as well as to maximize the frequency bandwidth, the distance between all components was minimised. This design was optimised using HFSS [4] (see Fig. 4(a)). The values of the output WEPHA025 3168 The overall transverse dimension of the device shown in Fig. 4 is about 160 mm. The second, even more compact generation with a different topology of the variable splitter is shown in Fig. 5. It is as small as 100 mm across the long side. In this modification, a waveguide cross-section of 20 mm x 12 mm was used. The maximum reflection in this design is less than -40 dB for any piston position. The bandwidth is between 370 MHz and 280 MHz depending on the division ratio. This is about 30% wider compared to the design shown in Fig. 4. For 100 MW RF peak power, the maximum electric field is 50 MV/m. The mechanical design of the variable splitter has been completed (see Fig. 6) and the fabrication has started. 7: Accelerator Technology T06 - Room Temperature RF IPAC2015, Richmond, VA, USA JACoW Publishing doi:10.18429/JACoW-IPAC2015-WEPHA025 Figure 6: Mechanical design of the variable RF splitter. CONCLUSION An X-band RF splitter with arbitrary division ratio was designed. The design uses waveguide-based components including RF splitters, H-bends and the RF polarizer. It was carefully tuned to provide a compact, low reflection, large bandwidth and low surface field design. Fabrication has started and once completed and tested the installation in the CERN X-band test stands is foreseen. REFERENCES [1] N. Catalan-Lasheras et al., WEPME016, Proc. of IPAC’14, Dresden, Germany (2014); http://www.JACoW.org [2] I. Syratchev et al., WEPE026, Proc. of IPAC’10, Kyoto, Japan (2010); http://www.JACoW.org [3] Alessandro Navarrini and Richard L. Plambeck, IEEE Trans. Microw. Theory Tech. 54(1) 272 (2006). [4] Ansys HFSS; http://www.ansys.com Figure 5: RF splitter design using 20 mm x 12 mm waveguide: (a) geometry; (b) power obtained at each port versus position of piston; (c) bandwidth plots. 7: Accelerator Technology T06 - Room Temperature RF WEPHA025 3169 Content from this work may be used under the terms of the CC BY 3.0 licence (© 2015). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI. 6th International Particle Accelerator Conference ISBN: 978-3-95450-168-7