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DEVELOPMENT OF TWO WAY HIGH POWER COMBINER AND RF MODULE FOR 10 KW, 352.21 MHZ SOLID STATE AMPLIFIER Sonal Sharma, B. V. Ramarao, S. Muthu, Manjiri Pande and Dr P Singh Ion Accelerator Development Division, Bhabha Atomic Research Centre, Mumbai-400085 Abstract In low energy high intensity proton accelerator (LEHIPA) two buncher cavities will be used to focus and match 3 MeV, 30 mA, proton beam from the radio frequency quadrupole (RFQ) to the drift tube linac (DTL). These buncher cavities require RF power of 10 kW at 352.21 MHz. For this a 10 kW solid state RF amplifier is being developed in BARC. It is planned to combine two 5 kW amplifiers to get final RF power of 10 kW with the help of a two way high power combiner which has been indigenously designed and developed using Wilkinson method. To achieve 5 kW, it is proposed to combine eight amplifier modules of 800 W, 352.21 MHz. Few of these amplifier modules have been tested up to output power of 900 W with efficiency of about 70% and gain of 21.5 dB. Wilkinson method providing high isolation between the ports. The coaxial combiner has high grade copper based output and input conductors. The input conductor is split into two parts which act as two λ/4 coaxial lines with each having a characteristic impedance of 70.7 Ω. The dimensions and spacing between the inner and outer conductor are designed to tolerate high power RF fields. This combiner provides very low level of loss and maintains a high level of isolation between the output ports. Initially the combiner was simulated using CST microwave studio. The simulated model is given in Fig 2. DESCRIPTION Two 5 kW, 352.21 MHz solid state amplifiers will be combined using a two way high power RF combiner to get final output of 10 kW. Each 5 kW RF amplifiers would require 8 amplifier modules of 800 W power output. The overall scheme of 10 kW amplifier is given in Fig 1. Fig 2: CST microwave model of RF combiner The s-parameters of the CST power combiner model are shown in Fig 3. The return loss at each port is better than -28 dB and transmission coefficient is about -3.01 dB. Fig 1: Overall scheme of 10 kW amplifier Two way high power coaxial RF combiner The two-way high power RF combiner is a coaxial combiner consisting of two 1-5/8” flange input ports and one high power 3-1/8” flange output port. The combiner is designed to accept 5 kW power at both the inputs and generate 10 kW at output. The design is based on Fig 3: s-parameters of CST model of RF combiner After simulation the RF combiner was fabricated using high grade copper with gold plating on inner surface. The photograph of the fabricated combiner is shown in Fig 4. 800 W, 352.21 MHz amplifier module Fig 4: Photograph of fabricated two way coaxial combiner The required s-parameters of the combiner were obtained on Vector Network Analyzer (VNA). Without gold plating, the return loss as seen on VNA was about 19dB, the transmission at one port was -3.14dB and other port -3.11dB. After gold plating, the measured RF characteristics on VNA showed a good input matching of -36 dB at the working frequency. Transmission coefficient at one port was -3.19 dB and at other port -3.03dB. The return loss at combined port is shown in Fig 5 and transmission coefficients are shown in Fig 6. Each 800 W solid state amplifier module has been built around high frequency push pull 50 V RF LDMOS (Lateral diffusion metal oxide semiconductor) transistor which have capability of delivering power as high as 1.2 kW at 352.21 MHz. The drain voltage is 50 V and gate bias is 2.3 V. The printed circuit board (PCB) used for the design is a high frequency Teflon laminate with thickness of 0.8 mm and dielectric constant of 3.5. The PCB is mounted on a properly designed water cooled heat sink to remove the excessive heat during operation. Each heat sink is shared between two amplifier modules. Each module test setup consists of a signal generator, a 10 W driver, directional couplers for power monitoring both at input and output. A 50 ohm RF termination and a power circulator are included inside every module to protect the device against excess of reflected power. The actual photograph of 800 W amplifier module is given in Fig 7. Fig 7: photo of 800 W, 352.21 MHz module Matching and performance of each module Fig 5: Return loss curve S11 at combined port Fig 6: Transmission coefficients The input and output circuit consists of micro strip transmission lines and capacitors for proper matching. Shorted quarter transformers are used at the input and output biasing points of the LDMOS to act as choke at the required frequency. A coaxial balun at input is used to provide balance out-of-phase input RF drive to the pushpull configured LDMOS. Similarly a coaxial balun at output combines the output of push-pull configuration to provide the final output of 800 W. For each of these amplifier modules, the input and output impedances were obtained using the datasheet of the LDMOS device and initial matching simulation was done in Motorola impedance matching program (MIMP). Matching was observed on the smith chart at the required frequency. The characterization of the module and low level power testing were carried out using vector network analyzer (VNA). Online input tuning at low power was done using variable high frequency capacitors to obtain better return loss at input. Finally, with the driver at input, output tuning of the amplifier was carried out to optimize the overall performance. The general specifications and results of the amplifier modules are given in Table1. Table 1: Specifications and results of 800 Watt module Sr.no Parameter value 1 Power output 900 Watt 2 Gain 21.5 dB 3 Efficiency 70% 4 Operating frequency 352.21 MHz 5 cooling Water cooled 6 VSWR at input 1.2:1 modules in parallel to obtain power of 5 kW using 8-way power combiner. Two such 5 kW RF system will be finally combined using the above mentioned high power coaxial combiner to obtain 10 kW. ACKNOWLEDGMENT The authors would like to thank Dr. S.L.Chaplot Director, Physics Group, BARC for his constant encouragement and support. Also special thanks to the team of RFSS, IADD, Physics group for their cooperation. REFERENCES CONCLUSION Till now low power testing of the two way RF combiner has been done using VNA with satisfactory results. High power testing will be done shortly. Individual amplifier modules have been tested up to power of 900W with efficiency of about 70% and gain of 21.5 dB. It is planned to combine eight such amplifier [1] “Classic works in RF engineering” by J.L.B. Walker, Daniel P. Myer, F.H. Raab, and C. Trask. [2] “Broadband VHF and UHF design using coaxial transformers” from High Frequency Electronics. [3] “Thermal design and evaluation methods for heat sink” by Donglyoul Shin. [4] “Systemizing RF power amplifier design” application note AN282A, www.freescale.com [5] “Microwave Engineering” by David M. Pozar, third edition.