Download development of two way high power combiner and rf module

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.