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176 MHZ SOLID STATE MICROWAVE GENERATOR DESIGN
A. Smirnov, A. Krasnov, K. Nikolskiy, N. Tikhomirova, E. Ivanov, S. Polikhov
Siemens Research Center, Moscow, Russia
O. Heid, T.Hughes, Siemens AG, Erlangen, Germany
Abstract
This paper concerns the R&D work upon design of a
compact RF amplifier to be used for linear accelerators.
The machine under development will operate at 176 MHz
with output power of 25 kW in continuous wave regime.
It consists of 50 push-pull PCB modules (approx. 500W
output power each), connected in parallel to several radial
filter rings, which both allow class-F operation and
combine the power from the modules, delivering it to a
single 50 Ohm coax cable. The CST simulations and the
design of 324 MHz test prototype are presented.
INTRODUCTION
High power RF sources are important elements for
most of linear accelerators that have found growing
number of applications in physics and medicine.
The main benefits of the generator under development
will be its smaller size, perspective of lower cost, better
reliability and higher efficiency, achieved with class-F
operation, compared to conventional RF power sources
like klystrons. The solid-state microwave power modules
based on SiC vJFET transistors arranged in parallel push-
pull circuits, will be designed on PCB boards.
All modules will be connected to a power combiner
with common output 50 Ohm coaxial cable.
This generator is planned to be a predecessor to the
µbig¶ 324 MHz machine with pulsed RF output power of
3 MW.
RF POWER MODULES
We have designed and manufactured compact RF
power modules with one pair of SiC transistors arranged
in circlotron topology [1] as shown on Fig.1.
Figure 1: Parallel push-pull circuit
The manufactured module layout is presented on Fig.2.
We used Rogers 4003C with İ=3.55 as a substrate
material. The transistors are fed with 1800 phaseshift,
provided with external balun. The module provides
maximum available gain of 18.9 dB at output power of
2.0 kW and with supply voltage of 150 V.
Figure 2: RF power module (heat sink is not shown):
1 ± DC supply voltage; 2 ± RF inputs; 3 ± input matching
circuit; 4 ± SiC transistors; 5 ± DC-blocking capacitors; 6
± quarter-wavelength lines; 7 ± symmetric output stripline
Each transistor will be mounted on a water-cooled heat-
sink with a sinter paste, as shown on Fig. 3, which can
dissipate up to 300 W average thermal power.
Figure 3: Transistor package mounted on a water-cooling
module with temperature distribution
MOPPA021 Proceedings of RUPAC2012, Saint|-|Petersburg, Russia
ISBN 978-3-95450-125-0
290
Copyright c
○2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
RF power structures and systems
POWER COMBINER
In [2], a possibility of parallel power combining with
use of a resonant cavity was shown. In this work, we
present a non-resonant power combiner concept based on
a stepped coaxial line, shown on Fig 4. It benefits in
avoidance of energy storage and thus in lower power
dissipation.
Figure 4: Stepped-line power combiner
The inner conductor represents sequence of coaxial
segments with impedances from 10 to 50 Ohm with 10
Ohm steps. The whole number of modules can be
distributed among 5 rings that are mounted on the outer
conductor of the coaxial cable and feed it through 5
circumferential slits, each ring having output impedance
of 10 Ohm.
Due to the offsets between the rings the drive amplifier
must provide the phase shift between the groups of RF
modules hooked up to each ring. Meanwhile, the distance
between the first and the last slits must not exceed
quarter-wavelength to avoid the resonance in the coaxial
cable.
Each ring has to be surrounded with metal housing that
introduces high shunt inductance which will serve to
reduce the power leakage.
RADIAL FILTER
In combination with a suitable resonant load, the RF
module can operate in class-F mode at very high
efficiencies (over 85%). In an ideal class-F operation,
current and voltage are shifted in phase by 1800, the
output voltage waveform having square shape and the
drain current being sine-like. A filter at the drain of the
transistor with the proper values of input impedances at
fundamental frequency and at odd harmonics is required
to shape the waveforms [3]. The shaping minimizes the
overlap of the voltage and current waveform which
reduces power dissipation in the transistor and increases
the efficiency.
Henceforth, we consider the third harmonic filter only.
Even harmonics are shorted inside each RF module.
The input impedance constraints for the filter are as
follows:
let
Zmodule
1=R1+jX1
and Zmodule
3=R3+jX3
be the output series impedances of a single RF power
module at fundamental frequency and at the third
harmonic respectively. Since we need to extract the power
at fundamental with high efficiency and to reflect the
third harmonic back in-phase, the input impedance Zfilter
has to
- compensate the imaginary part at both frequencies;
- provide power dissipation in the load higher than
in the transistors by a factor of 10 (~90%
efficiency);
- behave as an open-circuit at odd harmonic
or
Zfilter
1=10R1-jX1
and Zfilter
3=500R3-jX3
at the fundamental frequency and the third harmonic
respectively.
We design the filter which fulfills these two complex
constraints using IRXU Ȝ WUDQVPLVVLRQ OLQHV (four
segments) with different line impedances. We calculate
the line impedance of each filter segment using scattering
matrix approach, taking into account that the filter
eventually acts like an impedance transformer of the load
Rload; see Fig. 5.
Figure 5: Filter schematics
The preferable concept of the filter design is to use
segments formed from radial transmission lines, driven
with non-dispersive TEM-like wave. This leads to a filter
Proceedings of RUPAC2012, Saint|-|Petersburg, Russia MOPPA021
RF power structures and systems
ISBN 978-3-95450-125-0
291
Copyright c
○2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
that serves to a plurality of the RF modules connected in
parallel through horn antennae as shown on Fig.6.
We use CST Microwave Studio to optimize the filter
interior shape in order to get desired input impedance
values at the antennae tips, to which the modules are
connected.
Figure 6: Radial filter CST model
The numerical simulations showed that the radial filter
made of copper will dissipate 4.5 times less power than in
case of using stripline-based filters connected to the
output of each RF module individually. This filter
geometry can be easily embedded inside every ring of the
power combiner.
Test prototype
In order to verify the CST predictions we have
designed one filter ring with 16 horn antennae (see Fig.
7), which is currently under manufacture.
Figure 7: Test prototype CST model
SUMMARY
We have described our preliminary design of the basic
RF generator components. The next steps will improve
WKH 5) PRGXOHV¶ SHUIRUPDQFH LQ WHUPV RI RXWSXW SRZHU
and gain; reduce the size of the power combiner by
shortening each filter segment and by tuning each horn
antenna shape to make it work as the first filter segment
itself.
REFERENCES
[1] R. Irsigler, M. Back, O. Heid, Th. Kluge, J. Sirtl,
³&RPSDFW 6ROLG 6WDWH 5)-modules for Direct Drive
RF-/LQDFV´,3$&011, San Sebastian, Spain
>@ 2 +HLG 7 +XJKHV ³&RPSDFW 6ROLG 6WDWH 'LUHFW
'ULYH 5) /LQDF ([SHULPHQWDO 3URJUDP´ +%
Morschach, Switzerland
>@ ) + 5DDE ³$Q LQWURGXFWion to class-F power
amplifiers´ 5) 'HVLJQ YRO QR P SS -84,
May 1996
MOPPA021 Proceedings of RUPAC2012, Saint|-|Petersburg, Russia
ISBN 978-3-95450-125-0
292
Copyright c
○2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
RF power structures and systems