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Nuclear Instruments and Methods in Physics Research A

journal homepage: www.elsevier.com/locate/nima

Design of a variable X-band RF power splitter

Hao Zha⁎,1, Igor Syratchev, Dmitry Gudkov, Alexej Grudiev

CERN, Geneva CH-1211, Switzerland

A R T I C L E I N F O

Keywords:Radio frequency technologyAccelerating structurePower splitterRF polarizerHigh power

A B S T R A C T

This work presents the design of a novel X-band RF power splitter for high-power consumption. The RF powerdivision ratio is adjusted by mechanically changing the position of a special RF short circuit piston, which ismounted on a step-motor to provide precise linear movement. The RF splitter is well matched at any divisionratio with less than −40 dB reflection from the input port. The geometry of the splitter is optimized to obtain acompact (less than 12 cm) and large bandwidth (370 MHz) design. The surface electrical field throughout thedesign is minimized to transmit high RF power. The power splitter can also combine two inlet RF power units ofany amplitude. A prototype of the power splitter is manufactured and will be used in high-power test stands atCERN.

1. Introduction

High RF power X-band test stands are used at CERN to test theaccelerating structures of the compact linear collider main linac [1].These tests require precise adjustment of the input RF power ataccelerating structures [2,3]. However, klystron-based power sourcesrequire a considerable amount of time to reach a stable working stateafter changing their output power level. Therefore, a method foradjusting the driving RF power without changing the power level ofthe sources must be developed to increase the working efficiency of theaccelerating structures during testing and operation.

Fig. 1 shows a method for adjusting the driving power by using 3 dBhybrid to combine RF power generated from two klystrons [4]. Thepower from two klystrons exhibits the same amplitude but a variablephase difference. The output power at one port is determined by thephase difference (equations in Fig. 1), which can be adjusted bychanging the driving RF power level at the klystron. However, thismethod requires two klystrons and is inapplicable for facilities usingonly one klystron.

This work presents another approach of using an RF power splitterto divide power into two branches; this method is applicable for one-klystron systems. As shown in Fig. 2, one port is connected to thedevice under test, and the other port is connected to the matched RFload and absorbs excessive RF power. The division ratio is adjusted bymechanically changing the position of a special RF short-circuit piston.The detailed principle is explained in Section II. The piston is an RFcontact-free device and is mounted to a step-motor to provide precisemovement. Thus, the RF power is continuously adjusted.

A similar RF power splitter is used in other fields [5–9]. In thepresent work, the design of the RF power splitter must satisfy therequirements of X-band RF technologies in particle acceleratingstructures. These requirements include high power level (up to100 MW) and short pulse length (around 100 ns). The geometry ofthe splitter is designed to have a compact size and large bandwidth. Thesurface field is also minimized to reduce possible RF breakdown inhigh-power use. The splitter is well matched at any division ratio sothat the reflection from the input port is sufficiently small to protect thepower sources. The compact and large bandwidth design also enablesthe concatenation of such splitters, which are used to support manyaccelerating structures. The proposed design is described in section III.We manufactured this type of splitter and installed it in the X-band teststand at CERN. Prior to installation, the RF splitter was subjected to RFmeasurements using Vector Network Analyzer, as described in sectionIV.

2. Principle of operation

The schematic of the splitter RF circuit is shown in Fig. 3. Anadditional port is introduced in the network of the splitter to monitorthe function of the short-circuit piston. For known parameters sij in thescattering matrix [Eq. (1)], the outgoing power amplitude in each portyncan be calculated using Eq. (2). Here input power at port 1 isnormalized as 1. y1 is the reflection at the input port (also named as‘return loss’). y2 and y3 are the output power at the two output ports. y4is the RF power arrived at the short-circuit piston, and ρ y4 4 is the RFpower reflected back from this piston, where ρ4 is the reflection

http://dx.doi.org/10.1016/j.nima.2017.04.006Received 11 January 2017; Received in revised form 30 March 2017; Accepted 3 April 2017

⁎ Corresponding author.

1 Current address: Tsinghua University, Beijing, China.E-mail address: chah04@mails.tsinghua.edu.cn (H. Zha).

Nuclear Instruments and Methods in Physics Research A 859 (2017) 47–51

Available online 05 April 20170168-9002/ © 2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

MARK

coefficient.

yyyy

s ss s

s ss s

s ss s

s ss s ρ y

=

100

1

2

3

4

11 1221 22

13 1423 24

31 3241 42

33 3443 44 4 4

⎛

⎝

⎜⎜⎜

⎞

⎠

⎟⎟⎟

⎛

⎝

⎜⎜⎜

⎞

⎠

⎟⎟⎟

⎛

⎝

⎜⎜⎜

⎞

⎠

⎟⎟⎟(1)

yyy

sss

sss

sρ s

= +−

1

2

3

112131

142434

41

4−1

44

⎛⎝⎜⎜

⎞⎠⎟⎟

⎛⎝⎜⎜

⎞⎠⎟⎟

⎛⎝⎜⎜

⎞⎠⎟⎟

(2)

The outgoing power y1, y2, and y3 depends on the phase of ρ4. Thisphase can be changed by controlling the position of the RF short-circuitpiston. Thus, output power at ports 2 and 3 are then adjusted. The RFsplitter should be matched at any division ratio, which demands thaty1is equal to zero. Consequently, s11 and s14, should be equal to zero;however, s41 could not be zero, otherwise the dependence of y2 and y3 on

Fig. 1. Two klystrons used for power adjustment (‘j’ is the imaginary unit).

Fig. 2. RF power splitter.

Fig. 3. Four-port network proposal for the RF splitter.

Fig. 4. Five-port network proposal for the RF splitter.

Fig. 5. Orthogonal decomposition of the RF power.

Fig. 6. (a) RF polarizer (HFSS model); (b) short circuit piston (cut view).

H. Zha et al. Nuclear Instruments and Methods in Physics Research A 859 (2017) 47–51

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the reflection ρ4 is lost. This condition (s s≠14 41) breaks the symmetry ofthe RF network and needs components using anisotropic material(such as circulator) in the circuit. These anisotropic materials could notwork at the power level of hundreds of megawatts.

Matching at arbitrary ratio can be obtained using a five-portsymmetric network, where two ports (port 4 and port 5) are connectedto two pistons. The reflection factors for ports 4 and 5 are ρ4 and ρ5,respectively. The reflection y1 in this case is expressed in Eq. (3). y ≡01 .In the case of s s=14 15, Equation is converted into ρ ρ=−4 5 ands s s+ =244 55 45. The condition of ρ ρ=−4 5 requires simultaneous control oftwo pistons and provides a free dimension to adjust the power divisionratio.

A

A

y s s s ss

ρ s s

s ρ s

= + ( )

=−

−

1 11 14 154151

4−1

44 45

54 5−1

55

−1

⎛⎝⎜

⎞⎠⎟

⎛⎝⎜⎜

⎞⎠⎟⎟

(3)

sρ

sρ

s s s s s s s+ = + − 2142

4

152

544 14

255 15

245 14 15

(4)

Fig. 4 shows a five-port RF network for the splitter. The networkuses three RF impedance combiners to split the input port to fouroutput branches. Each RF impedance combiner is designed such thatevery port sees the combined impedance of the two others. Thedistance from the first impedance combiner to the second or thirdone is Nλ/2+λ/4, where λ is the propagating wavelength, and N is anintegral number. According to transmission line theory, the totalimpedance seen by the input port is Z Z Z Z1/( + )+1/( + )2 4 3 5 , where Zi isthe impedance of each port by itself. The scattering matrix of this five-

port network is expressed in Eq. (5).

S = 14

0 2 2 2 22 1 1 − 3 12 1 1 1 − 32 − 3 1 1 12 1 − 3 1 1

5

⎛

⎝

⎜⎜⎜⎜

⎞

⎠

⎟⎟⎟⎟(5)

As shown in Fig. 4, ports 2 and 3 are output ports and areconsidered matched in the design. Thus, Z Z= =12 3 . Ports 4 and 5 areconnected to two pistons, and the impedance is calculated usingreflection factors: Z ρ ρ=( +1)/( −1)n n n , where n is 4 or 5. In the conditionof ρ ρ=−4 5, the total impedance in the input port is always one and thesystem is consequently matched. The scattering matrix of the networkin this case is:

Fig. 7. RF splitter design using WR90 waveguide: (a) geometry (HFSS model); and (b)power at each port versus position of the piston.

Fig. 8. Attenuation factor (per 1 cm waveguide) of high-order propagating modes versus(a) waveguide width a; (b) waveguide height b.

Fig. 9. Waveguide dimensions versus maximum surface field (100 MW power flow).

H. Zha et al. Nuclear Instruments and Methods in Physics Research A 859 (2017) 47–51

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S

ρ ρρ ρ ρ

ρ ρ ρ= 1

4

0 2 − 2 2 + 22 − 2 (1+ ) 1 −

2 + 2 1 − (1− )3

4 4

4 42

42

4 42

42

⎛

⎝

⎜⎜⎜

⎞

⎠

⎟⎟⎟(6)

Reflection factor ρ4 is a complex number with an amplitude of one;as such, the phase is adjustable. If ρ4 is expressed as e jφ2 , theny j φ=−e sinjφ

2 and y φ= e cosjφ3 . Output RF power amplitudes y2 and

y3 range from 0 to 1, indicating that the power division ration isarbitrary. y2 and y3 have a 90° phase difference, thus the function of thispower splitter is similar to orthogonal decomposition to a vector

(Fig. 5). The RF power splitter can also be used as an RF combiner,which merges the power from ports 2 and 3 to port 1. The RF powerfrom ports 2 or 3 should have 90° phase difference, and the amplitudecould be arbitrary. The position of pistons should be adjusted accordingto the ratio of two amplitudes to achieve the matching state and ensurefull power transmission.

The system shown in Fig. 4 can be simplified if one port canaccommodate two different waveguide modes, similar to two TE11

modes in a circular waveguide with orthogonal polarization. In thiscase, only one short-circuit piston is used to reflect both modes. The RFcircular polarizer, as shown in Fig. 6(a), can provide such modemerging [10–20]. Here, RF signals coming from left (port 4) and right(port 5) will excite modes with orthogonal polarization in the con-nected circular waveguide. The circular waveguide is terminated by theshort-circuit piston [Fig. 6(b)], which reflects both polarizationsequally. Ports 4 and 5 are completely decoupled; thus, the phase ofthe reflected signals will be identical for both ports. This RF circularpolarizer is used to substitute the two channels with short-circuitpistons in the five-port network system shown in Fig. 4.

3. RF design and optimization

Fig. 7 shows the design of an RF variable splitter working at11.994 GHz. The splitter implements the concept of RF circularpolarizer and consists of several waveguide-based components: threeRF impedance combiners, two H-bended waveguides, and one circularRF polarizer. All components are designed using WR90 waveguidedimensions (22.86 mm×10.16 mm), and the diameter of the circularwaveguide is 18 mm. The distance among all components is minimizedto provide a compact design and maximize the frequency bandwidth.This design was optimized using HFSS code [21] (a high-frequency RFsimulation tool). The values of the outgoing RF power at each portversus the piston position are shown in Fig. 7(b). After carefully tuningall components, the system achieves a good matching state, where the

Fig. 10. RF splitter design using new waveguide dimensions: (a) geometry (HFSSmodel); (b) power at each port versus position of the piston; and (c) bandwidth of outputports.

Fig. 11. RF splitter before being installed: (a) half piece; and (b) brazed together.

Fig. 12. Measured bandwidth of RF splitter.

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maximum reflection is about −38 dB. The bandwidth of the devicevaries between 180 and 270 MHz depending on the division ratiobetween the two ports. For 100 MW peak RF power, the maximumsurface field is 48 MV/m. This field level is considered safe for X-bandRF pulses shorter than 2 μs.

The RF splitter could not reach the good matching state (less than−30 dB) for all power division ratios because of the reduced overalltransverse dimension of the device. The mismatch is due to the high-orderpropagating modes, such as TE11 or TE20, of the working frequency in thewaveguide. These high-order propagating modes are derived from thegeometrical asymmetry in some components and will disturb the circuit ofthe RF splitter. The high-order modes are cut off in the WR90 waveguide(e.g., cutting off frequency of TE20 is 13.7 GHz) to become attenuated whenpropagating in the waveguide. Therefore, this RF splitter design requires asufficient length of waveguide to suppress high-order modes.

The suppression of high-order propagating modes can be improvedusing a small waveguide; as a result, the cutting-off frequencies of thesemodes are high, and their attenuations in the waveguide are large.Fig. 8 shows the attenuation of each mode versus the waveguidedimensions, where a is the waveguide width and b is the waveguideheight. Among all high-order propagating modes in the geometry of theRF splitter, TE20 mode exhibits the largest contribution to themismatch. Thus, reducing a will enhance the suppression to TE20

mode and lead to a compact design. However, a small dimension ofwaveguide leads to a higher surface electrical field, as shown in Fig. 9.We finally select the waveguide dimensions: a =20 mm, b =12 mm. Thediameter of the pipe in the circular polarizer is reduced to 17 mm.

Fig. 10 shows the design using the new waveguide dimensions. Thedesign has a diameter of 100 mm and is more compact than the designshown in Fig. 4. The maximum return loss is less than −45 dB. Thebandwidth is 370 MHz for port 2 and 280 MHz for port 3. Themaximum electric field is 50 MV/m when transporting 100 MW power.The design has a more compact size, larger bandwidth, smallerreflection of input port than previous designs using WR90 dimensions.

4. Experimental measurements

We manufactured the designed RF splitter for the X-band high powertest stand at CERN, as shown in Fig. 11. The splitter was brazed from twohalf pieces and installed into the test stand. Prior to installation, a test(i.e., cold test) was carried out to measure the scattering parameters of thesplitter by using a vector network analyzer. The network analyzer canachieve a precision of 0.001 (−60 dB) in measuring the scatteringparameters after well calibrated. The measurement aims to ensure thearbitrary division ratio and good matching state of the splitter.

During the measurement, the position of the piston was moved toachieve different division ratios. A good matching state (less than−35 dB) was found for different positions of the piston. The bandwidthwas measured in two ratios, namely, full power transmission at port 2and full power transmission at port 3 (Fig. 12). The maximum powertransmission from port 1 to port 2 or 3 is at the level of −0.035 dB,corresponding to 99.6% transmission. The measured bandwidths arebenchmarked against the simulated values shown in Fig. 10(c).

5. Conclusion

An X-band RF splitter with arbitrary power division ratio wasdesigned using waveguide-based components including RF impedancecombiners, H-bend, and RF polarizer. The geometry was optimized toprovide a compact, low reflection, large bandwidth and low-surfacefield design. The splitter was manufactured, cold tested, and installedin the X-band high power test stand in CERN.

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