Proceedings of IBIC2012, Tsukuba, Japan

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RF FRONT END FOR HIGH BANDWIDTH BUNCH ARRIVAL TIMEMONITORS IN FREE-ELECTRON LASERS AT DESY
A. Penirschke#, A. Angelovski, M. Hansli, R. Jakoby, Institut für Mikrowellentechnik und Photonik,
TU Darmstadt, Darmstadt, Germany
C. Sydlo, M. K. Czwalinna, M. Bousonville, H. Schlarb, DESY, Hamburg, Germany
A. Kuhl, Universität Hamburg, Institut für Experimentalphysik Gruppe Beschleunigerphysik,
Hamburg, Germany
S. M. Schnepp, Laboratory for Electromagnetic Fields and Microwave Electronics,
ETH Zürich, Switzerland
T. Weiland, Institut für Theorie Elektromagnetischer Felder, TU Darmstadt, Darmstadt, Germany

High gain free-electron lasers (FELs) can generate ultra
short X-ray pulses in the femtosecond range. For a stable
operation of the FEL, the precise knowledge of the bunch
arrival time is crucial. The bunch arrival time monitors(BAMs) at FLASH achieve a resolution less than 10 fs forbunch charges higher than 500 pC which is sufficient forthe beam based stabilization system of the Free-Electron
Laser in Hamburg FLASH [1]. Increased demands forlow bunch charge operation mode down to 20 pC at
FLASH II and the European X-ray free-electron laser
XFEL require an upgrade of the existing beam diagnostic
equipment. A new high bandwidth BAM with newdeveloped cone shaped pickups [2] promises sub-10 fs
resolution for both, the high and low bunch charge
operation mode. This paper adresses the RF signal path of
the high bandwidth BAMs for FLASH II and XFEL. It
comprises radiation resistant coaxial cables, combiners
and limiters up to a frequency of 40 GHz from the pickup
electrodes to the Mach Zehnder Modulator (EOM).
Detailed investigations of the signal path using
measurements and simulations with AWR Microwave
Office allows for a good prediction of the signal quality
and shape at the EOM.

INTRODUCTION
In order to achieve femtosecond stability in thesynchronization process, the Free Electron Laser in
Hamburg (FLASH) is equipped with Bunch Arrival-time
Monitors (BAM) which are part of the electro-opticaldetection system [1]. The BAM comprises an RF-pickup,
an electro-optical front-end and read-out electronics [3].
The resolution of the BAM is proportional to the slope
steepness at the zero crossing of the pickup signal [4].
The slope steepness is a function of the peak-to-peakvoltage and the duration of the voltage response. These
two parameters are related to the geometrical parametersof the pickup structure such as the diameter of the coneshaped pickup and the diameter of the cut-out.

CONE-SHAPED PICKUP DESIGNIn [5] a cone-shaped pickup was introduced whichallows the detection of the arrival time with sub-fs
resolution for the low charge operation mode in FLASHII and XFEL. The simulated BAM pickup system has aslope at the zero crossing of approximately 382 mV/ps
and operates in the frequency range of DC - 40 GHz.
Figure 1 shows a sketch of the cross-section of the coneshape pickup.
d
c

h
e

a
b
a = 2.42 mm
40.50 mm (XFEL)e=
34 mm (FLASH)
b = 5.60 mm
c = 0.70 mm h = 6 mm
d = 1.62 mm

Figure 1: Cross-section of the cone-shaped pickup [5].
The cone represents a continuous transition from thebutton to the pin of the connector matched to the cable
impedance of 50 Ω.

BAM RF-FRONT END
Due to higher operation frequencies the RF front end of
the current BAM needs to be upgraded for the highbandwidth BAM. A sketch of the proposed RF front end
is shown in Figure 2.
For high- and low-charge operation, two different
channels are necessary. The low charge and the highcharge channel consist of two opposite arranged pickups(either horizontally or vertically arranged) which areconnected to a combiner via phase-matched rf-cables.

___________________________________________*Work supported by the Federal Ministry of Education and Research(BMBF).
#
penirschke@imp.tu-darmstadt.deISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractMOPA43

Proceedings of IBIC2012, Tsukuba, Japan0

low charge
channel-10|SXX| / dB

combiner
PS2-55-450

high charge
channelcombiner
ATM 213H-30

Port 1 - Port 2
Port 1 - Port 3
Port 2 - Port 3

0

10

BAM Boxsplitter
ATM 213H

20 30 40 50
frequency / GHz

60

Figure 3: Transmission and reflection coefficient versus
frequency of the RF-combiner PS2-55-450.
limiter
N9355

EOM 1

EOM 2

EOM 3

The combiner, a Multi-Wilkinson type from Pulsar
Microwave (Type PS2-55-450) ensures a maximum phasedifference of 8 degrees at 40 GHz between both input
ports. The limiter Agilent N9355 (10 dBm, up to 50 GHz)
protects EOM 3 from overvoltage in the high charge
operation mode. The low charge channel operates up to
40 GHz.
For high charge operation a bandwidth of 13 GHz issufficient to observe sub-10fs accuracy. Two different
EOMs are proposed for the coarse and fine channel. The
combiner and splitter are ATM 213H, which ensure amaximum operation frequency of 20 GHz and amaximum phase difference of 2.5 degrees. The limiter
Agilent N9356 (26 dBm, 26 GHz) protects EOM 1 of thefine channel. The coarse channel also consists of anvariable attenuator to adjust the slope at EOM 2.
To ensure a proper operation of the RF front end, the Sparameters of all components were measured prior to theinstallation.

Low Charge Channel Components
To combine the pickup signals a Multi-Wilkinson typecombiner from Pulsar Microwave (Type PS2-55-450) waschosen. The measurement results, shown in Figure 3,
exhibit reflection coefficients S11 less than -12 dB in afrequency range from 3 GHz up to 40 GHz andtransmission coefficients S21 of less than -6 dB up to
40 GHz for both components. The combiner show a low
pass characteristic with -10 dB at 45 GHz.
The limiter N9355C from Agilent technologies waschosen, since it offers a bandwidth up to 50 GHz with anacceptable power threshold of 10 dBm. The measurement
results displayed in Figure 4, exhibit refection coefficients

S11 less than -10 dB up to 52 GHz and transmission
coefficients S21 better than 3 dB up to 40 GHz and better
than -4 dB up to 50 GHz. The transmission factor of bothdevices is approximately linear up to 50 GHz.

High Charge Channel Components
The high charge channel comprises a much lower
bandwidth up to 13 GHz so that a combiner with a cutoff
frequency of about 20 GHz is sufficient. Here the
combiner ATM 213H was chosen since it is already in use
in the RF front end of the current BAMs. The
transmission coefficient, shown in Figure 5, exhibits a flat
frequency response up to 20 GHz and an acceptabletransmission up to 32 GHz.
The reflection coefficient is lower than -15 dB up to
20 GHz and lower than -5 dB up to 32 GHz.
To prevent EOM 1 from overvoltage, the limiter N9356C
from Agilent technologies was chosen. It limits the signal
to 25 dBm in a frequency range between 0.01GHz and
26.5GHz. The measurement results of an existing limiterat DESY and two ordered limiters are shown in Figure 6.
The existing limiter # MY45450217 shows a reflection
coefficient S11 lower than -15 dB up to 35 GHz and atransmission coefficient S21 better than -3 dB up to
32 GHz and -6 dB up to 36 GHz.

0

0

-5

-5

-10

-10
S11 N9355C_1-15

S21 N9355C_1
S11 N9355C_2

-20

-15

reflection S11 / dB

attenuator
BW-S15W2transmission S21 / dBlimiter
N9356

Figure 2: Proposed cabling plan and components for the
40 GHz BAM.
c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○-20

S21 N9355C_2-20
20 30 40 50 60
frequency / GHz
Figure 4: Transmission coefficient for the limiters Agilent
N9355C.
0

10

ISBN 978-3-95450-119-92Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan|SXX| / dB

-10

-20

-30

port1 - port2
port1 - port3
port2 - port3

0

10

20
30
frequency / GHz

40

Figure 5: Transmission coefficient of the combiner ATM
213H.
Since the S-parameter measurements of the existing
limiter exhibits excellent results up to 36 GHz thechannel, since its power threshold of 25 dBm would allow
the EOM to operate in the specified voltage range.
Therefore two additional limiters were ordered. Both
new limiters unfortunately have a much lower cutoff
frequency of 30 GHz (S11 -15 dB up to 30 GHz, S21 -3 dB@ 28 GHz and -6 dB @ 30 GHz). The cutoff frequency iswith 30 GHz about 6 GHz lower than that for the existing
device and therefore not useful for the 40 GHz path.

Cabling

0

0

-10

-10
|S11| MY45450217 (DESY)

-20

-20

|S21| MY45450217 (DESY)|S11| MY45452129 (New)

-30

-30

|S21| MY45452129 (New)|S21| / dB

|S11| / dB

For the cabling from the BAM pickups to the EOMs
silicon dioxide (SIO2) semi-rigid cables from Times
Microwave were chosen. The distance between the BAM
pickups and the BAM box is approximately two meters.
The outer jacket of the SIO2 cable consists of stainlesssteel, the outer and the inner conductor of copper and thedielectric is low density, high purity silicon dioxide(SIO2). The temperature range of the cables with standard
connectors is -200°C up to 200°C and the cable offers a
excellent radiation resistance (not further specified).
Times Microwave offers different cable diameters from

|S11| MY45452133 (New)-40|S21| MY45452133 (New)

0

10

20
30
frequency / GHz-40
40

6.58 mm diameter for low frequency applications(fcutoff=19 GHz) down to 2.286 mm for high frequency
applications up to 65 GHz.
Two different cable types are suitable for the BAM
cabling. The 38 GHz cable (3,58 mm diameter) has anattenuation of 2.75 dB/m @ 40 GHz whereas the 65 GHz
cable (2,286 mm diameter) has about 50 % more
attenuation (4.3dB/m).
For the BAM cabling the question raised, if a cutoff
frequency of 38 GHz is sufficient, since the 3 dB roll off
factor of the combiner is about 43 GHz. A conservative
choice would be to use the thinner cable to prevent
multimode propagation on the cable, but this would costadditional attenuation of the signal and thus a lower signal
at the EOMs.
Using the software tool Microwave Office from AWR,
simulations with different cable configurations were
performed. The simulations include the simulation resultsof the BAM Pickups obtained from CST Particle Studioand the S-parameter measurements of the combiner andthe limiters. The SIO2 cables were simulated by a lossy
coaxial cable model with the parameters given in Table 1.
Table 1: Dimensions of SIO2 Cables [6]Outer Diameter / mm
Inner diameter / mm
Attenuation / dB
fcutoff / GHz3.58 mm
cable
2.77
0.99
2.75
38

2.286 mm
cable
1.55
0.56
5.57
65

It is noticeable, that due to the extrapolation of SParameters and the exiting signal, the simulations with theMicrowave office simulations sometimes show nonphysically results. Nevertheless the simulation results
clearly shows the trend and can therefore be used tochoose the optimum setup.
To simulate the frequency response of the high charge
and the low charge channel each of them was modeledwith S-parameter blocks as shown in Figure 7.
All simulations are carried out with a cable length of
20 cm from the pickups to the combiner and 200 cm fromthe combiner to the EOM. Figure 8 shows the simulation
results of the frequency response for different cable types.
It can be seen that for the red and green curve the cutoff frequency of the limiter limits the frequency response
to 30 GHz. The usable bandwidth of the high charge
channel is limited to 20 GHz. At 20 GHz the setup with
6.58 mm cables offers about 5. 32 dB attenuation whereas
the 3.58 mm cable offers 7.19 dB of attenuation. A
crosscheck with a 3.58 mm cable for the connection
Input

Cable
20 cm

Figure 6: Transmission coefficient for Agilent N9356
limiters.

6.86 mm
cable
6.25
2.21
1.115
19

Combiner

Cable

Limiter

Output200 cmFigure 7: Block circuit diagram of the BAM RF front end.

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○0

MOPA43

MOPA43

Proceedings of IBIC2012, Tsukuba, Japan

Table 2: Slope and Voltage Reduction of the High Charge
ChannelParameter
Slope / mV/ps
Vpp / Vfcutoff / GHz

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 8: Simulation results for the low charge channel ofthe RF front end.
between pickup and combiner and a 6.86 mm cable fromthe combiner to the limiter shows nearly the same results
as for the 6.58 mm cable setup, since the cable length
between pickup and combiner is only 1/10 of the cable
length from the combiner to the limiter.
When using the voltage signal obtained by simulationsof the pickup with CST Particle Studio with a bunchcharge of 20 pC the voltage response can be simulated.
Figure 9 shows the voltage response of the single pickup
and the high charge channel with 6.86 mm and 3.58 mm
cables.
Using cables with higher losses as for the 3.58 mm
cable causes additional attenuation and thus a reduction of
the slope. Due to the fact, that within the high and low
charge channel, two pickup signals are combined, theamplitude after the limiter can be higher than that for the
single pickup. The slope of 394 mV/ps at a peak-to-peakvoltage Vpp=3.19 V reduces to 207 mV/ps at Vpp=3.28 V
and reduces further to 164 mV/ps at Vpp=2.61 V for the
3.58 mm cable. The results are summarized in Table 2.
The obtained spectrum is shown in Figure 10. Due tothe lowpass characteristic of the 26 GHz limiter theusable frequency band is limited to 30 GHz. It isnoticeable that the spectrum of the 3.58 mm cables isabout 3 dB less than that for the 6.86 mm cables.

Figure 9: Voltage response of the high charge channel fordifferent cable types for a bunch charge of 20pC.
ISBN 978-3-95450-119-9
4

singlePickup394
3.19
40

6.86 mm
cable
207
3.28
19

3.58 mm
cable
164
2.61
38

The frequency response for the low charge channel isshown in Figure 10. A linear attenuation is obtained up to
43 GHz. Above 43 GHz the lowpass characteristic of the
combiner dominates and limits the bandwidth. At 40 GHz
the 3.58 mm cable channel offers about 12.8 dB
attenuation and the 2.286 mm cable 16.2 dB. Thus, thethinner cable has 3.4 dB more losses and thus offers asignificant reduction in the slope.
The voltage response of the low charge channel isshown in Figure 11. Due to good performances of the
combiner and the limiter a slope of 264 mV/ps at
Vpp=3.22 V is obtained for the 3.58 mm cables.
Simulation results using the 2.286 mm cables show a
reduced slope of only 146 mV/ps at Vpp = 2.42 V. The
slope reduction is dominated by the high attenuation of
the long cable with 2.86 mm diameter.
The simulation results for the low charge channel aresummarized in Table 3.
Table 3: Slope and Voltage Reduction of the Low Charge
ChannelParameter
Slope / mV/ps
Vpp / Vfcutoff / GHz

singlePickup394
3.19
40

3.58 mm
cable
264
3.22
38

2.286 mm
cable
146
2.42
65

The spectra of all simulated channels are shown in
Figure 12. One can see that the combiners and limiters areproperly chosen for the application. Choosing the3.58 mm cable to connect the pickups to the combiners,
using the 6.85 mm cable for the connection from the
combiner to the limiter for the High charge channel andFigure 10: Simulated frequency response of the low
charge channel for 3.58 mm cables and 2.286 mm cables.

Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japanthe 3.58 mm cable for the low charge channel offers thebest results.

CONCLUSION
In this paper, the proposed upgrade of the RF front end
for the high bandwidth BAM is presented. It comprises a
new low charge channel with increased bandwidth up to
40 GHz. All necessary components were ordered andcharacterized with S-parameters.
Using the software tool AWR Microwave Office thelow and high charge signal path could be analyzed. The
pickup signal, obtained from CST Particle studio for abunch charge of 20 pC was used as input signals to the RF
front end and the attenuation and distortion was
simulated. For the cabling semi-rigid silicon dioxidecables with different cutoff frequencies were investigatedin order to find the optimum cable type for the low andhigh charge channel.
Simulations of the low and high charge channel showthat the 3.58 mm cable offers a sufficient voltage response
and thus a sufficient slope for the low and high charge
channel. Since the attenuation of the 2 meter long
6.85 mm cable offers about 1.8 dB more losses one canthink about using this cable if Vpp is to low in the highcharge channel. Simulations of the low charge channel
clearly demonstrated that the thinner cable reduces theslope dramatically. It is worth using the 3.58 mm cableeven if higher order modes can decrease the signal
strength above 40 GHz.

Figure 12: Simulated spectrum of the low and high charge
channel.

ACKNOWLEDGMENT
The authors would like to thank the company CST for
providing the CST MICROWAVE STUDIO software
package. The authors would like to thank P. Gessler, S.
Vilcins-Czvitkovits, M. Siemens of DESY, Hamburg and
J. Rönsch-Schulenburg, J. Roßbach of the Institute forExperimental Physics at Hamburg University, Hamburg
for the support.

REFERENCE[1] F. Löhl, et al., "Electron Bunch Timing with
Femtosecond Precision in a Superconducting FreeElectron Laser", Physical Review Letters, Vol. 104,
Issue 14, id. 144801, April 2010.
[2] A. Angelovski et al., " Realization of a High
Bandwidth Bunch Arrival time Monitor with Coneshaped Pickup Electrodes for FLASH and XFEL ",IPAC 2011, San Sebastian, Spain, 4-9 September,
2011.
[3] F. Loehl, "Optical synchronization of a free-electron
laser with femtosecond precision", DESY-TESLAFEL-2009-08 and DESY-Thesis-2009-031, 2009.
[4] M.K. Bock, et al., "Recent Developments of the
Bunch Arrival Time Monitor with Femtosecond
Resolution at FLASH", IPAC 2010, Kyoto, Japan,
23-28 May 2010.
[5] A. Angelovski, et al., "Pickup Design for High
Resolution Bunch Arrival time Monitor for FLAH
and XFEL", DIPAC 2011, Hamburg, Germany, 1618 May 2011.
[6] Times Microwave: www.timesmicrowave.com

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 11: Simulated voltage response of the low charge
channel for 3.58 mm cables and 2.286 mm cables for abunch charge of 20pC.

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