Proceedings of IBIC2012, Tsukuba, Japan

MOPA15NEW ELECTRONICS DESIGN FOR THE EUROPEAN XFEL RE-ENTRANT
CAVITY MONITOR
C. Simon#, CEA-Saclay/DSM/Irfu, Gif sur Yvette, France
N. Baboi, DESY, Hamburg, Germany
R. Baldinger, B.Keil, R. Kramert, G. Marinkovic, M. Roggli, M. Stadler, PSI, Villigen, Switzerlandequipped with a beam position monitor connected to aquadrupole at the high-energy end of the cavity string. 31
cold BPMs will be re-entrant RF cavities which have tooperate in a clean and cryogenic environment. Pickuprealisation and mounting in the cryomodule are aCEA/DESY collaboration but are not discussed here [2].

About one third of the beam position monitors (BPMs)
in the European XFEL (E-XFEL) cryomodules will be reentrant cavities. The BPM mechanics and RadioFrequency front-end (RFFE) electronics are developed byCEA/Saclay. Two RFFEs and a digital back-end with two
ADC mezzanines are integrated into a compact
standalone unit called MBU (modular BPM unit)
developed by PSI.
The signal processing uses hybrids and a single stage
down conversion to generate the signals sum and delta.
Every RF/analog component of the re-entrant BPM
electronics has been simulated with a Mathcad model andtested independently on test benches. The very low Q of
the cavity monopole mode allows the new electronics tofilter this mode at the dipole mode frequency, and an I/Q
demodulation for delta and sum channels allow the digitalback-end to determine the sign of the beam position justby comparing the phases of the channels, independentlyof beam arrival time jitter and external reference clockphase. This paper describes the design and architecture ofa new re-entrant BPM electronics, including results ofbeam tests at FLASH that were performed to validate thechosen design.

The electronics of the European XFEL BPM system [3]follows a modular design approach [4]. This customized
crate, called Modular BPM Unit (MBU) (Fig. 1), contains
a generic digital back-end (GPAC = generic PSI ADCcarrier) with two ADC mezzanine boards, and either four
button [3] or two re-entrant or undulator cavity [3] BPM
RFFEs, or any combination of those types, as well aspower supplies, fans, and a rear IO module with digitaland multi-gigabit fiber optic IOs.

INTRODUCTION

Figure 1: Modular BPM Unit.

The European XFEL [1] is an X-ray free electron laser
user facility currently under construction in Hamburg,Germany. This accelerator has a superconducting
17.5GeV main linac and its parameters are summarized inTab 1.

RF/ANALOG ELECTRONICS

Table 1: E-XFEL Accelerator Parameters

MODULAR BPM UNIT CONCEPTThe signal processing of the re-entrant BPM uses asingle stage down conversion to obtain Δ/Σ. The
difference Δ and sum Σ signals are obtained from apassive 4- ports 180° hybrid which will be installed in abox mounted at the side of the cryomodule. Each coupler
is connected to each pair of opposite antennas andtransmits the signals to the radio-frequency front-end(RFFE) electronics via some 30 m long semi-rigid cables.
This RFFE electronics, based on a Printed Circuit Board(PCB) with surface mount components, uses the VME64x
form factor as required by the Modular BPM Unit. A firstelectronics was realized in 2010 and tested with beam [2],and a new RFFE with additional functionality is currently
being developed.

Parameter

Value

Normalized projected emittance

1-2 mm mrad

Typical beam sizes (RMS)

20 – 200 µm

Nominal bunch charge

0.1 – 1 nC

Bunch spacing≥ 222 ns

Macro-pulse length

600 µsNumber of bunches within macro-pulse

1 - 2700

New Design

Nominal macro-pulse repetition rate

10 Hz

Maximum macro-pulse repetition rate

25 Hz

The new RFFE analog electronics design, presentedFig. 2, has three channels to perform single-stage
downconversion of X position, Y position and reference(charge) RF signals from L-band to an intermediate (IF)frequency. Because of the low external quality factor, theThe BPM system is developed by a collaboration ofCEA/Saclay/Irfu, DESY and PSI. Each cryomodule is___________________________________________#

claire.simon@cea.fr

ISBN 978-3-95450-119-9
Beam Position Monitor System

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstractc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○MOPA15

Proceedings of IBIC2012, Tsukuba, Japan

single bunch response of the cavity has to be broadened
via band-pass filter before its acquisition.
Monopole and dipole mode frequencies of the pickup
are respectively 1255 MHz and 1724 MHz [5]. To
determine the sign of the beam position, the sum Σchannel of the RFFE will work at the same frequency as
the difference Δ channel. The very low external quality
factor Q (Q = 24) and the amplitude level of the cavity
monopole mode [5] allows the new electronics to filter
this mode at the dipole mode frequency 1724 MHz andstill obtain a sufficiently large signal level.
On the Δ channel, the rejection of the monopole mode
signal, which does not depend on the beam position, is
produced by a band-pass filter limited to a 3 dB
bandwidth of 110 MHz around the dipole mode frequency
1724 MHz with about 60 dB of attenuation to reject thehigher order modes and monopole mode.
Because of the planned operation of the E-XFEL atlower charges from 20 pC to 100 pC, this new design has
one input gain range from 0.1 to 1 nC and one from 20 pC
to 0.1 nC bunch charge, for the Δ channel. In this second
input gain range, the signal is amplified with a low noise
amplifier (LNA) and the gain is adjusted thanks tovariable attenuators to protect the LNA in different
operating modes of the accelerator. The Σ channel has
only one gain range and uses the same variable attenuatorand LNA. Dynamic range is extended from - 10 dB to
20 dB by switching the gain.
To protect mixers from possible high output powerfrom the antennae, an RF limiter is used on Δ channels.
The signal is, then, translated to a lower IF by mixing
with a local oscillator (LO) and an in-phase/quadraturephase (I/Q) demodulation on each channel. The LO drives
all three channels with an output power +17 dBm and islocked to the reference signal 216,66 MHz from the linac
machine. To generate the LO signal around the dipole

mode frequency, a phase-locked loop (PLL) is combined
to a divider using an intermediate frequency 9.028 MHz.
The I/Q demodulator is composed of mixers used forsystems with a high dynamic range, and of 90 ° hybrids
couplers chosen for their low phase and amplitude
imbalances. This I/Q demodulation for delta and sumchannels allow the digital back-end to determine the signof the beam position just by comparing the phases of thechannels, independently of beam arrival time jitter andexternal reference clock phase. With an 216.66 MHz onboard oscillator integrated to the electronics, the LO canalso operate without external reference clock in a freerunning mode.
The ADC clock, integrated on the PCB, is also locked
to the reference signal and uses a programmable phaseshifter and PLL.
Every RF/analog component of the re-entrant BPM
electronics has been simulated with a Mathcad model [6]and tested independently on test benches.
The output I/Q IF signals delivered to the ADCs aredifferential signals. Their shape is close to Gaussian withduration of around 40 ns as shown Fig. 3. The amplitude
range is ±1.25V.

Figure 3: Output I/Q IF signals simulated with a Mathcad
model.

Figure 2: Signal processing electronics.

ISBN 978-3-95450-119-92Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, Japan

Beam Tests
Beam tests at FLASH have been performed in order tovalidate the new RFFE design for the sum (Σ) signalprocessing channel.
A simple RF processing electronics composed of aband-pass filter at the dipole mode frequency(1724 MHz), an 90° hybrid and two mixers to do the I/Q
demodulation were installed to measure the beam charge.
This electronics showed Fig. 4 was connected to cablesfrom the BPM pickup installed in the FLASH linac [5]. A
9.028 MHz machine reference clock signal was used togenerate the LO signal for the I/Q demodulation.

MOPA15phase. The re-entrant RFFE output signals for the Σchannel are sampled at its peak and for the Δ channelssampled at their top for a significant offset.

Figure 4: Schematics of the sum (Σ) signal processing.
Signals IΣ and QΣ, generated by the electronics, wereconnected to the digital electronics composed of fast
ADCs with 14 bits resolution [7].
To test the electronics with beam, a re-entrant BPM
installed between two toroids, was used to carry out beam
tests. The coefficient to determine the charge read by theBPM was obtained from the charge readings of the twotoroids. The beam position was moved with steerer
magnets in order to see if the Σ channel signal isindependent of the beam position. The beam charge wasvaried via the gun laser energy.
Figure 5 illustrates the charge read by the BPM
electronics setup vs charge read by the both toroids. It
shows a good linearity of the BPM in a range from 0.1 nC
to 1 nC.

Figure 5: Charge read by reentrant BPM vs charge readby toroids 1TCOLand 5DBC3.

DIGITAL BACKEND
The final version of the re-entrant BPM electronics will
use a digital back-end designed by PSI that employs 16bits ADCs operating at 160 MSPS. The FPGAs on thedigital back-end will automatically adjust the ADC clock

On the present prototype of the digital back-end, twoVirtex-5 FPGAs (Fig. 6) on the GPAC calculate beam
position and charge from the ADC data, perform
automated adjustment of ADC clock phase, RFFE gain
and read RFFE status data like temperatures, PLL lock
flags, etc... A third Virtex-5 FPGA contains genericfirmware/software to control timing and machineinterlock system. The electronics supports both an
external trigger mode and a self trigger mode where a
programmable threshold for the reference channel, i.e.
bunch charge, is used as trigger condition.
The MBU has a customized low-cost MBU backplane
PCB with direct connections between the different boards
plugged into the MBU. However, operation of the boards
in normal VME64x crates is also supported, where GPAC
and RFFE backplane connectors are interfaced via VME
rear transition modules.
SFP multi-gigabit fiber optic transceivers at front andrear side of the MBU, connected to FPGAs on the GPAC,
are used for control and timing system interface as well as
for exchange of low-latency (microsecond timescale) datawith other subsystems like LLRF.
Due to the similar architectures of re-entrant andundulator cavity BPM RFFE, both systems can use thesame ADC, digital back-end and nearly identical FPGA
firmware. Firmware concept, design and beam tests
results are described in detail in ref. [3,8].

CONCLUSION
New electronics design for the re-entrant BPM cavity is
being developed in the E-XFEL project. Beam tests weredone to check a new design approach for the Σ channel,
showing good linearity in a range from 0.1 nC to 1 nC.

ISBN 978-3-95450-119-9
Beam Position Monitor System

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Figure 6: Digital Back-End FPGA Signal Processing
Board Prototype [4]. The two large Virtex-5 FPGAs
receive the ADC data from the digitizer mezzanines (viaadjacent 500-pin connectors, mezzanines removed onphoto).

MOPA15

Proceedings of IBIC2012, Tsukuba, JapanThe next step will be the production of the final RFFE
board and its test in the MBU, using the GPAC and 16-bit
ADCs for digitization and signal processing. Lab tests andbeam tests will be carried out to check if the performancemeets the E-XFEL requirements.

ACKNOWLEDGMENT
We thank our colleagues, D. Nölle, B. Lorbeer, O.
Hensler from DESY for their support and their help
during the measurements done on the FLASH linac and S.
Steve for the fruitful discussions. We, also, wish toacknowledge our colleagues from CEA Saclay, inparticular Michel Luong for their advices and their help
on this project.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○REFERENCES[1] R. Brinkmann, “The European XFEL Project”,
Proc.FEL´06, Berlin, Germany, 2006..
[2] C. Simon et al., “Status for the re-entrant cavity
Beam Position Monitor for the XFEL project”,
BIW10, Santa Fe.
[3] B. Keil et al., “The European XFEL Beam Position
Monitor System”, Proceedings IPAC’10, p. 1125 ff.
[4] B. Keil et al., “A Generic BPM Electronics Platform
for European XFEL, SwissFEL and SLS”, theseproceedings[5] C. Simon, et al., “Performance of a reentrant cavity
beam position monitor”, Physical review special
topics – accelerators and beams 11, 082802 (2008).
[6] C. Simon et al.,” High resolution BPM for linearcolliders”, EPAC06, Edinburgh.
[7] J. Andruszkow, P. Jurkiewicz, and F. Tonisch, 8channel FastADC with 14 bit resolution, Reference
Manual.
[8] M. Stadler et al., “Beam Test Results of Undulator
Cavity BPM Electronics For The European XFEL“,these proceedingsISBN 978-3-95450-119-9
4Beam Position Monitor System