Proceedings of IBIC2012, Tsukuba, Japan

TUPA19FIRST TESTS OF A LOW CHARGE MTCA-BASED ELECTRONICS FOR
BUTTON AND STRIP-LINE BPM AT FLASH
B. Lorbeer∗ , N. Baboi, F. Schmidt-Föhre, Deutsches Elektronen-Synchrotron† , Hamburg, Germany
Abstract

OVERVIEWINTRODUCTION
Free-electron-laser (FEL) user facilities like FLASH at
DESY and LCLS at SLAC have been established as very
useful sources in the VUV and X-Ray regime. Apartfrom the laser-like features of these sources, the short lightpulses produced by these machines are very attractive forusers. Recently it turned out that the typical pulse lengthof about 100fs can be shortened substantially. The operation of these FEL facilities have demonstrated that beamcharges smaller than 100pC allow the generation of very
short electron bunches resulting in FEL pulse lengths in thefew fs regime [2, 3]. Since existing electronics for buttonand strip-line beam position monitors (BPM) of FLASH
have been designed for a charge of about 1 nC, they show
insufficient resolution for machine operation below 300pC.
Therefore, a new type of button and strip-line electronics
is under development. The dynamic range has been extended to lower charges to match the increasing need for
low charge operation. At FLASH the Micro Telecommunication Computing Architecture (MTCA) for physics standard will replace the old Versa Module Eurocard (VME)
based electronics of the old BPM system. The standard
introduces a new environment for the analog front-end
and digitizer electronics. The specifications for the newFLASH BPM electronics match with the ones from European XFEL [4]. The requirement for single-bunch resolution is 50μm in an aperture of +/- 3mm of center with abeam line diameter of 40mm [5]. In this paper the first prototype BPM electronic system based on the MTCA.4 standard is introduced. The analog electronics are described indetail. The first measurements with single-bunch operation
are presented.
∗ bastian.lorbeer@desy.de† DESY

The MTCA-based button and strip-line BPM system isschematically shown in Fig. 1.
   '(*')+ !

 (         ')*' ('(+"#$$%% & Figure 1: BPM conceptual system setup. One signal ofeach plane from the BPM is combined with the other after
a delay of ca. 100ns. The combined signals are then transported over a long RF cable from the tunnel to the electronic racks.
It follows the well-known Delay Multiplex Single Path
Technology (DMSPT), originally developed in 1986 for theHERA accelerator at DESY[6]. The pick-ups of the buttonand strip-line BPMs are oriented in the vertical and horizontal plane in the beam line. The strip-line monitor under test is the main type of standard BPM currently in operation at FLASH. The beam line diameter is 34mm andthe strip-line length is 200mm. The button monitor usedhere is a prototype button BPM for the E-XFEL. It is animproved HERA type button pick-up design to match theneeds in linear machines such as FLASH, and the E-XFEL.
The button diameter is 16mm and the beam line diameter is34mm. It has been installed and studied in the last year atFLASH [7]. The signals Vx1 and Vx2 in the horizontal, andVy1 and Vy2 in the vertical plane are concatenated with thehelp of a broadband radio frequency (RF) combiner. Thisis done by inserting a delay of 100ns for Vx1 and Vy1 withrespect to the other signal in each plane. Finally the signals
are travelling on a 80 m long 3/8” RF-cable from a patch
panel in the tunnel to a patch panel at an electronic rack outside the tunnel. This method has a superior common mode
electromagnetic interference (EMI) rejection compared toa single RF cable connection for both signals in each plane.
Typical signals from a button and a strip-line monitor are
shown in Fig. 2. A zoom of the signals is shown in Fig 3.
The rise-time of both signals is in the order of a few hundred picoseconds. Though the signal shape for both typesof BPM is entirely different, the front-end can measureboth types of waveforms. With a beam charge of 100pC
and a centered beam the signal strength is 180mVpeak forthe strip-line and 270mVpeak for the button BPM. The

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 ○Current FEL based light sources foresee operation withvery short electron bunches. These can be obtained withcharges of 100pC and lower. The typical charge range for
FLASH, DESY, Hamburg spans a range from 100pC up
to 1nC. The electronics currently installed at button andstrip-line BPMs have been designed for best performanceat higher charges and have reached their limits. Currently,for low charges a new type of electronics is being developed to overcome these limitations. These electronics areconforming with the MTCA.4 for physics standard [1]. The
next generation of FLASH BPM electronics is suitable formeasurements of button and strip-line BPMs. Furthermore,
the first measurement results taken with beam at FLASH,DESY are presented here.

TUPA19

Proceedings of IBIC2012, Tsukuba, Japan        

    



Figure 2: Button and strip-line monitor signals measured
in electronic racks. The amplitude is normalized to pC.

2

Figure 4: The prototype electronics for one BPM are located on a standarMTCA.4 for physics rear transition module.
receiver for each plane is based on a peak detector that isdriven by an alternating cascade of a low noise amplifier
and a digital step attenuator. The step attenuator can be
adjusted over a wide range from 0...31.25dB in steps of
0.25dB. The maximum small signal gain of the front-end isin the order of 40dB. The block diagram of one channel isshown in Fig. 5.

buttonstrip−line

1.5
1amplitude / mV/pC   

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○ 0.5
0−0.5



−1−1.5            "−2130

132

134
time /ns136

138

Figure 3: A zoom of the strip-line and button BPM.

 # $      !
    aquisition takes place on an analog front-end module, a digitizer with a generic FPGA firmware, and a CPU module.
All boards are designed to be compatible with the MTCA.4
standard. The analog front-end on a rear transition module (RTM), the digitizer on double size advanced mezzanine card (AMC), and the MTCA crate will be described in
more detail in this paper.

ANALOG FRONT-END
The signals from either button or strip-line BPM are received by an analog front-end. A picture of the prototype
board is shown in Fig. 4.
The standard size for a MTCA.4 compatible RTM is
180mm x 162mm. The module is powered by a single+12V supply. Other voltages are generated by DC/DC converters and voltage regulators. The connector on the righthand side of the picture is the so called Zone3 connector.
It carries the interface to the Advanced Mezzanine Card(AMC) digitizer card. A single receiver consists of twochannels. Each channel measures one plane of a BPM. TheFigure 5: Block diagram of the front-end for the position
measurement in one plane.
The purpose of the first step attenuator in the measurement chain of each plane is to adjust the power level forthe different beam charges, while the second step attenuator optimizes the signal level for the different transverse
offset’s. The output of the second amplifier drives a fastdetector composed of a Schottky diode and a peak hold capacitor. The hold capacitor is actively discharged with afast discharge circuitry. The discharge circuit is driven by
an external pulse generator that is synchronized with themachine timing. The hold capacitor is followed by a buffer
operation amplifier and a fanout with four outputs. This
fanout offers the opportunity to sample in an inter-leaved
mode and hereby increasing the number of sample-points
per waveform, resulting in an improved resolution. An additional feature of the front-end is the possibility for inter macro pulse calibration of the electronics using either
an external pulse or a pulse signal sent from a field pro-ISBN 978-3-95450-119-9
2Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, Japan

MTCA CRATE
For the test measurements a five slot MTCA fromSchroff has been used (Fig. 6).



 !  "#$$! $ Position Calculation
The expressions to calculate the beam offset positions in
the horizontal and vertical plane are given by Eq. (1) andEq. (2):xoﬀset ∼Vx1 − Vx2
Δx=Vx1 + Vx2
Σx(1)yoﬀset ∼Vy1 − Vy2Δy=
Vy1 + Vy2Σy(2)The values of the Δ/Σ - term is normalized to thecharge. They lie in the range between +1 and -1. The proportionality factor in Equ. (1) and (2) is mainly given bybeam line diameter, button size or strip-line length. Thefactor has been obtained from electromagnetic field simulation of a beam excited structure considering the entire
geometry or can be obtained experimentally from calibration of the monitor in the machine. At the point of themaximum slope, where the sign of the Δ/Σ - term flips
the monitor constant k is defined. From simulations we obtain kstrip =19.8mm for the strip-line monitor and from a
combination of simulations and measurements we obtainkbutton =12.3mm for the button BPM [7].

MEASUREMENT DATA
During recent beam studies it was possible to make position sweeps for typical button and strip-line BPM using thenew electronic system in a MTCA crate. A typical raw data
trace from hundred subsequent measurements for a stripline BPM is shown in Fig. 7.
21.81.6
1.4
amplitude / V

Figure 6: Crate used for the tests including management
carrier hub (MCH), hard disk (HD), processor (CPU), digitizer SIS8300, and timing module x1timer. The BPM frontend RTM is on the rear side and is not shown here.

1.2
10.8
left button/
strip−line
Vx2

0.6

The analog signals from the RTM are digitized on a commercially available 10 channel digitizer SIS8300 [8]. Afterthe digitization the raw data is collected on a control system server that is running on the crate CPU. The server isread out by a Matlab program running on a remote PC via
ethernet. The beam offset is calculated here. The synchronization of the measured data is done with a timing module x1timer. The x1timer receives a 108MHz clock signal from the master oscillator and a machine trigger fromthe old VME based timing system. The x1timer redistributes the trigger and clock to the digitizer card to clock
and trigger ADC’s and FPGA. At the moment a generic
FPGA firmware has been used to obtain the raw data sample points from the ADC’s. The development of a custom
made firmware is under way.

0.4

right button/
strip−line
Vx1

0.20offset
050

100150200time / ns

250300350

Figure 7: Raw data for a centered beam measured at 500pC
for the strip-line monitor. The sampling time step is 9.23ns.
The data is taken for 100 subsequent pulses in single-bunchmode.
The graph shows the ADC data for hundred subsequent
macro-pulses in single-bunch mode. The same signal
strengths for Vx1 and Vx2 indicate that the beam is centered. The fluctuations in the hundred subsequent measurements originate from the beam jitter as well as fromISBN 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 ○grammable gate array (FPGA) on the digitizer AMC. In
calibration mode the RF switch is switched to calibration
mode. Moreover the front-end board has an interlock interface to the Machine Protection System and an I2C bus
interface for optional purposes. In the AC coupled part of
the front-end the lower cut-off frequency is <5MHz and theupper bandwidth limit is at the -3dB point is 350MHz. After the peak detector the signal path is DC-coupled and thebandwidth is <160MHz at the fanout. The measured noise
figure from input jack to the input of the detector diode is<7dB. In order to have a sufficient drive voltage on thepeak detector the minimum input peak voltage is 20mV.
The strip-line and button BPM types tested with this newelectronic deliver a sufficient signal strength to measure theposition at charges down to 30pC.

TUPA19TUPA19

Proceedings of IBIC2012, Tsukuba, Japanlimitation of the electronic resolution. For the calculationof the Δ/Σ - term only the maxima of Vx1 and Vx2 havebeen considered. Consequently, all horizontal offset positions deliver hundred points.

0.4

0.3

Delta Over Sumc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○

0.2

0.1

0−0.1

−0.2

−0.5

0.15

00.5
steerer current / A

11.5

Figure 8: Position sweep at 20pC for the strip-line BPM.

Button Monitor
The button BPM under test is installed in a dump section prior to the undulator section in the FLASH facility.
The electron beam has been moved using a dipole magnet.
The beam has been detected on a screen at the end of thesection. The distance between the BPM and the screen is
0.6m. The measured charge was 80pC. With the calibration
constant of the screen the relative movement at the BPM
under test could be estimated. The measurable aperture isapproximately +/- 4mm with an error of 10%. The position
sweep is shown in Fig. 9.

PERFORMANCE ESTIMATIONThe resolution for the strip-line and button BPM havebeen estimated using the monitor constants kstrip andkbutton respectively multiplied by the measured Δ/Σ values. The standard deviation for the strip-line BPM is well
below 50μmrms at all offset positions. The measurable

Delta Over Sum

The strip-line under test is positioned after the first bunch
compressor in FLASH. The quadrupoles nearby have beenswitched off for these measurements in order to have a driftline. With the help of a steering magnet in this section
the beam could be moved tranversely by approximately +/3mm at the position of the strip-line under test. The position sweep detected with the new electronics is shown inFig. 8. The Δ/Σ is plotted against the steerer current. The
charge was approximately 20pC. In the circuit configuration for these measurements the maximum drive level on
the detector diode is 1Vpeak and a minimum of 0.5Vpeak .
Using Eq. 1 this leads to a measurable aperture of approximately +/- 3.5mm.

−10.2

0.1

Strip-line Monitor

−0.3
−1.5

0.25

0.05
0−0.05
−0.1
−0.15
−0.2

123456
beam position at screen / mm

78Figure 9: Position sweep at 80pC for the button BPM.
range fulfills the requirements. The expected resolution
will improve when considering several points for the position calculation. In the case of the button BPM, the standard deviation at all positions is on the order of 100μmrms.
The measurement range is larger than required. One possible reason explaining that the resolution is worse than required might be that the dipole transfers the energy jitter
into a position jitter. A cure for this is to perform future
measurements in a dispersion free section of the machinewith the same type of BPM. In order to have a real resolution measurement excluding the beam jitter one needs several BPM systems of this new type. The individual BPM’s
can then be compared with all other BPM’s. Plans to do
this in the future are under way.

SUMMARY
The prototype of the first MTCA compatible electronics
for button and strip-line BPM has been tested in FLASH.
The measurements show promising results for a new typeof BPM to be used in the future at FLASH, DESY in Hamburg. The analog electronics have successfully been integrated on a MTCA.4 for physics RTM. The test setup forthe strip-line BPM has shown sufficient measurement performance of 50μmrms at charges below 100pC, even in anon optimized readout configuration. The measurement ofthe button BPM probably includes a huge amount of beamjitter. With a removed beam jitter in a future measurement
the resolution will improve significantly.

OUTLOOK
The drive voltage on the detector circuit will be increased in order to obtain higher accuracy, linearity, andan extended measurement range for transverse offsets. A
redesign of this prototype is planned for this year. Reference measurements in the laboratory and calibration of theanalog front-end are in progress. The readout configuration will be further improved by optimized sample timing,
attenuation control, and correction of acquired data.

ISBN 978-3-95450-119-9
4Beam Position Monitor System

Proceedings of IBIC2012, Tsukuba, Japan

TUPA19ACKNOWLEDGMENT
Sincere thanks go to Rudolf Neumann and J.LundNielsen for many discussions, advice, circuit ideas, concerns and inspiration. We also acknowledge the good collaborative work with K.Rehlich and colleagues and the support from the DESY FEB department.

REFERENCES[1] PICMG MTCA.4 Revision 1.0, ”MicroTCA Enhancements
for Rear I/O and Precision Timing”, August 22, 2011,
http://www.picmg.com[2] J. Rönsch-Schulenburg et. al., ”Generation of ultra-shortelectron bunches at FLASH”, THPD33, Proceedings ofFEL2012, Nara, Japan.
[3] Y.Ding, ”Measurements and Simulations of Ultralow Emittance and Ultrashort Electron Beams in the Linac Coherent
Light Source”, Physical Review Letter 102, 254801 (2009).
[4] D. Nölle, ”The European XFEL Beam Position Monitor System”, Conceptual Design Report, Rev.1.00, June 15 2010[5] N. Baboi private communication.

[7] D. Lipka et. al., ”Button BPM Development for the European XFEL”, MOPD19, Proceedings of DIPAC2011, Hamburg, Germany.
[8] SIS8300, ”MTCA for Physics digitizer”, August 2011,
http://www.struck.deISBN 978-3-95450-119-9
Beam Position Monitor System

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○[6] F. Schmidt-Föhre et. al, ”BPM system upgrades in the Petra 3
Pre-accelerator chain during the 2008 shutdown”, MOPD20,
Proceedings of DIPAC09, Basel, Switzerland.