HIGH POSITION RESOLUTION BPM READOUT SYSTEM WITH
CALIBRATION PULSE GENERATORS FOR KEK e+/e- LINAC
F. Miyahara#, K. Furukawa, M. Satoh, Y. Seimiya, T. Suwada, KEK, Tsukuba, Ibaraki, Japan
R. Ichimiya, JAEA, Rokkasho, Aomori, Japan
H. Saotome, KIS, Tsuchiura, Ibaraki, Japan

Abstract
The KEK e+/e- injector linac will be operated in
multiple modes for the electron beam injection to three
independent storage rings, the SuperKEKB HER, Photon
Factory (PF) Ring and the PF-AR, and the positron beam
injection into the damping ring and the SuperKEKB LER.
The beam current ranges between 0.1 and 10 nC/bunch.
The beam current depends on the beam mode. The
operation modes can be switched every 20 milliseconds.
The injector linac is under upgrade for the SuperKEKB,
where the required resolution of beam position
measurement is less than σ=10 µm. However, the current
beam position monitor (BPM) readout system based on
oscilloscopes for stripline beam position monitors has the
position resolution of 50 µm approximately. Thus, wehave developed a new BPM readout board with narrow
band pass filter, 16-bit, 250 Msps ADCs and calibration
pulse generators. The system is based on VME standard
and the beam position is calculated by FPGA on board.
The calibration pulse follows every position
measurement. The calibration pulse is used for the gain
correction and the integrity monitor of the cableconnection. We will report details of the system.

INTRODUCTION
In the SuperKEKB, the injector linac is required to
inject 5 nC/bunch, 7GeV electron and 4 nC/bunch, 4 GeV
positron beams with the emittance less than 20 mm-mrad.
The electron beam with emittance of about 10 mm-mrad,is generated by a photocathode RF gun [1]. The initialemittance is low enough for the requirement, but anemittance growth due to the short range transverse
wakefield which is caused by misalignment is not
negligible. The emittance growth can be suppressed byusing the offset injection method [2]. Numerical studies
show the emittance at the end of the linac is less than
20 mm-mrad under the condition of alignment error of
0.1 mm [3,4]. To perform the offset injection in the linac,
at least resolution of 10 µm is required. However, theresolution of current oscilloscope based BPM readout
system [5] is about 50 µm. The beam is also transported
to the PF ring. The bunch charge for the PF ring is
0.2 nC/pulse during the top-up operation. By contrast,
primary electron beam for positron production will be________________________________________#fusashi.miyahara@kek.jp

10 nC/bunch. Those operation modes are switched every
20 ms. Thus, the wide dynamic range for beam currents isrequired. The dynamic range can be ensured by setting
attenuation of the BPM signal for every beam modes orcurrents. That means several gain parameter sets and acalibration system to fix those parameters are needed. The
calibration system allows the monitoring of a variation of
the gain and integrity of the cable connection. The newly
developed board was designed to have the high position
resolution with dynamic range from 0.1 to 10 nC/bunch
and to be equipped calibration pulse generators. A
position resolution of σ=3 µm for 0.1 nC/bunch beam wasachieved [6]. The board is also designed to have wide
dynamic range for the position. The detail of the design isreported in previous studies [6,7]. There are 93 stripline
BPMs on the linac, 20 VME crates with 3-8 readoutboards are set on klystron gallery. The old system will be
replaced with the new one stepwise after this summer. In
this paper, we report a whole system of the BPM,
calibration scheme and stability of the gain.

BPM SYSTEM
The BPM readout system consists of a VME CPU
board (MVME5500), a RAS (Reliability, Accessibility,
Serviceability) board, an Event Receiver (VME-EVR230RF) [8] and BPM readout boards. An EPICS IOC [9]runs on the CPU. The RAS board monitor status (such as
power, temperature) and control the fan of the VME crate.
The event receiver is used for the distinction of the beammode and the generation of triggers to BPM boards. The
readout board has 4 signal input channels for one stripline
BPM. The calibration signal is also sent from one of thosechannels to the corresponding electrode.

BPM Readout Board
The BPM readout board is composed of four RF units
which detect the signal from BPM electrodes, 16-bit 250
MSPS ADCs, sub FPGAs for the ADC and a main FPGA.
The position and charge of the beam are calculated in themain FPGA. Those results and parameters for the
calculation and the operation of the board are recorded in
the registers on main FPGA. Figure 1 shows the block
diagram of the board and the RF unit. The setting value ofthe variable attenuator, sum of ATT1A and ATT1B,
depends on bunch charge. In particular, the beam is awayfrom BPM center, a nonlinearity of the ADC gain causesa measured position drift. The readout board has a goodlinearity, less than ±5 µm drift in position, for the relative
input signal of ±10 dB.

static signal balance between electrodes and depends on
signal attenuation in the coaxial cable and a variation of
circuit element. The G’, dynamic gain factor, represents
gain drift which strongly depends on the environmental
temperature. Those factors are discussed in the next
section.

Calibration Pulse

Figure 1: Schematics the BPM readout board (a) and the
RF Unit (b).
The signal from the stripline BPM is filtered by 4thorder Butterworth filter which has a bandwidth (BW) of
60 MHz with center frequency (fcenter) of 180 MHz andthe 2nd-order Bessel filter (BW=22 MHz, fcenter=180
MHz). The double bunch operation with time interval of
96 ns is required in the operations for SuperKEKB. Thus,
the Bessel filter is used to convert a bipolar signal toringing sinusoidal waveform with full-width of ~90 ns.
The Butterworth filter is used to compensate rather broad
cut-off characteristic of the Bessel filter. The beam
position is given by,
3

3

x = A0 + ∑∑ a mn (∆ H / Σ H ) (∆ V / Σ V )mnm =0 n =03

3

The calibration pulse is used for a correction of gainbalance between electrodes and a monitor of gain drift
and cable connection. The signal is shown in Fig. 2. The
signal is sent to one of the four electrodes, and then return
signals are measured by the readout system. Figure 3
shows ADC signals for the output pulse which was sentfrom Ch1. Both Ch3 and Ch4 correspond to adjoining
electrodes and detect large signal at around 2400 ns. The
small signal after the large signal, delayed about 350 ns,
represents reflection. The calibration pulse can be set
from a short pulse to CW. The pulse width is controlled
by the switch after the RF module. Due to response to the
CW contains superposition of refection signals; a shortpulse is adopted for the calibration. The attenuator after
the calibration module, ATT(CAL) in Fig. 1, is used to
avoid saturation of the amplifiers and ADC. The power ofthe RF at the BPF1 input is 36 dBm in ATT(CAL)=0 dB.
In case of a largest attenuation setting which corresponds
to 10 nC/bunch, we confirmed that signal level enoughfor the calibration can be obtained.

(1)

y = B0 + ∑∑ bmn (∆ H / Σ H ) (∆ V / Σ V ) ,mnm =0 n =0

where
Figure 2: Calibration pulse signal.

∆ H =W 1−W2 , Σ H =W 1−W2
∆V =W 3−W4 , Σ H =W 3−W4

Wi = Gi Gi′ ∑ (Vi , j − Vi , ped ) .
2

(2)

(3)

j

The coefficients amn and bmn were given by fitting thefunction to the mapping data taken from stretched wiremeasurement. The variables A0 and B0 stand for the offset
of electrical center from the quadrupole magnet centerand are determined by beam based alignment. An output
signal from an electrode is given by root sum square of
ADC values within a window which covers thebeam/calibration signal and correction factor G and G’.
The variable Vi,j and Vi,ped, are ADC value of the i-th
electrode (see Fig. 1), where j represents ADC channel,
and the pedestal, respectively. The factor G stands for

Figure 3: ADC response to the calibration pulse. The
pulse was sent to the BPM from Ch1(X+).

Control and Operation
The readout system is controlled by EPICS framework
which was utilized for most devices in the injector linac.
The EPICS IOC works on VxWorks 5.5.1. All parameters
for the operation and results of a measurement such as
position, charge and waveform can be taken from EPICS
records. To reduce data acquisition (DAQ) time, thewaveform of the signal is not transferred in normal
operation. It takes about 3.5 ms to take four waveforms inthe board. The waveform is used for setting/confirming
the ADC gate. The development of the software isongoing. The detail of the software will be reported inelsewhere [10].
DAQ scheme is the following:
1. External Trigger→Beam Data acquisition→Writing parameters for calibration
2. Internal Trigger (6.6 ms after the ext. trigger)→ Calibration Data acquisition for Horizontal→Writing parameters for calibration
3. Internal Trigger (13.2 ms after the ext. trigger)→ Data acquisition for calibration (vertical)→Writing parameters for a next beam.
The external triggers are synchronized the beam timing
which is distributed by the EVR. The writing parameterrequired for each process because parameters such asattenuation setting, gain factor could be changed. The
repetition rate of the linac is 50 Hz; three DAQ processes
are performed in 20 ms interval. Thus, one DAQ process
must be less than 6 ms. Figure 4 shows total elapsed times
for one DAQ process in case of maximum load with 8
readout boards. The DAQ time is much faster thanrequired time.

Figure 4: Total elapsed time for one data acquisition
process with 8 BPM readout boards.

SIGNAL STABILITY AND GAINBALANCE
Studies of the gain drift and gain balance between
opposite electrodes have been performed by using thecalibration pulse. The drift of signal level and gain

Figure 5: Signal level and gain balance variation.
Temperature is shown in the top. Signal levels of
CH3(Y+) and CH4(Y-) are plotted by red and blue dots,
respectively. The green dots show the ratio of bothsignals. The Att(In) and Att(Cal) stand for attenuation
value for the input (ATT1A+ATT1B) and output of thecalibration.
balance lead to variation of measured charge and position,
respectively. The gain balance reflects variation in circuit
elements. In particular, the amplifier has a rather large
temperature-dependence compared with other elements.
Thus, the temperature dependence of the system and thegain drift has been studied. The response to thecalibration signals fordifferent environmental
temperatures measured by putting a BPM, 30 m coaxial
cables and the readout system in a thermostatic chamber.
The signal level W3, W4 and gain balance W3/W4 for thecalibration pulse from CH1 are plotted with thetemperature in Fig. 5. Where, G =G’=1 in Eq. 3. Threesets of the attenuation settings correspond to different
bunch charge, 0.5, 1 and 5 nC/bunch. In case ofAtt(In)=25 dB, in order to keep the signal level to be ingood linearity region, the attenuator just after the pulse
generator was set to 9 dB.
Each data point shown in
Fig. 5 is an average of 500 events, and a statistical error isless than 1 ch. The environmental temperature waschanged every ~5 hours. The signal and gain balance arevery stable and shows quick response and good
reproducibility to the temperature, except in the case 9 dB
attenuation for the calibration pulse. It appears that theattenuator needs much more time to be an equilibrium
state due to high output power of the pulse generator. Thisvariation is less than 1% and the absolute value of thesignal does not affect the beam position but the charge.
The temperature dependence of the signal level is -1%/°C
in the system.

Figure 6 shows temperature dependence of the gainbalance. The data are averages of 10 minutes just beforechanging the temperature setting. It has been found thatthe temperature dependence of the gain is very small, lessthan 0.02%/°C which corresponds to about 1 µm/°C, andshows same gradient for different attenuation settings. In
the injector linac, the room temperature is kept in 23 to
26 °C. As a result, the position drift due to a temperaturefluctuation is negligible and a correction of the gain drift,G’, is not required. By contrast, the value of the gainbalance depends on the attenuation value, Att(In), that
corresponds the beam charge. The static gain factor G
will be set so that the signal ratio of opposite electrodes
becomes 1 for any attenuation setting. The calibration
pulse can be powerful equipment to perform the gain
calibration in our linac. More precise gain correction canbe performed by using the beam.

Figure 6: The gain ratio vs. the environmental
temperature.

CONCLUSION
In order to suppress the emittance growth in the KEK
e+/e- linac, a new BPM system with high position
resolution and wide dynamic range in beam charge andposition is required. We have developed the new VMEbased BPM readout system which consists of CPU board,
RAS board, EVR for event timing system and the new
readout board with a calibration pulse generator. The

system shows negligible gain drift, less than 1 µm, in theenvironmental temperature of the injector linac. The
calibration system gives easy ways to correct the gainbalance for any attenuation settings that correspond to
beam currents.

ACKNOWLEDGMENT
We greatly thank Dr. Steve Smith (SLAC) and Dr.
Andrew Yang (SLAC) for providing us with theimportant information of the developing of BPM readout
system and meaningful suggestions. The authors would
like to express thank to Dr. Makoto Tobiyama (KEK) for
useful suggestions. We grateful Kyosuke Yamada(DIGITEX LAB. CO.) for his support in the fabricationof the readout board.

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