HIGH POSITION RESOLUTION AND HIGH DYNAMIC RANGESTRIPLINE BEAM POSITION MONITOR (BPM) READOUT SYSTEM FOR
THE KEKB INJECTOR LINAC TOWARDS THE SuperKEKB
R. Ichimiya#, T. Suwada, M. Satoh, F. Miyahara, K. Furukawa, KEK, Tsukuba, Japan
Abstract
The SuperKEKB accelerator is now being upgraded tobring the world highest luminosity (L=8x1035/cm2/s).
Hence, the KEKB injector linac has to produce low emittance and high charge electron (20 mm mrad, 7 GeV/c2,5 nC) and positron (20 mm mrad, 4 GeV/c2, 4 nC) beam,respectively. In order to achieve these criteria, the accelerator structure has to be aligned within 0.1 mm position
error. Since BPMs are essential instruments for beam
based alignment (BBA), it is required to have one magnitude better position resolution to get enough alignment
results. We have begun to develop high position resolution BPM readout system with narrow bandpass filters (fc= 180 MHz) and 250 MSa/s 16-bit ADCs. It handles twobunches with 96 ns interval separately and has a dynamic
range from 0.1 nC to 10 nC. To compensate circuit drift,
two calibration (x-direction and y-direction) pulses are
output to the BPM electrodes between beam cycles(20 ms). Since it needs to achieve not only high position
resolution but also good position accuracy, overall nonlinearity within ±0.02 dB is required and the system has
to have more than ±5 mm accurate position range. We
confirmed the system performance with a 3-BPM resolution tests at KEK Injector Linac and it turned out that thesystem has 3 μm position resolution. We plan to install
this system during 2015 summer shutdown.

INTRODUCTIONKEK Injector Linac is now being upgraded as a part ofSuperKEKB accelerator complex [1] to achieve
L = 8x1035 /cm2/s luminosity, both electron and positron
beam emittance have to be 20 mm mrad. To perform theBBA stably, BPMs are required to have one magnitude
better position resolution than the required alignmentaccuracy. To accomplish <10 μm position resolution, we
have designed dedicated narrow bandpass filter (BPF)
type readout circuitry with 250 MSa/s 16 bit pipeline
ADC (AD9467-250 [2]). As the SuperKEKB injector
injects electron / positron beams into four different energyrings (SuperKEKB HER/LER, PF and PF-AR) simultaneously, we employed electrical attenuators to keep
enough dynamic range.*
BPM readout system has to have not only high position
resolution (σ < 10 μm) but also good position accuracy.
There are many sources of position error, especially input
channel gain drift from each BPM electrode is one ofdominant sources. To compensate this, calibration pulse
generator [3] is implemented. A calibration pulse fromone electrode is equally divided into adjacent electrodes* ryo@post.kek.jpwith coupling capacitance (see Fig. 1). By calculating
both channel output power, channel gain ratio can bemonitored and compensated.

Figure 1: BPM calibration scheme is illustrated. Calibration tone is provided to the top electrode and induced
pulses are generated on both adjacent electrodes withcoupling capacitance.

HARDWARE IMPLEMENTATION
Baseline design of this BPM readout system has beenreported at previous IBIC2013 conference [4]. Hence, wereport updates from the last design here.
As discussed in previous section, we need to handle thesignal with high linearity to achieve target position resolution (σ < 10 μm). It turned out that first prototype has
following rooms to be improved:
At first, helical coils are needed to precise tuning, so it
is not suitable for high linearity application. Therefore,
we changed BPF centre frequency to 180 MHz so that we
can use normal LC filters. Figure 2 shows the photographof the prototype of the BPM readout board, Figure 3
shows block diagram of 2nd prototype BPF board. Thelatter BPF performs waveform shaping (burst length =60 ns) and limits the bandwidth (22 MHz). The former
BPF (BW=60 MHz) is for anti-aliasing at the ADC.
In addition, the latter amplifier is the critical device thatlimits the overall linearity. Therefore, we tested three RF
amplifiers shown in Table 2. As shown in Fig. 4 anADL5536 (Analog Devices (AD)), which have the highest output third-order intercept point (OIP3), has optimal(flat) linearity.
Figure 5 shows the BPF output waveform and ADC
sampled data points. Waveform is stretched to about

60 ns length (<96 ns) so that enough data points can besampled. 180 MHz as BPF center frequency locates at
250 MHz ADC[2]’s 2nd Nyquist zone (Under samplingcondition).

Figure 2: An photograph of prototype of BPM readoutboard (called 18K12A) is shown. This is a double widthmodule. A BPF board is attached on bottom of a main(ADC) board. The output from BPF board is fed to themain board via four coaxial cables.
Then, we changed ADC input circuit from active circuit(with a RF amplifier) to passive circuit (with transformers) not to limit the linearity.
In addition, we took following improving steps to increase effective number of bits (ENOBs) of the ADC,
which relates to the position resolution:
1. It is known that smaller ADC clock jitter contributes
higher ENOBs. Hence, we changed standard AT cut
crystal oscillator to SAW oscillator (known as low jitter output).
2. Eliminates ADC Clock delay chip (EP195) that gives
additional clock jitter.
Finally, we increased calibration pulse generator output
power to more than 40 dBm so that we can get enoughS/N with the large inner diameter BPM for positron(ϕ = 63 mm, normal BPM has ϕ = 27 mm).

Table 1: Design DifferencesPrototype

1st (18K12)

2nd (18K12A)

BPF Frequency

300 MHz180 MHzBPF structure

Bessel Filter withhelical coils +anti-aliasing filter

Bessel filter with
LC + Butterworth filter foranti-aliasing

Cal. power> 36 dBm> 40 dBm

ADC ClockAT cutSAW (low jitter)

Clock delayYes

NoADC Input

Active circuit

Passive circuit

Figure 3: Block diagram of Band Pass Filter board (onechannel is shown). It equips calibration pulse input selector. First Butterworth filter (60 MHz) cut tails that exceed
ADC’s Nyquist window (125 MHz – 250 MHz) andsecond Bessel filter (22 MHz) forms total bandwidth andburst length (60 ns). Each stage has an electrical attenuator and an amplifier to set an adequate level.

Figure 4: Amplifier linearity curve in the BPF circuitry isshown. AD (Blue) shows wide linearity, Avago (Red)shows moderate characteristics and RFMD (Yellow)
shows narrow linearity.

Table 2: Amplifier Comparison
Device

P1dB

Gain

OIP3

NF

Technology

SBB5089Z (RFMD) [5]20 dB

20.8 dB37.5 dB

4.3 dB

InGaP HBT.

ADL5536 (AD) [6]

19.6 dB

19.8 dB

45.0 dB

2.6 dB

GaAs HBT.

MGA-30689 (Avago) [7]22.4 dB

14.3 dB

43.0 dB

3.0 dB

GaAs pHEMT.

20 ns

200 mV

Figure 5: (a) BPF output waveform and (b) ADC sampled data points (2nd Nyquist zone under sampling).

PERFORMANCE TEST
Linearity / Resolution Check
In order to estimate linearity and position resolution of
the BPM readout system, we performed linearity andresolution test. Instead of beam emulated impulse generator signal, 180 MHz CW is used. Because we’d like tokeep the signal level stable as much as possible. Since
accurate signal level is the key of these test, we used anAgilent J7211B Step attenuator, which has 0.03 dB repeatability. We calibrated it with a network analyzer. To
provide beam equivalent ADC sampled data points, weused 96 ns Hanning window, which gives similar waveform with Fig. 5 (b).
Figure 6 shows test results with this setup: position resolution (upper plot) and channel power linearity (lowerplot). At best condition, we can get about 1 μm position
resolution. However, actual beam passes through at offset
position from the BPM center, which gives unequal channel power between opposite channels. Therefore, we haveto set an appropriate operation point, which allows ±5mm position range (14 dB range is required).
Our requirement conditions for this system are:1.
Position resolution is less than 10 μm,
2.
Linearity is within ±0.02 dB.

We find 24 dB ranges is agreed to these condition in theFig. 6, so we can set an appropriate operation point thatallows ±5 mm position range.

Amplifier Gain Time Drift
For the BPF output amplifier candidates, we also measured amplifier gain time drift in a thermostatic chamber(25±0.1°C) for 286 hours (11.9 days).
Measurement results are shown in Fig. 7. Although eachchannel power time drift ratio is up to 1.6% order, this isvery curious phenomenon that these amplifiers have longtime range gain drift even they are stored in a thermostatic chamber.
MGA-30689 (Avago) shows largest gain drift(1.6% (0.14 dB) / 11.9 days). On the other hand,ADL5536 (Analog Devices) shows smallest gain drift(0.6% (0.05 dB) / 11.9 days). Only MGA-30689 has
pHEMT (FFT structure) and ADL5536 and SBB5089
have HBT (Bipolar structure). The difference of the amplifier structure may contribute these gain drift differences.
With this result and linearity measurements discussed inthe previous section, we have decided to employ theADL5536 for the BPF amplifier.

3-BPM Resolution Tests
In order to make a decision to fabricate this readoutsystem, we performed a 3-BPM resolution tests at KEKLinac. We used adjacent three BPMs (SP52-4, SP54-4
and SP56-4).
When we assume linear lattice, arbitrary three BPM
positions x1, x2 and x3 are given as:.
Here, we can get (A, B, C) with a multiple regression
analysis from N set of (x1n, x2n, x3n ) with N-times trials.
Therefore residual can be given (corresponds to Fig. 8right) :1.

Assuming three BPM’s position resolutions are same, theposition resolution is given:

1Figure 6: Upper plot shows BPM readout system resolution at ideal condition. Lower plot shows system linearity: we have 24 dB with ±0.02 dB linearity range.

.

We took 3500 events data at 12.5 Hz, with changing
Y steering magnet current in each 500 events. Figure 8
shows (left) reconstructed position to measured position,(right) residual distribution.
From the last formula and data, we got 3 μm position
resolution.

SUMMARY AND FUTURE PLAN
We have developed a BPM readout system with highposition resolution (σ < 10 μm) and high accuracy with again calibration system. In order to achieve these criteria,
high linearity (±0.02 dB linearity) and large dynamicrange (>14 dB) is required.
We performed 3-BPM resolution tests to demonstrate
the readout system can be used at real environment andwe got 3 m position resolution.
We plan to fabricate total 100 units within this fiscal
year (till March 2015) and install at 2015 summer shutdown.

ACKNOWLEDGMENT

Figure 7: Amplifier gain drift is shown. Vertical axis isnormalized channel power and horizontal axis is elapsedtime. DUTs were tested in a thermostatic chamber.

Through this BPM readout system development, wethank Dr. Steve Smith, Dr. Andrew Young at SLAC andDr. Makoto Tobiyama at KEK for their kind suggestions
and advice in design and development. We also thank
Mr. Kyosuke Yamada of Digitex Lab. Co. Ltd. (PCB),Mr. Morishige Aoyama of Sanko-sha Inc. (RF) and
Mr. Takehito Koui of Skywave Co. Ltd. (FPGA) for developing this readout system with us.

Figure 8: Left plot shows reconstructed position vs. measured position. Right plot shows residual error distribution
histogram.

REFERENCES[1] M. Masuzawa, in Proceedings of the First International
Particle Accelerator Conference (IPAC’10), Kyoto, 2010,
pp. 4764–4768.
[2] Analog Devices, 16-bit, 200 MSPS/250 MSPS Analog-toDigital Converter AD9467 Data Sheet: http://www.analog.com/static/imported-files/data_sheets/AD9467.pdf[3] S. R. Smith et al., “LCLS Stripline BPM System Commissioning”, in Proceedings of the 23rd Particle Accelerator Conference (PAC’09), Vancouver, Canada, 2009,
pp4027-4029.

[4] R. Ichimiya et al., “Development of High Precision Beam
Position Monitor Readout System with Narrow Bandpass
Filter for the KEKB Linac towards the SuperKEKB”,
Proceedings of the IBIC2013, pp.698-701.
[5] RFMD, 50 MHz to 6000 MHz, Cascadable Active Bias
InGaP HBT MMIC Amplifier SBB5089Z Data Sheet:
http://www.rfmd.com/CS/Documents/SBB5089ZDS.pdf[6] Analog Devices, 20 MHz to 1.0 GHz, IF Gain BlockADL5536 Data Sheet: http://www.analog.com/static/
imported-files/data_sheets/ ADL5536.pdf[7] Avago Technologies, 40 MHz to 3000 MHz, Flat Gain
High Linearity Gain Block MGA-30689 Data Sheet:
http://www.avagotech.co.jp/docs/AV02-1876EN