The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Improvement of a Clipping Circuit in the BPM Readout Electronics
for the A TF Damping Ring
Mik:io TAKANO, Fujio HINODE*, Junji URAKAWA**
Toho University, 2-2-1 Miyama, Funabashi, Chiba 275, Japan*Laboratory for Nuclear Science, Tohoku University, 1-2-1 Mikamine, Taihaku, Sendai, Miyagi 982, Japan**High Energy Accelerator Research Organization, 1--1 Oho, Tsukuba, Ibarak:i 305, Japan

Abstract
The ATF damping ring has been operated since January in1997 at KEK, and several beani studies have been started.
For the orbit measurement, button-type beam position
monitors (BPM) are installed in the ATF damping ring together with a conventional readout electronics. This readout
electronics includes clipping circuits with Schottky diodes
to measure a charge induced in each button-type electrode ofa BPM by a beam. But Schottky diodes can hardly respondto high frequency components in a BPM signal. This fact
finally limits the position resolution of BPM. However, thehigh position resolution will be increasingly required. Hence
we tried to use an FET as a clipping device instead of aSchottky diode, and found out a few good properties to improve performance of the clipping circuit.

1 Introduction
The accelerator test facility (ATF) [ 1] was constructed inorder to research and to develop necessary technology for
future linear colliders. It consists of a 1.54-GeV injector
linac, a damping ring, a beam transport line from the linac
to the ring, and an extraction line for the beam diagnostics
and several beam studies. We have started to operate theATF damping ring since January in 1997.
To achieve as low an emittance as possible is one of critical issues to obtain a high luminosity in future linear colliders, as well as to achieve a high acceleration efficiency
and a strong final focus. Future linear colliders require a very
low vertical emittance, typically rer = 30 nm. A damping
ring is the most feasible method for obtaining such a lowemittance. The ATF damping ring has a race-track shape,and consists of 36 normal cells with a combined-function
bending magnet and eight wiggler magnets to reduce thedamping time. Normal cells form two arc sections, andwiggler magnets are put in two straight sections. The circumference is 138.6 m.
In order to achieve an extremely low emittance, we mustcorrect the dispersion of the orbit, which is small ( 1] < 2mm) in the long wiggler section of the A TF damping ring.
A precise measurement of the dispersion is indispensable.
We usually obtain the actual dispersion by comparing eachclosed-orbit distortion under the conditions of different RF
frequency (4f"" 10 kHz). For this reason, the resolution ofthe beam position monitor (BPM) must be better than 5jlm.

mm in diameter. BPM signals induced by a beam are sent toclipping modules through long cables (RG223/U), typically
40 m in length. Each cable length is determined by taking
account of time of flight so as to detect BPM signals in asame turn by a common trigger signal synchronized withthe beam revolution. Bipolar BPM signals are clipped in theclipping modules. Then unipolar output signals are integrated and digitized in a 14-bit charge-sensitive ADC. A gate
signal for each ADC is generated from the common trigger
signal. This BPM readout system allows us to measure thebeam orbit in any turn with only one passage of the beam
by changing delay time of the common trigger signal [2].
We have investigated signals of a button-type BPM induced by a beam with a bunch length of 20 ps (6 mm) in
the ATF linac by using a digital sampling oscilloscope Tektronix 11802 with a 20-GHz sampling head SD-24. BPM
signals passing through a 33-m cable have a negative swing
with a pulse width of 200 ps in FWHM followed by alonger positive tail. BPM signals have main frequency
components around a few GHz. Thus the clipping module isrequired to operate at as high a frequency as possible.

3 Tests of Clipping Circuits
In general, a diode is used to clip a bipolar pulse. But apn-junction diode can not respond to fast pulses because ofstorage of minority carriers. The pn junction under the forward bias stores minority carriers diffused across the junction and not recombined with a major carrier. When the bias
is reversed, these minority carriers diffuse in the reverse direction.. Whereas a Schottky diode which has a metalsemiconductor junction has no storage of minority carriers.
Therefore, we used Schottky diodes in the clipping module.
Nevertheless, the clipping module needs a low-pass filter of
30 MHz for keeping the operation stable. In addition, theclipping module needs amplification of a signal passing
through a low-pass filter, because a frequency component of
30 MHz in a BPM signal is minor. If a clipping circuit
which can operate at a higher frequency is available, theposition resolution of BPM will be improved together with
the S/1\r ratio. Furthermore, the production cost of clipping
modules will be able to be reduced, because number of parts
will be reduced.
In this paper, we study the performance of another clipping device. We tried to make use of a junction field-effecttransistor (FET) under the class B operation. A bipolar junction transistor was not tried because of the same reason as apn-junction diode.

2 BPM Readout Electronics
3.1 Clipping Circuit using Schottky Diode
There are 96 button-type BPMs in the ATF damping ring.
Most of BPMs have a cylindrical shape of inner diameter of
24 mm, and have four button-type pickup electrodes of 12-426-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

The essential part of the clipping module is investigated tocompare with an FET clipping circuit. The curren~-voltage
characteristic of a Schottky diode can be written as ·eV

I= / 0 { ek•r -1 },current, e is the electron charge, k 8
saturation
theiswhere / 0is Boltzmann's constant, and T is the absolute temperature.
This is known as the rectifier equation. Figure 1 shows twocomponents in the current-voltage characteristic of a Schottky diode (MATSUSHITA MA700A). The rectifier equationfits the data with / 0 = 6.7 ± 0.6 nA and k 8 T/e = 27.8 ± 0.3
mV in a region of the forward voltage less than 300 mV.
Exponential increasing of the forward current gets slow at ahigher forward voltage. It seems to be due to an effect ofseries resistance in the diode.

,,!

~·

>>··r.... ·r~+-----------~------------~---------"----------------'------------•-----------'
''
LL

·····

I =·I {exp[eV/ k BT]-1} ...

Figure 3. Typical output signal of Schottky clipping ci rcuit (top) and its bipolar input pulse (bottom).
The linearity of the Schottky clipping circuit is shown inFigure 4. In this measurement, the same setup as that of thewaveform measurement was used but the input square pulse
with a pulse width of 8 ns. The clipped pulse appears when
the bipolar input pulse has· a pulse height over 400 mV, .but
the clipped pulse is smaller than the input pulse. The undershoot due to the junction capacitance has a pulse height of
20-30% of the clipped pulse. The pulse height of the undershoot is as high as that of the clipped pulse for an input
pulse below 400 mV. The Schottky diodelooks like an AC
coupling capacitorin this region.

010 = 6.7±0.6 nA

250,~~~rr~~~~rr~~OOA I L L.

...... -+··---k 8T/e = 27.8±0.3 mV ...

! Outpu~ pulse, height!

~ 150 r-·········:············(·········:········-":··········-VF [mV]

Figure 1. Current-voltage characteristic of a Schottky diode (MA700A) in the forward bias.
A test circuit using two Schottky diodes is shown in Figure 2. An RC differentiating circuit in this test circuit
makes a bipolar input pulse from an input square pulse. Atypical ou~ut signal (50 mV/div, 10 ns/div) is shown at thetop of Figure 3 together with a bipolar input pulse (500mV/div) at the bottom. In this measurement, a 50-MHz
function generator HP 8116A was used to generate the input
square pulses with a pulse width of 20 ns and a 400-MHz
oscilloscope Tektronix 2467B with a probe P6133 (10 MQ,13.5 pF) to pick up an input pulse. The clipped pulse isfollowed by a differentiatedpulse of the negative swing owing to the junction capacitance of the Schottky diode.
!OpFMA700A500.......

200

1 o· 4 L....~.w.......;..L..._._....J_.._._-"-'-_._~.l....<._~
800 1 000
600
200 4000i-..0> 100.....

i; •i

f~·

j·+ .... +· .. ·+ ... +·· ·····~..

··········i.············f.······-"···r... Unde~shoot:so:-00i•• :. • •:l·•j•.ll0.20.40.60.8vi [VJFigure 4. Clipped pulse V 0 versus input pulse V;.
Response to a faster pulse generated by a 500-MHz pulse
generator HP 8131A was measured with a 1-GHz digitalreal-time oscilloscope. Tektronix TDS684B. When a positive unipolar pulse which has a pulse width of 2.0 ns and apulse height of 1.0 V was directly injected to the Schottkyclipping circuit without RC differentiating circuit, the output pulse was a positive unipolar pulse with a pulse heightof 0.5 V. When a reversed pulse was injected, a negative
unipolar pulse with a pulse height of 0.5 V appeared at theoutput. For such a fast pulse, the Schottky diode seems tobehave like a capacitor.

Figure 2. Clipping circuit using two Schottky diodes.
3.2 Clipping Circuit using Junction FET

We tried a source follower of a junction FET in the class
B operation, but it had the same problem as the Schottkyclipping circuit for its pn-junction capacitance. A commonsource amplifier was investigated instead. Figure 5 shows
the transfer characteristic of an FET (HITACHI 2SK291).
The drain current can be theoretically written as-427-

The II th Symposium on Accelerator Science and Technology, Harima Science Garden City, I997

2_(Vas )Io - loss 1- Vpis less than 20% of the clipped pulse for an input pulse over
200 mV.

'where foss is the saturation drain current and Vr is the pinchoff voltage. A theoretical curve fits the data very well with
foss= 27.8 ± 0.2 rnA and Vp = -0.97 ± 0.01 V.

800> 600

2SK291 @V 05 = 5.0 V.s250> 40020;;('.s 152000100.20.60.81 .2v; [V]5Figure 8. Clipped pulse V 0 versus input pulse V;.

oLy~~~w_~~_w~_w~~- 1 .2

0.4

-1- 0 .8 - 0 .6 - 0 .4 - 0 .2

0V GS (V]Figure 5. Transfer characteristic of a junction FET(2SK291).
A test circuit using this FET is shown in Figure 6. Samemeasurements as the Schottky clipping circuit were made tothis circuit with the exactly same setup. This circuit has an
operating point at Vas = 0.904 V. A typical output signal(200 mV/div, 10 ns/div) is shown at the top of Figure 7
together with a bipolar input pulse (500 mV/div) at the bottom. The overshoot is very small in comparison with theclipped pulse.

IO.I~FFigure 6. Clipping circuit using an FET.

Response to a faster pulse generated by a 500-MHz pulse
generator was measured. When a positive unipolar pulse
which had a pulse width of 2.0 ns and a pulse height of 1.0
V was directly injected to the FET clipping circuit without
RC differentiating circuit, the output pulse was a positive
unipolar pulse with a pulse height of 1.7 V. When a reversedpulse was injected, a bipolar differentiatedpulse witha small pulse height of 170 mV appeared at the output. It
seems to be due to the gate-draincapacitance.
4SummaryWe compareda clipping circuit using a junction FET withthat using two Schottky diodes. The FET clipping circuit
can respond to an input pulse with a pulse height over 100mV, and has almost no overshoot in contrast with the Shottky clipping circuit. Both clipping circuits have nonlinearity, but that of the FET clipping circuit is much simpler than the Schottky clipping circuit. The non-1inearity of
the FET circuit can be corrected with a quadratic function.
Furthermore, the FET clipping circuit could respond to afast pulse with a pulse width of 2 ns, but the Schottky clipping circuit could not. An FET has a few abilities to improve performanceof the clipping circuit and to improve theposition resolution of BPM.
We will test a few kinds of MOS FETs as well, because
MOS FETs have smaller capacitances. In addition, the cascodeconnection will be tried to reduce an effect of the gatedrain capacitance. We will make a prototype clipping module using a junction or MOS FET, and will test it withbeams in this autumn.

References[1] F. Hinode etal., "ATF Design and Study Report", KEKInternal 95-4 (1995).
Figure 7. Typical output signal of FET clipping circuit[2] F. Hinode et al., "A Conventional Read-out Electronics(top) and its bipolar input pulse (bottom).
for the Button-type BPM in the ATF Damping. Ring"- (submitted to the 1997 Particle Accelerator Conference),
The linearity of the FET clipping circuit is shown in Fig- KEK Preprint 97-101 (1997).
ure 8. The clipped pulse appears clearly when the bipolar
input pulse has a pulse height over 100 mV. The overshoot-428-