JAERI-Conf 95-021

PA41
DEVELOPMENT OF AN ANALOG SWITCHGATING A SINGLE BUNCH
T. Ieiri, K. Tanimura* and M. Suzuki*
KEK, National Laboratory for High Energy Physics
Oho 1-1, Tsukuba-shi, lbaraki-ken, 305 Japan
E-mail: IEIRIT@kekvax.kek.jp* NTT Electronics Technology Co.
*Morinosato-Wakamiya 3-1, Atsugi-shi, Kanagawa-ken, 243-01 Japan

1. Introduction
Most storage machines require a multi-bunch
operation in order to achieve a high intensity. KEKB[1]also requires a multi-bunch operation, where about 5000
bunches will be filled in rf buckets with a period of 2 ns.
It is not guaranteed that stored bunches have always thesame intensity and the same position. Therefore, it isimportant to measure the beam parameters of eachbunch. There would be two methods to obtain the beam
information on each bunch. The first one is to sample
successive bunches with a rate of more than 500 MHz
and store data in a memory bank after digitalization.
The second as an alternative method is to pick up a
specified bunch with a ultra-fast gating switch. A gatedsignal is slowly processed within one revolution. Aswitching time should be much shorter than a bunch
Changing timing of a gate pulse
spacing time.
responds to other bunches. The former is applied for thebunch-by-bunch feedback system[2,3]. However, it
would be technically difficult to convert sampled data toa normalized position signal in real-time. The latter
technique using a gate is described here. This techniqueis adequate for measuring a specified bunch in real-time,
but cannot measure all bunches at the same time.

2. Analog Switch
In order to pick up one bunch under a multi-bunch
operation, a switching time of less than 1 ns is requiredin the KEKB. However, commercially available analog
switches using PIN diodes and GaAs FETs do notsatisfy the requirement. A fast analog switch using Sibipolar transistors with fT of 10 GHz has beendeveloped[4] for a burst modulator. A switching time
was obtained to be 1.2 ns. The on/off isolation was
more than 60 dB at 200 MHz. Circuit configuration ofthe switch has been kept and the transistors used in thecircuit were replaced by those with higher fT of 35 GHz
in order to shorten the switching time. Fig.l shows thecircuit of the ultra-fast analog switch. The circuit
consists 12 transistors. The transistors from T3 to T6
control the switch by comparing the voltage externallyapplied at J1 with the In order to pick up one bunch
under a multi-bunch operation, a switching time of lessthan 1 ns is required voltage VR 1. The switch is off
when the voltage at J1 is zero and on at -0.7 V. An rf
signal passes through from J2 to J3. The bias voltages
VCS 1 and VCS2 are used for adjusting offset voltages ofan output signal.

J1VR2INPUT
J22800

T8T7VEE
10030600

60600Fig. 1 Circuit of analog switch.

3. Performance
The switch has been manufactured as a monolithic ICand packaged in a case of 9.3 mm square with 21 pins. Afrequency response was measured using a network analyzer.
Fig. 2 shows transmission loss (S21) with on and off.
The insertion loss is about 11 dB up to 1 GHz. The
on/off isolation is about 40 dB at 500 MHz and is getting
worse as frequency increases. The isolation is worse thanthat expected in a simulation, which may be due to stray
capacitances.
A transient response was tested. A continuous rf waveof 508 MHz was prepared and divided into two ways. One
is applied to the switch input J2 and the other to a
The divider produces a pulse
frequency divider.
corresponding to the revolution frequency by doing 1/5120,
where 5120 is the harmonic number of KEKB. An output
pulse of the divider is used as a trigger of a gate pulse.
Rise and fall times of the gate pulse which synchronizes
with the rf wave should be much faster than the bunch
spacing time. The switching time was less than 200 ps.
Switching noises were observed in an output signal of theswitch. The noises shaping a spike were reduced by alow-pass filter of 800 MHz. Fig. 3 shows an example ofthe transient response. We may notice that a continuous
wave is converted to a mono-pulse. Table 1 summarizes
specifications of the analog switch.

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JAERI-Conf 95-021

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Fig. 2 Frequency characteristics. Horizontal frequency
range is 0.05 GHz to 2.05 GHz. Vertical scale is
10dB/div.
Table 1 Specifications of the switch.
Input/Output Impedance
Input Amplitude
BandwidthControl
Switching TimeON/OFF Isolation
Insertion Loss
Power Supply

50 n0.7 V p-p max.
1.0 MHz to 1.0 GHz
0 to -0.7 v
200 ps max.
40 dB min. at 500MHz

band-pass filter. The other is a wide-band technique,where a pulse height or area of a pulse is detected. Let'scompare the SIN in both techniques assuming that one
bunch is gated with 2 ns gate width and the signal process
is carried out within one tum (10 JlS). The bandwidth is
The thermal noise level ismore than 100 kHz.
proportional to square root of the bandwidth. On thecontrary, a signal amplitude in the frequency domain isapproximately reduced to the duty factor(2xlOE-4) in thetime domain. Therefore, the wide-band detection in thetime domain has an advantage for the SIN ratio in a tumby-tum measurement.
One example of the tum-by-tum measurement is shownin Fig. 4. The apparatus will be able to measure thebetatron tunes as is used in TRISTAN[5]. A beam pulse
is picked-up by a button electrode and will be stretched out
by a coaxial cable. Low-pass filters also stretch the beampulse further so as not to overlap successive bunches. A
low noise amplifier is needed in order to compensate theloss occurred at the gate. The BOD[6l(Bunch OscillationDetector) samples the gated beam pulse with a selfproduced pulse and holds its peak voltage. The BOD
can detect AC components of a normalized beam position
with a help of an AGC (Automatic Gain Control) loop.
The betatron tune measurement is very important anespecially in the components of a normalized beam
position with a help of an KEKB since the beam-beamtune shift gives us a transverse beam size at a colliding
point.

lldB
200mW

RF/508MHz

Divider•
115120"wRh variable delay
analog switd11

BOD: Bunch Oscillation Detector

Fig. 4 Block diagram for detecting the betatron tunes.

pper traceFig. 3 Transient response of theshows an input rf signal of 500MHz with 400mV/div.
Middle one is a gated output signal with 50 mv/div. A
low-pass filter of 800 MHz is used. Lower one is agate pulse with 1V/div. All traces are swept with
1ns/div.

4. Signal Processing
Two techniques to detect a beam pulse are widelyused. One is a narrow-band method, where a specified
frequency component of a beam pulse is detected with a

As a summary, we have developed an analog switch
for gating a single bunch. The gating switch has theswitching time of 200 ps. The on/off isolation was 40
This switch would be used for
dB at 500MHz.
monitoring a bunch intensity and the betatron tunes inreal-time.

References[1] KEKB Design Report, KEK Peport 95-7 (1995).
[2] E. Kikutani andY. Minagawa, KEK Report 945(1994).
[3] J.D. Fox et al., Proc. of 1993 PAC, Washington
D.C., (1993)p.2076.
[4] H. Kikuchi, NTT Co., private communication.
[5] T. Ieiri et al., NIM A265 (1988)p.364.
[6] T. Ieiri et al., Proc. of the 5th Symp. on Accelerator
Science, Tsukuba (1984)p.l57.

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