ATTENUATOR FOR OBSERVATION OF FAST RISE-TIME HIGH VOLTAGE PULSES
Hatsuo Yamazaki, Akira Homma and Shinichi Yamaki
Department of Nuclear Engineering, Hokkaido University
When observing fast signals of high voltage, we have to use
an attenuator between the signal source and an oscilloscope. If
we want to observe signals of 1 nsec rise-time, the attenuator
has to be what works well in a frequency range reaching 300 MHz.
Besides, it has to endure the peak currents of pulses and theheat produced in it.
If the input power of the attenuator isless than 100 mW, we can use resisters whose size is much smaller than the wavelength of VHF electromagnetic wave. Attenuatorswith good characteristics in VHF range can be obtained by a ~­shape or a T-shape network of resisters put in a coaxial line.
Attenuators of high wattage cannot be made by this method because of the larger size of resisters. A special considerationis necessary in this case. We have made an attenuator for fast
pulses using a 200 watt resister for RF load which is in themarket, and its structure and performance are given in the following.
The RF load resister is such that carbon is coated on a ceramic cyJinder and there is no spiral cut on the surface. Thestructure of the attenuator is shown in ::ig.l. The input coaxial cable is connected to the flared section which is mode ofsheet copper. The characteristic impedance is held 50 onms
throughopt the flared section. The 50 ohm RF resister, its permissible power being 200 watts, is connected to the larger endof the inner conductor of the flared ~ection. The outer condu~­
tor over the re:'siste* is tapered toward the output end of theresister. Kobayashi ) reported the method to calculate the optimum rate of tapering for minimizing the reflection of signals
in the case of VHF dummy loads. The tapering rate calculatedby the method is exponential. We adopted a linear tapering rateby approximation instead of the exponential one. The output
end of the 200 watt resister is connected to the outer cylinder
by a parallel combination of eight 4-ohm resisters of 1/8 watt.
These resisters are arranged radially as shown in fig.l in orderto reduce the inductance of this part. The signals across theparallel combination of resisters are fed through a 50 ohm
resister to the inner conductor of the output coaxial cable.
The outer conductor of the cable is connected to the output endof the tapered cylinder over the 200 watt resister by a cone~------260

unit: mmfig.l

Structure of the attenuator.

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son

son

200vJ

1/BW

INPUT

o.sn

OUTPUT
fig.2

l/8vJx8

Circuit diagramof the attenuator

Vert:
20V/div.
Horiz: lOnsec/div.

Vert:O.lV/div.
Horiz: 10 nsec/div.

(a) Wave form of source
signal.

(b) Wave form of attenuatedsignal.
fig.3

made of sheet copper. The circuit diagram of the attenuator isshown in fig.2. When the output is connected to an oscilloscopewith a 50 ohm termination, signals are attenuated by 46 db.
As a measure of reflection of signals caused by the attenuator, we measured the VSWR sending continuous waves ranging from80 MHz to 2 GHz toward the input of the attenuator. The measured VSWR distributes between 1.02 and l.S against frequencies.
To see the fidelity of the attenuated wave forms to the originalones, we took the both pictures by oscilloscope. The wave forms
are shown in fig.3. As the signal source, we used a pulse
generator which generates pulses by avalanche operation of transistors. We can conclude that the fidelity of wave form isfairly good. The permissible wattage for continuous operationis less than 200 watts, because the resister is enclosed in thetapered cylinder. However, if some ventilation holes are punched or if a forced air cooling is adopted, the wattage can beraised easily.

*) Kobayashi,s.

Denkitsushingakkai-Zass hi 34 (19Sl) 391.

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