SRS Labs SR430, Руководство пользователя

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110 110
Using Photomultiplier Tubes
flections from the input to the preamplifier.
A good starting point for a snubber network is a 10
inch piece of RG174/U coax cable with a small 50
Ohm pot connected to the end so that the termi-
nating impedance may be adjusted from 0 to 50
Ohms. ( A 10 inch cable will have a round trip time
of about 5 ns -- be sure your PMT has a risetime
less than this.) The other end of this cable is con-
nected to the anode of the PMT, together with the
output signal cable.
Output current pulses will split, 50% going out the
signal cable, and 50% going into the snubber. If
the snubber pot is adjusted to 50 Ohms there will
be no reflection -- the only affect the snubber has
is to attenuate the signal by a factor of two.
The reflection coefficient for a cable with a charac-
teristic impedance R0, terminated into a resistance
Rt, is given by:
Reflection Coefficient = ( Rt - R0 ) / ( Rt + R0 )
If the pot is adjusted to a value below 50 Ohms,
then some portion of the signal will be inverted
and reflected back toward the anode. This reflect-
ed (and inverted ) signal is delayed by the round
trip time in the snubber cable and sent out the sig-
nal cable. The amount of the reflection is adjusted
for the best pulse shape as shown in the figure be-
low.
The round trip time in the snubber cable may be
adjusted so that the reflected signal cancels anode
signal ringing. This is done by using a cable length
with a round trip time equal to the period of the
anode ringing.
PMT BASE CONCLUSIONS:
(1) Taper voltage divider for higher gain in first
stages.
(2) Bypass last few dynodes in pulsed applica-
tions.
(3) Use a snubber circuit to shape the output
pulse.
CATHODE SHIELDING
Head-on PMT's have a semitransparent photoca-
thode which is operated at negative high voltage.
Use care so that no objects near ground potential
contact the PMT near the photocathode.
MAGNETIC SHIELDING
Electron trajectories inside the PMT will be affect-
ed by magnetic fields. A field strength of a few
Gauss can dramatically reduce the gain of a PMT.
A magnetic shield made of a high permeability ma-
One would like to use a large resistor to get a
large voltage pulse, however in photon counting it
is important to maintain a high bandwidth for the
output signal. Since charge on the anode is re-
moved by the load resistance, smaller load resis-
tances increase the bandwidth. The bandwidth of
a 10 pF anode with a 100 Ohm load is 300 MHz.
For convenience, 50 Ohm systems are usually
used. The current pulse from the PMT travels
down a 50 Ohm cable which is terminated by the
50 Ohm input impedance of a preamplifier. The at-
tenuation of RG-58 coax cable at 300 MHz is
about 1 dB/ 10 ft. and so it does not significantly
degrade performance in this application.
To allow counting to 200 MHz, a preamplifier with
a bandwidth which is somewhat larger than 200
2 ns
PMT PULSE
WITHOUT
SNUBBER
PMT PULSE
WITH
SNUBBER
A SLOW FALLTIME PULSE SHAPE IS
IMPROVED WITH A SNUBBER
2 ns
RINGING
WITHOUT
SNUBBER
SNUBBER
CANCELS
RINGING
ANODE RINGING CANCELED WITH
SNUBBER CABLE
terial should be used to shield the PMT.
PREAMPLIFIERS
The output of a PMT is a current pulse. This cur-
rent is converted to a voltage by a load resistor.