1.
Introduction
1.1. General
The Vibration Pick-up Preamplifier Type
2625
is provided with integration
networks
for
measurement
of
velocity and displacement in addition to accelera-
tion. There are three input sockets connected to a selector switch, with in-
dividual sensitivity adjustment of each input. The two inputs not in use are
connected to ground. The function selector controls three different gain ranges
for
the three inputs: 1: Variable from
-40
to
-20
dB, 2: Variable from 0 to
+
20
dB, 3: Fixed at 0 dB. The integration networks are passive
RC
networks
with three different
lower
frequency limits.
1,
10
and
100Hz
for
velocity, and six
limits,
1,
3,
10,
30,
100 and 300 Hz
for
displacement measurements.
The output signal is taken from a standard microsocket
10-32 NF
or
a B & K
coaxial socket at the
front
panel,
or
from a microsocket at the back.
A field effect transistor stage in the
input
circuit
gives extremely high input
impedance, variable from about
3000
MQ at zero gain to about
450
MQ at
20
dB gain.
The preamplifier has a built-in battery compartment
for
battery operation or
may be powered by external source,
28
V
DC.
The Type
2625
Vibration Pick-up Preamplifier is a high
input
impedance
amplifier
for
use with piezoelectric accelerometers.
A piezoelectric device possesses a small capacitance, which develops a
small
charge across
it
when subjected to an acceleration. If the accelerometer were
connected to a large capacitance, such as a long cable to an amplifier, the
sensitivity would be much reduced, because
of
the potential dividing action
of
the cable capacitance and accelerometer capacitance. The main function
of
the preamplifier is as an impedance transformer, between the high input
impedance required
for
the accelerometer, and low output impedance re-
quired
for
feeding the· signal into relatively high capacitance
of
a cable.
3
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