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Vibration Pick-up Preamplifier
X Z
Input
Goln · Goin
v-·z-
Output
~
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Type 2625
A fully transistorised preamplifier
of
·
modular design
measurements. Integration
are included to give an output
proportional to acceleration, velocity,
or
displacement, from an accelero-
meter input. Fixed gains,
individually adjustable gains are
available from three separate inputs.
for
vibration
networks
or
Vibration Pick-up Preamplifier
Type 2625
Reprint
february
1970
Contents
1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.
Block
Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.
Front Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3.
Batt:ery Compartment
2.4.
Rear pane·l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.5.
Module System
2.6.
Input Characteristics
2.7.
Frequency Characteristics
2.8. Output Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.9.
Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.10. Distortion
2.11. Noise
3.
Operation . .
3.1.
Power Requirements
3.2.
Input and Output
3.3.
Integration Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.
Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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0 0
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10
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14
1.
Introduction
1.1. General
The Vibration Pick-up Preamplifier Type
for
networks
tion. There are three input sockets connected to a selector switch, with individual 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
+
20
with three different
limits,
The output signal is taken from a standard microsocket
coaxial socket at the
A field effect transistor stage in the
impedance, variable from about
20
dB gain.
The preamplifier has a built-in battery compartment
may be powered by external source,
The Type
amplifier
A piezoelectric device possesses a small capacitance, which develops a
charge across
connected to a large capacitance, such as a long cable to an amplifier, the
sensitivity would be much reduced, because
of
the cable capacitance and accelerometer capacitance. The main function
of
the preamplifier is as an impedance transformer, between the high input
impedance required
quired
measurement
dB, 3: Fixed at 0 dB. The integration networks are passive
1,
3,
10,
30,
100 and 300 Hz
2625
Vibration Pick-up Preamplifier is a high
for
use with piezoelectric accelerometers.
it
when subjected to an acceleration. If the accelerometer were
for
feeding the· signal into relatively high capacitance
of
velocity and displacement in addition to accelera-
lower
frequency limits.
for
front
panel,
or
3000
for
the accelerometer, and low output impedance re-
2625
is provided with integration
-40
to
-20
dB, 2: Variable from 0 to
RC
1,
10
and
100Hz
for
velocity, and six
displacement measurements.
10-32 NF
from a microsocket at the back.
input
circuit
gives extremely high input
MQ at zero gain to about
for
battery operation or
28 V DC.
input
of
the potential dividing action
of
a cable.
networks
or
a B & K
450
MQ at
impedance
small
3
2.
Description
2.1.
Block
Diagram
A
block
One
diagram
of
three inputs is selected by the selector switch, which also controls
of
the
circuit
is given in Fig.
2.1.
the gain setting. The input stage is a field-effect transistor, and the main
amplifier stages are followed by the integration networks which are controlled from the fro.nt panel.
Selector
2.2.
Capacitive
Switch Attenuotor
Front Panel
The front panel
Differential
Input
Stage
Amplifier
Fig. 2.1.
of
Block
the 2625 is shown in Fig. 2.2. The controls are as follows:
Output
Stage
Feedback
Adjustement
diagram
Integration
Networks
of
preamplifier
Amplifier
circuit.
Power
ACCELERATION, VELOCITY, DISPLACEMENT. This knob selects the integration networks.
In the ACCELERATION position no integrating networks are
connected. Three velocity and six displacement positions are possible, giving
lower limiting frequencies as indicated. The low frequency
limit
should be set
as high as possible to obtain the best possible signal to noise ratio.
INPUT SELECTOR. This knob has twelve positions, and selects which
3 inputs,
X,
Y
or
Z is used. The two inputs
not
in use are connected to
of
the
ground.
of
Three
whereby the gain
20
different accelerometers
the twelve switch positions are labelled
X
Gain Adj.
y
0--20
z
of
the amplifier can be adjusted individually between 0 and
dB
for
the three inputs. Thus
or
it
from a triaxial accelerometer to the same value
dB
is possible to scale the signals from three
4
Fig.
2.2.
Front panel
of
the preamplifier.
of mV/g for each channel. Accelerometers with sensitivities from 1 to
can be scaled to 10 mV/g and those with sensitivities from 10 to
can be scaled to 100 mV/g.
Also in the input stages is a capacitive attenuator which gives
tion. This gives six more switch positions, labelled
X
Gain Adj.
y
-40
to-
z
20 dB
40
dB attenua-
10
100
mV/g
mV/g
5
and
X
Y
-40
dB
z
giving the possibility
a fixed gain
The remaining three positions
of
- 40 dB.
of
adjusting the gain between
of
the selector switch are labelled
X
Y 0 dB
z
in which no sensitivity adjustment is possible.
-40
and
-20
dB
or
giving
GAIN. These three potentiometers are used
dividually for the three inputs, as described under
INPUT.
for connection to the accelerometer via mininoise cable . The two inputs not
in use are always grounded.
OUTPUT. There are two output sockets available on the front panel, electrically
identical.
GROUND. The small unmarked terminal is connected to the instrument chassis.
The three input sockets require miniature coaxial plugs Type JP 0012,
One is a standard microsocket, the
for
adjusting
INPUT SELECTOR.
other
a B & K coaxial socket.
the
sensitivity in-
2.3. Battery Compartment
Batteries
shown in Fig.
on the rear
for
the
instruments internal supply are kept in the
2.3. The compartment is accessible by removing the
of
the
equipment (Fig. 2.4), and sliding the top panel backwards.
top
compartment
top
screw
3 X 9 V batteries are required, size 25.5 X 48.5 mm. (IEC Recommendation
86-2.6
F 22). See specifications.
2.4. Rear Panel
The rear panel
POWER SWITCH. This connects the power to the instrument by selecting either
the internal batteries
are
labelled OFF. BATT. EXT. POWER and ZR 0024, respectively. In position
ZR
0024 the output is switched from the power amplifier to the integration net-
works, thus reducing the power requirement from
2 mA.
+ 28
V.
microsocket (centre pin positive).
OUTPUT. This is the third output socket and is electrically identical to the two
on the front panel.
of
the instrument is shown in Fig. 2.4.
or
the external supplies. Four positions are available and
+ 28
V.
7
rnA
to + 28
External power, including that from the ZR 0024 is supplied to this
V.
2.5.
Module System
The preamplifier is part
shown in Fig.
very convenient.
2.5. This system makes rack mounting
6
of
the B & K module system
of
instrumentation as
of
instrument combinations
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