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ball resting on three points in a hermetically sealed chamber in the vibration
table is used. When the ball just commences to vibrate the acceleration level
of the table will be 1 g peak.
To isolate the vibration system from the chassis of the instrument and
thus minimise interference from external vibrations, the system is hung
from a spring suspension which has a resonant frequency of approximately
15Hz.
4.2. PREAMPLIFIER SECTION
4.2.1. Input Stage
There are two alternative input sockets to the preamplifier section of the
4292.
One accepts the standard B & K coaxial plugs Type JP
0101
whilst
the other is fitted with a
10-
32 NF thread and accepts the miniature
coaxial plugs Type JP
0012.
The sockets are connected in parallel and lead
to the input stage which uses a FET (field effect transistor) to give it an
extremely high input impedance (450
Mn
to 2 GQ, depending on setting of
the SENSITIVITY control). This is especially important as it permits a
low-lower limiting frequency to be obtained for a given combination of
accelerometer capacitance and preamplifier input impedance. The FET also
helps give the preamplifier a high signal-to-noise ratio.
4.2.2. 0 - 20 dB Amplifier
The output of the input stage is coupled to a two stage DC amplifier. To
give the amplifier a good temperature stability the input stage consists of
two transistors connected to form a differential pair. The overall gain of the
amplifier may be adjusted from
0
to
20
dB by adjustment of the SENSITIV-
ITY control which is a potentiometer in the main feedback loop of the
amplifier.
4.2.3. Integration Networks
For vibration measurements using an accelerometer with the
4292,
the
output from the
0
to
20
dB amplifier will be proportional to the accelera-
tion of the vibration measured. However, it will often be required to obtain
an output which is proportional to the velocity, or displacement of the
vibration. Since the quantities acceleration, velocity and displacement of a
16
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