Table of Contents
Advertisement
5.1.2.
Calibration Accuracy
For calibration the 1 g peak vibration level produced by the vibration
table corresponds to the earth's gravitational force at the place where
calibration is carried out. At sea level it represents an acceleration of
9.81 m/sec
2 .
It is indicated when the bronze ball in the table begins to
rattle.
Although this condition can readily be determined by listening to the
noise generated by the ball, by far the most accurate method is to monitor
the vibration level of the table using an oscilloscope (see section 3.2 item
6 d). This method is especially recommended when using the 4292 for the
first time, since by monitoring the wavefvrm displayed on the oscilloscope
(1 g peak is indicated when distortion first occurs on the negative slope of
the waveform) and by listening to the the table, the user can become familar
with the type of noise generated by the ball when an acceleration level of
1 g peak is reached. Future calibrations can then be carried out quite
accurately by listening.
5.2.
PREAMPLIFIER SECTION
5.2.1.
Input Characteristics
An equivalent circuit of the input to the preamplifier section of the 4292
is shown in Fig.5.2. The input sockets are AC coupled via a 1000 pF cou-
pling capacitor Cc to a FET (field effect transistor) input stage. The FET
gives the preamplifier a low input capacitance
c
1
and a high input resistance
R
1 ,
which is especially important when considering the low frequency
Cc
r
o----1
I
cl
Rl
EOL
El
l
27~.:199
Fig.5.2. Equivalent circuit of preamplifier input of 4292
22
Advertisement
Table of Contents
