BRUEL & KJAER 4292 Instructions And Applications page 20

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Although for periodic vibrations both velocity and displacement can be
calculated mathematically from the acceleratiQn, this is not possible for
complex vibrations and therefore integration has to be performed electrical-
ly. For this reason the 4292 employs nine passive R-C integration networks
which may be selected using the INTEGRATION SWITCH. In the velocity
modes of this switch the acceleration signal from the output of the 0 to
20 dB amplifier is integrated using the type of network shown in Fig.4.4, in
which the voltage across the capacitor is given by:
e
= -
1
J
i dt
c
c
and the total input voltage is:
e
=
e R
+
ec
=
R
i
+
i-f
i dt
4.4.
4.5.
To obtain the correct integration. the values of R and Care chosen such
t~at eR ~ec, and consequently i
=
W
which, substituted into equation 4.4
g1ves
1
ec
=
RC
f
e dt
4.6.
A specific combination of resistor and capacitor can only be used for
integration over a limited frequency range. At high frequencies the signal
level across the capacitor will decrease to a too low value and consequently
the signal-to-noise ratio will be too small. At frequencies less than the lower
limiting frequency
(-7
dB point on frequency characteristics for velocity
~eR-.f
I
... CJ
CI
r
R
ec
eoutput
1
I
l
2U25S
Fig.4.4. Schematic of an R-C integration network used for velocity meas-
urement
18