CMG-3V
4 Calibrating the 3V
4.1 Calibration methods
Velocity sensors such as the 3V are not sensitive to constant DC levels,
either as a result of their design or because of an interposed high-pass
flter. Instead, three common calibration techniques are used.
•
Injecting a step current allows the system response to be
determined in the time domain. The amplitude and phase
response can then be calculated using a Fourier transform.
Because the input signal has predominantly low-frequency
components, this method generally gives poor results. However,
it is simple enough to be performed daily.
•
Injecting a sinusoidal current of known amplitude and frequency
allows the system response to be determined at a spot frequency.
However, before the calibration measurement can be made the
system must be allowed to reach a steady state; for low
frequencies, this may take a long time. In addition, several
measurements must be made to determine the response over the
full frequency spectrum.
•
Injecting white noise into the calibration coil gives the response of
the whole system, which can be measured using a spectrum
analyser.
You can perform calibration either using a Güralp DM24 digitizer,
which can generate step and sinusoidal calibration signals, or by feeding
your own signals into the instrument through a handheld control unit.
Before you can calibrate the instrument, its calibration relays need to be
activated by pulling low the
CAL ENABLE
line on the instrument's
connector for the component you wish to calibrate. Once enabled, a
calibration signal provided across the
CAL SIGNAL
and
SIGNAL
GROUND
lines will be routed through the feedback system. You can
then measure the signal's equivalent velocity on the sensor's output
lines. Güralp Handheld Control Units provide a switch for activating the
CAL ENABLE
line.
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