Guralp Systems CMG-3T - Manual (Page 23)

Operator's guide
3.2 Calibration methods
Velocity sensors such as the 3T are not sensitive to constant DC levels,
either as a result of their design or because of an interposed high-pass
filter. 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 hand-held
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 Hand-held Control Units provide a switch for activating
the CAL ENABLE line.
3.3 Calibration with Scream!
Güralp digitizers provide calibration signal generators to help you set
up your sensors. Calibration is most easily done through a PC running
Güralp's Scream! software.
Depending on the digitizer type, sine-wave, step and broadband noise
signal generators may be available. In this section, broadband noise
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