Guralp Systems CMG-40T - Manual (Page 26)

CMG-40T
4.2 Calibration methods
Velocity sensors such as the 40T 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 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.
4.3 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 calibration will be
used to determine the complete sensor response in one action. Please refer to the
digitizer's manual for information on other calibration methods.
1. In Scream!'s main window, right-click on the digitizer's icon and select
Control.... Open the Calibration pane.
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