Calibrating the 40T
Note:
If you have a digital instrument, separate the serial numbers of the
analogue part and the digital part with a space, as shown.
5.2
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 digitiser, 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.
5.3
Calibration with Scream!
Güralp digitisers 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 digitiser 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
digitiser's manual for information on other calibration methods.
28
Issue F - June 2019
